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vector-toc-level-2"> <a class="vector-toc-link" href="#Case_sensitivity"> <div class="vector-toc-text"> 2.1 Case sensitivity </div> </a> <ul id="toc-Case_sensitivity-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Whitespace_and_semicolons" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Whitespace_and_semicolons"> <div class="vector-toc-text"> 2.2 Whitespace and semicolons </div> </a> <ul id="toc-Whitespace_and_semicolons-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Comments" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Comments"> <div class="vector-toc-text"> 2.3 Comments </div> </a> <ul id="toc-Comments-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> <li id="toc-Variables" class="vector-toc-list-item vector-toc-level-1"> <a class="vector-toc-link" href="#Variables"> <div class="vector-toc-text"> 3 Variables </div> </a> <button aria-controls="toc-Variables-sublist" class="cdx-button cdx-button--weight-quiet cdx-button--icon-only vector-toc-toggle"> Toggle Variables subsection </button> <ul id="toc-Variables-sublist" class="vector-toc-list"> <li id="toc-Scoping_and_hoisting" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Scoping_and_hoisting"> <div class="vector-toc-text"> 3.1 Scoping and hoisting </div> </a> <ul id="toc-Scoping_and_hoisting-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Declaration_and_assignment" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Declaration_and_assignment"> <div class="vector-toc-text"> 3.2 Declaration and assignment </div> </a> <ul id="toc-Declaration_and_assignment-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Examples" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Examples"> <div class="vector-toc-text"> 3.3 Examples </div> </a> <ul id="toc-Examples-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> <li id="toc-Primitive_data_types" class="vector-toc-list-item vector-toc-level-1"> <a class="vector-toc-link" href="#Primitive_data_types"> <div class="vector-toc-text"> 4 Primitive data types </div> </a> <button aria-controls="toc-Primitive_data_types-sublist" class="cdx-button cdx-button--weight-quiet cdx-button--icon-only vector-toc-toggle"> Toggle Primitive data types subsection </button> <ul id="toc-Primitive_data_types-sublist" class="vector-toc-list"> <li id="toc-Undefined" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Undefined"> <div class="vector-toc-text"> 4.1 Undefined </div> </a> <ul id="toc-Undefined-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Number" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Number"> <div class="vector-toc-text"> 4.2 Number </div> </a> <ul id="toc-Number-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-BigInt" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#BigInt"> <div class="vector-toc-text"> 4.3 BigInt </div> </a> <ul id="toc-BigInt-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-String" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#String"> <div class="vector-toc-text"> 4.4 String </div> </a> <ul id="toc-String-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Boolean" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Boolean"> <div class="vector-toc-text"> 4.5 Boolean </div> </a> <ul id="toc-Boolean-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Type_conversion" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Type_conversion"> <div class="vector-toc-text"> 4.6 Type conversion </div> </a> <ul id="toc-Type_conversion-sublist" class="vector-toc-list"> <li id="toc-Boolean_type_conversion" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#Boolean_type_conversion"> <div class="vector-toc-text"> 4.6.1 Boolean type conversion </div> </a> <ul id="toc-Boolean_type_conversion-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> <li id="toc-Symbol" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Symbol"> <div class="vector-toc-text"> 4.7 Symbol </div> </a> <ul id="toc-Symbol-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> <li id="toc-Native_objects" class="vector-toc-list-item vector-toc-level-1"> <a class="vector-toc-link" href="#Native_objects"> <div class="vector-toc-text"> 5 Native objects </div> </a> <button aria-controls="toc-Native_objects-sublist" class="cdx-button cdx-button--weight-quiet cdx-button--icon-only vector-toc-toggle"> Toggle Native objects subsection </button> <ul id="toc-Native_objects-sublist" class="vector-toc-list"> <li id="toc-Array" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Array"> <div class="vector-toc-text"> 5.1 Array </div> </a> <ul id="toc-Array-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Date" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Date"> <div class="vector-toc-text"> 5.2 Date </div> </a> <ul id="toc-Date-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Error" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Error"> <div class="vector-toc-text"> 5.3 Error </div> </a> <ul id="toc-Error-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Math" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Math"> <div class="vector-toc-text"> 5.4 Math </div> </a> <ul id="toc-Math-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Regular_expression" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Regular_expression"> <div class="vector-toc-text"> 5.5 Regular expression </div> </a> <ul id="toc-Regular_expression-sublist" class="vector-toc-list"> <li id="toc-Character_classes" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#Character_classes"> <div class="vector-toc-text"> 5.5.1 Character classes </div> </a> <ul id="toc-Character_classes-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Character_matching" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#Character_matching"> <div class="vector-toc-text"> 5.5.2 Character matching </div> </a> <ul id="toc-Character_matching-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Repeaters" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#Repeaters"> <div class="vector-toc-text"> 5.5.3 Repeaters </div> </a> <ul id="toc-Repeaters-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Anchors" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#Anchors"> <div class="vector-toc-text"> 5.5.4 Anchors </div> </a> <ul id="toc-Anchors-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Subexpression" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#Subexpression"> <div class="vector-toc-text"> 5.5.5 Subexpression </div> </a> <ul id="toc-Subexpression-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Flags" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#Flags"> <div class="vector-toc-text"> 5.5.6 Flags </div> </a> <ul id="toc-Flags-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Advanced_methods" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#Advanced_methods"> <div class="vector-toc-text"> 5.5.7 Advanced methods </div> </a> <ul id="toc-Advanced_methods-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Capturing_groups" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#Capturing_groups"> <div class="vector-toc-text"> 5.5.8 Capturing groups </div> </a> <ul id="toc-Capturing_groups-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> <li id="toc-Function" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Function"> <div class="vector-toc-text"> 5.6 Function </div> </a> <ul id="toc-Function-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> <li id="toc-Operators" class="vector-toc-list-item vector-toc-level-1"> <a class="vector-toc-link" href="#Operators"> <div class="vector-toc-text"> 6 Operators </div> </a> <button aria-controls="toc-Operators-sublist" class="cdx-button cdx-button--weight-quiet cdx-button--icon-only vector-toc-toggle"> Toggle Operators subsection </button> <ul id="toc-Operators-sublist" class="vector-toc-list"> <li id="toc-Arithmetic" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Arithmetic"> <div class="vector-toc-text"> 6.1 Arithmetic </div> </a> <ul id="toc-Arithmetic-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Assignment" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Assignment"> <div class="vector-toc-text"> 6.2 Assignment </div> </a> <ul id="toc-Assignment-sublist" class="vector-toc-list"> <li id="toc-Destructuring_assignment" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#Destructuring_assignment"> <div class="vector-toc-text"> 6.2.1 Destructuring assignment </div> </a> <ul id="toc-Destructuring_assignment-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Spread/rest_operator" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#Spread/rest_operator"> <div class="vector-toc-text"> 6.2.2 Spread/rest operator </div> </a> <ul id="toc-Spread/rest_operator-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> <li id="toc-Comparison" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Comparison"> <div class="vector-toc-text"> 6.3 Comparison </div> </a> <ul id="toc-Comparison-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Logical" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Logical"> <div class="vector-toc-text"> 6.4 Logical </div> </a> <ul id="toc-Logical-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Logical_assignment" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Logical_assignment"> <div class="vector-toc-text"> 6.5 Logical assignment </div> </a> <ul id="toc-Logical_assignment-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Bitwise" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Bitwise"> <div class="vector-toc-text"> 6.6 Bitwise </div> </a> <ul id="toc-Bitwise-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Bitwise_Assignment" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Bitwise_Assignment"> <div class="vector-toc-text"> 6.7 Bitwise Assignment </div> </a> <ul id="toc-Bitwise_Assignment-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-String_2" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#String_2"> <div class="vector-toc-text"> 6.8 String </div> </a> <ul id="toc-String_2-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-??" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#??"> <div class="vector-toc-text"> 6.9 ?? </div> </a> <ul id="toc-??-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> <li id="toc-Control_structures" class="vector-toc-list-item vector-toc-level-1"> <a class="vector-toc-link" href="#Control_structures"> <div class="vector-toc-text"> 7 Control structures </div> </a> <button aria-controls="toc-Control_structures-sublist" class="cdx-button cdx-button--weight-quiet cdx-button--icon-only vector-toc-toggle"> Toggle Control structures subsection </button> <ul id="toc-Control_structures-sublist" class="vector-toc-list"> <li id="toc-Compound_statements" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Compound_statements"> <div class="vector-toc-text"> 7.1 Compound statements </div> </a> <ul id="toc-Compound_statements-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-If_..._else" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#If_..._else"> <div class="vector-toc-text"> 7.2 If ... else </div> </a> <ul id="toc-If_..._else-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Conditional_(ternary)_operator" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Conditional_(ternary)_operator"> <div class="vector-toc-text"> 7.3 Conditional (ternary) operator </div> </a> <ul id="toc-Conditional_(ternary)_operator-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Switch_statement" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Switch_statement"> <div class="vector-toc-text"> 7.4 Switch statement </div> </a> <ul id="toc-Switch_statement-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-For_loop" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#For_loop"> <div class="vector-toc-text"> 7.5 For loop </div> </a> <ul id="toc-For_loop-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-For_..._in_loop" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#For_..._in_loop"> <div class="vector-toc-text"> 7.6 For ... in loop </div> </a> <ul id="toc-For_..._in_loop-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-While_loop" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#While_loop"> <div class="vector-toc-text"> 7.7 While loop </div> </a> <ul id="toc-While_loop-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Do_..._while_loop" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Do_..._while_loop"> <div class="vector-toc-text"> 7.8 Do ... while loop </div> </a> <ul id="toc-Do_..._while_loop-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-With" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#With"> <div class="vector-toc-text"> 7.9 With </div> </a> <ul id="toc-With-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Labels" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Labels"> <div class="vector-toc-text"> 7.10 Labels </div> </a> <ul id="toc-Labels-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> <li id="toc-Functions" class="vector-toc-list-item vector-toc-level-1"> <a class="vector-toc-link" href="#Functions"> <div class="vector-toc-text"> 8 Functions </div> </a> <button aria-controls="toc-Functions-sublist" class="cdx-button cdx-button--weight-quiet cdx-button--icon-only vector-toc-toggle"> Toggle Functions subsection </button> <ul id="toc-Functions-sublist" class="vector-toc-list"> <li id="toc-Async/await" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Async/await"> <div class="vector-toc-text"> 8.1 Async/await </div> </a> <ul id="toc-Async/await-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> <li id="toc-Objects" class="vector-toc-list-item vector-toc-level-1"> <a class="vector-toc-link" href="#Objects"> <div class="vector-toc-text"> 9 Objects </div> </a> <button aria-controls="toc-Objects-sublist" class="cdx-button cdx-button--weight-quiet cdx-button--icon-only vector-toc-toggle"> Toggle Objects subsection </button> <ul id="toc-Objects-sublist" class="vector-toc-list"> <li id="toc-Creating_objects" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Creating_objects"> <div class="vector-toc-text"> 9.1 Creating objects </div> </a> <ul id="toc-Creating_objects-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Methods" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Methods"> <div class="vector-toc-text"> 9.2 Methods </div> </a> <ul id="toc-Methods-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Constructors" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Constructors"> <div class="vector-toc-text"> 9.3 Constructors </div> </a> <ul id="toc-Constructors-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Inheritance" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Inheritance"> <div class="vector-toc-text"> 9.4 Inheritance </div> </a> <ul id="toc-Inheritance-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> <li id="toc-Exception_handling" class="vector-toc-list-item vector-toc-level-1"> <a class="vector-toc-link" href="#Exception_handling"> <div class="vector-toc-text"> 10 Exception handling </div> </a> <ul id="toc-Exception_handling-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Native_functions_and_methods" class="vector-toc-list-item vector-toc-level-1"> <a class="vector-toc-link" href="#Native_functions_and_methods"> <div class="vector-toc-text"> 11 Native functions and methods </div> </a> <button aria-controls="toc-Native_functions_and_methods-sublist" class="cdx-button cdx-button--weight-quiet cdx-button--icon-only vector-toc-toggle"> Toggle Native functions and methods subsection </button> <ul id="toc-Native_functions_and_methods-sublist" class="vector-toc-list"> <li id="toc-eval_(expression)" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#eval_(expression)"> <div class="vector-toc-text"> 11.1 eval (expression) </div> </a> <ul id="toc-eval_(expression)-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> <li id="toc-See_also" class="vector-toc-list-item vector-toc-level-1"> <a class="vector-toc-link" href="#See_also"> <div class="vector-toc-text"> 12 See also </div> </a> <ul id="toc-See_also-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-References" class="vector-toc-list-item vector-toc-level-1"> <a class="vector-toc-link" href="#References"> <div class="vector-toc-text"> 13 References </div> </a> <ul id="toc-References-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Further_reading" class="vector-toc-list-item vector-toc-level-1"> <a class="vector-toc-link" href="#Further_reading"> <div class="vector-toc-text"> 14 Further reading </div> </a> <ul id="toc-Further_reading-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-External_links" class="vector-toc-list-item vector-toc-level-1"> <a class="vector-toc-link" href="#External_links"> <div class="vector-toc-text"> 15 External links </div> </a> <ul id="toc-External_links-sublist" class="vector-toc-list"> </ul> </li> </ul> </div> </div> </nav> </div> </div> <div class="mw-content-container"> <main id="content" class="mw-body"> <header class="mw-body-header vector-page-titlebar"> <nav aria-label="Contents" class="vector-toc-landmark"> <div id="vector-page-titlebar-toc" class="vector-dropdown vector-page-titlebar-toc vector-button-flush-left" > <input type="checkbox" id="vector-page-titlebar-toc-checkbox" role="button" aria-haspopup="true" data-event-name="ui.dropdown-vector-page-titlebar-toc" class="vector-dropdown-checkbox " aria-label="Toggle the table of contents" > <label id="vector-page-titlebar-toc-label" for="vector-page-titlebar-toc-checkbox" class="vector-dropdown-label cdx-button cdx-button--fake-button cdx-button--fake-button--enabled cdx-button--weight-quiet cdx-button--icon-only " aria-hidden="true" > Toggle the table of contents </label> <div class="vector-dropdown-content"> <div id="vector-page-titlebar-toc-unpinned-container" class="vector-unpinned-container"> </div> </div> </div> </nav> <h1 id="firstHeading" class="firstHeading mw-first-heading">JavaScript syntax</h1> <div id="p-lang-btn" class="vector-dropdown mw-portlet mw-portlet-lang" > <input type="checkbox" id="p-lang-btn-checkbox" role="button" aria-haspopup="true" data-event-name="ui.dropdown-p-lang-btn" class="vector-dropdown-checkbox mw-interlanguage-selector" aria-label="Go to an article in another language. Available in 8 languages" > <label id="p-lang-btn-label" for="p-lang-btn-checkbox" class="vector-dropdown-label cdx-button cdx-button--fake-button cdx-button--fake-button--enabled cdx-button--weight-quiet cdx-button--action-progressive mw-portlet-lang-heading-8" aria-hidden="true" > 8 languages </label> <div class="vector-dropdown-content"> <div class="vector-menu-content"> <ul class="vector-menu-content-list"> <li class="interlanguage-link interwiki-ar mw-list-item"><a href="https://ar.wikipedia.org/wiki/%D9%86%D8%AD%D9%88_%D8%A5%D8%AE%D8%B7%D8%A7%D8%B7%D8%A9_%D8%AC%D8%A7%D9%81%D8%A7" title="نحو إخطاطة جافا – Arabic" lang="ar" hreflang="ar" data-title="نحو إخطاطة جافا" data-language-autonym="العربية" data-language-local-name="Arabic" class="interlanguage-link-target">العربية</a></li><li class="interlanguage-link interwiki-fa mw-list-item"><a href="https://fa.wikipedia.org/wiki/%D9%82%D9%88%D8%A7%D8%B9%D8%AF_%D8%AC%D8%A7%D9%88%D8%A7%D8%A7%D8%B3%DA%A9%D8%B1%DB%8C%D9%BE%D8%AA" title="قواعد جاوااسکریپت – Persian" lang="fa" hreflang="fa" data-title="قواعد جاوااسکریپت" data-language-autonym="فارسی" data-language-local-name="Persian" class="interlanguage-link-target">فارسی</a></li><li class="interlanguage-link interwiki-fr mw-list-item"><a href="https://fr.wikipedia.org/wiki/Syntaxe_JavaScript" title="Syntaxe JavaScript – French" lang="fr" hreflang="fr" data-title="Syntaxe JavaScript" data-language-autonym="Français" data-language-local-name="French" class="interlanguage-link-target">Français</a></li><li class="interlanguage-link interwiki-arz mw-list-item"><a href="https://arz.wikipedia.org/wiki/%D8%A8%D9%86%D8%A7%D8%A1_%D8%AC%D8%A7%D9%81%D8%A7_%D8%B3%D9%83%D8%B1%D9%8A%D8%A8%D8%AA" title="بناء جافا سكريبت – Egyptian Arabic" lang="arz" hreflang="arz" data-title="بناء جافا سكريبت" data-language-autonym="مصرى" data-language-local-name="Egyptian Arabic" class="interlanguage-link-target">مصرى</a></li><li class="interlanguage-link interwiki-ms mw-list-item"><a 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</div> <div id="contentSub"><div id="mw-content-subtitle"></div></div> <div id="mw-content-text" class="mw-body-content"><div class="mw-content-ltr mw-parser-output" lang="en" dir="ltr"><div class="shortdescription nomobile noexcerpt noprint searchaux" style="display:none">Set of rules defining correctly structured programs</div> <style data-mw-deduplicate="TemplateStyles:r1251242444">.mw-parser-output .ambox{border:1px solid #a2a9b1;border-left:10px solid #36c;background-color:#fbfbfb;box-sizing:border-box}.mw-parser-output .ambox+link+.ambox,.mw-parser-output .ambox+link+style+.ambox,.mw-parser-output .ambox+link+link+.ambox,.mw-parser-output .ambox+.mw-empty-elt+link+.ambox,.mw-parser-output .ambox+.mw-empty-elt+link+style+.ambox,.mw-parser-output .ambox+.mw-empty-elt+link+link+.ambox{margin-top:-1px}html body.mediawiki .mw-parser-output .ambox.mbox-small-left{margin:4px 1em 4px 0;overflow:hidden;width:238px;border-collapse:collapse;font-size:88%;line-height:1.25em}.mw-parser-output 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role="presentation"><tbody><tr><td class="mbox-image"><div class="mbox-image-div"><img alt="" src="//upload.wikimedia.org/wikipedia/commons/thumb/b/bd/Ambox_current_red_Asia_Australia.svg/42px-Ambox_current_red_Asia_Australia.svg.png" decoding="async" width="42" height="34" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/b/bd/Ambox_current_red_Asia_Australia.svg/63px-Ambox_current_red_Asia_Australia.svg.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/b/bd/Ambox_current_red_Asia_Australia.svg/84px-Ambox_current_red_Asia_Australia.svg.png 2x" data-file-width="360" data-file-height="290" /></div></td><td class="mbox-text"><div class="mbox-text-span">This article needs to be updated. The reason given is: New features/versions now in JavaScript. Please help update this article to reflect recent events or newly available information. (November 2020)</div></td></tr></tbody></table> <figure class="mw-default-size" typeof="mw:File/Thumb"><a href="/wiki/File:Source_code_in_Javascript.png" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/e/e4/Source_code_in_Javascript.png/220px-Source_code_in_Javascript.png" decoding="async" width="220" height="138" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/e/e4/Source_code_in_Javascript.png/330px-Source_code_in_Javascript.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/e/e4/Source_code_in_Javascript.png/440px-Source_code_in_Javascript.png 2x" data-file-width="804" data-file-height="503" /></a><figcaption>A snippet of <a href="/wiki/JavaScript" title="JavaScript">JavaScript</a> code with keywords highlighted in different colors</figcaption></figure> The <a href="/wiki/Syntax_(programming_languages)" title="Syntax (programming languages)">syntax</a> of <a href="/wiki/JavaScript" title="JavaScript">JavaScript</a> is the set of rules that define a correctly structured JavaScript program. The examples below make use of the log function of the console object present in most browsers for <a href="/wiki/Standard_streams#Standard_output_.28stdout.29" title="Standard streams">standard text output</a>. The JavaScript <a href="/wiki/Standard_library" title="Standard library">standard library</a> lacks an official standard text output function (with the exception of <code>document.write</code>). Given that JavaScript is mainly used for <a href="/wiki/Client-side_scripting" class="mw-redirect" title="Client-side scripting">client-side scripting</a> within modern <a href="/wiki/Web_browser" title="Web browser">web browsers</a>, and that almost all Web browsers provide the alert function, <code>alert</code> can also be used, but is not commonly used. <meta property="mw:PageProp/toc" /> <div class="mw-heading mw-heading2"><h2 id="Origins">Origins</h2>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=1" title="Edit section: Origins">edit</a>]</div> <a href="/wiki/Brendan_Eich" title="Brendan Eich">Brendan Eich</a> summarized the ancestry of the syntax in the first paragraph of the JavaScript 1.1 specification<a href="#cite_note-1">[1]</a><a href="#cite_note-2">[2]</a> as follows: <style data-mw-deduplicate="TemplateStyles:r1244412712">.mw-parser-output .templatequote{overflow:hidden;margin:1em 0;padding:0 32px}.mw-parser-output .templatequotecite{line-height:1.5em;text-align:left;margin-top:0}@media(min-width:500px){.mw-parser-output .templatequotecite{padding-left:1.6em}}</style><blockquote class="templatequote">JavaScript borrows most of its syntax from <a href="/wiki/Java_(programming_language)" title="Java (programming language)">Java</a>, but also inherits from <a href="/wiki/Awk" class="mw-redirect" title="Awk">Awk</a> and <a href="/wiki/Perl" title="Perl">Perl</a>, with some indirect influence from <a href="/wiki/Self_(programming_language)" title="Self (programming language)">Self</a> in its object prototype system.</blockquote> <div class="mw-heading mw-heading2"><h2 id="Basics">Basics</h2>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=2" title="Edit section: Basics">edit</a>]</div> <div class="mw-heading mw-heading3"><h3 id="Case_sensitivity">Case sensitivity</h3>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=3" title="Edit section: Case sensitivity">edit</a>]</div> JavaScript is <a href="/wiki/Case_sensitivity" title="Case sensitivity">case sensitive</a>. It is common to start the name of a <a href="#Constructors">constructor</a> with a <a href="/wiki/CamelCase" class="mw-redirect" title="CamelCase">capitalised</a> letter, and the name of a function or variable with a lower-case letter. Example: <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>var a = 5; console.log(a); // 5 console.log(A); // throws a ReferenceError: A is not defined </pre></div> <div class="mw-heading mw-heading3"><h3 id="Whitespace_and_semicolons">Whitespace and semicolons</h3>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=4" title="Edit section: Whitespace and semicolons">edit</a>]</div> Unlike in <a href="/wiki/C_(programming_language)" title="C (programming language)">C</a>, whitespace in JavaScript source can directly impact <a href="/wiki/Semantics_(computer_science)" title="Semantics (computer science)">semantics</a>. <a href="/wiki/Semicolon" title="Semicolon">Semicolons</a> end statements in JavaScript. Because of <a href="/wiki/Lexical_analysis#Semicolon_insertion" title="Lexical analysis">automatic semicolon insertion</a> (ASI), some statements that are well formed when a newline is parsed will be considered complete, as if a semicolon were inserted just prior to the newline. Some authorities advise supplying statement-terminating semicolons explicitly, because it may lessen unintended effects of the automatic semicolon insertion.<a href="#cite_note-3">[3]</a> There are two issues: five tokens can either begin a statement or be the extension of a complete statement; and five restricted productions, where line breaks are not allowed in certain positions, potentially yielding incorrect parsing. The five problematic tokens are the open parenthesis "<code>(</code>", open bracket "<code>[</code>", slash "<code>/</code>", plus "<code>+</code>", and minus "<code>-</code>". Of these, the open parenthesis is common in the <a href="/wiki/Immediately_invoked_function_expression" title="Immediately invoked function expression">immediately invoked function expression</a> pattern, and open bracket occurs sometimes, while others are quite rare. An example: <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>a = b + c (d + e).foo() // Treated as: // a = b + c(d + e).foo(); </pre></div> with the suggestion that the preceding statement be terminated with a semicolon. Some suggest instead the use of leading semicolons on lines starting with '<code>(</code>' or '<code>[</code>', so the line is not accidentally joined with the previous one. This is known as a defensive semicolon, and is particularly recommended, because code may otherwise become ambiguous when it is rearranged. For example: <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>a = b + c ;(d + e).foo() // Treated as: // a = b + c; // (d + e).foo(); </pre></div> Initial semicolons are also sometimes used at the start of JavaScript libraries, in case they are appended to another library that omits a trailing semicolon, as this can result in ambiguity of the initial statement. The five restricted productions are <code>return</code>, <code>throw</code>, <code>break</code>, <code>continue</code>, and post-increment/decrement. In all cases, inserting semicolons does not fix the problem, but makes the parsed syntax clear, making the error easier to detect. <code>return</code> and <code>throw</code> take an optional value, while <code>break</code> and <code>continue</code> take an optional label. In all cases, the advice is to keep the value or label on the same line as the statement. This most often shows up in the return statement, where one might return a large object literal, which might be accidentally placed starting on a new line. For post-increment/decrement, there is potential ambiguity with pre-increment/decrement, and again it is recommended to simply keep these on the same line. <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>return a + b; // Returns undefined. Treated as: // return; // a + b; // Should be written as: // return a + b; </pre></div> <div class="mw-heading mw-heading3"><h3 id="Comments">Comments</h3>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=5" title="Edit section: Comments">edit</a>]</div> <style data-mw-deduplicate="TemplateStyles:r1236090951">.mw-parser-output .hatnote{font-style:italic}.mw-parser-output div.hatnote{padding-left:1.6em;margin-bottom:0.5em}.mw-parser-output .hatnote i{font-style:normal}.mw-parser-output .hatnote+link+.hatnote{margin-top:-0.5em}@media print{body.ns-0 .mw-parser-output .hatnote{display:none!important}}</style><div role="note" class="hatnote navigation-not-searchable">Main article: <a href="/wiki/Comment_(computer_programming)" title="Comment (computer programming)">Comment (computer programming)</a></div> <a href="/wiki/Comment_(computer_programming)" title="Comment (computer programming)">Comment</a> syntax is the same as in <a href="/wiki/C%2B%2B" title="C++">C++</a>, <a href="/wiki/Swift_(programming_language)" title="Swift (programming language)">Swift</a> and many other languages. <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>// a short, one-line comment /* this is a long, multi-line comment about my script. May it one day be great. */ /* Comments /* may not be nested */ Syntax error */ </pre></div> <div class="mw-heading mw-heading2"><h2 id="Variables">Variables</h2>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=6" title="Edit section: Variables">edit</a>]</div> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1236090951"><div role="note" class="hatnote navigation-not-searchable">Main article: <a href="/wiki/Variable_(programming)" class="mw-redirect" title="Variable (programming)">Variable (programming)</a></div> <a href="/wiki/Variable_(programming)" class="mw-redirect" title="Variable (programming)">Variables</a> in standard JavaScript have no <a href="/wiki/Type_system" title="Type system">type</a> attached, so any value (each value has a type) can be stored in any variable. Starting with <a href="/wiki/ECMAScript#6th_Edition_–_ECMAScript_2015" title="ECMAScript">ES6</a>, the 6th version of the language, variables could be declared with <code>var</code> for function scoped variables, and <code>let</code> or <code>const</code> which are for <a href="/wiki/Block_scope" class="mw-redirect" title="Block scope">block level</a> variables. Before ES6, variables could only be declared with a <code>var</code> statement. Values assigned to variables declared with <code>const</code> cannot be changed, but their properties can. <code>var</code> should no longer be used since <code>let</code> and <code>const</code> are supported by modern browsers.<a href="#cite_note-4">[4]</a> A variable's <a href="/wiki/Identifier_(computer_languages)" title="Identifier (computer languages)">identifier</a> must start with a letter, underscore (<code>_</code>), or dollar sign (<code>$</code>), while subsequent characters can also be digits (<code>0-9</code>). JavaScript is case sensitive, so the uppercase characters "A" through "Z" are different from the lowercase characters "a" through "z". Starting with JavaScript 1.5, <a href="/wiki/ISO_8859-1" class="mw-redirect" title="ISO 8859-1">ISO 8859-1</a> or <a href="/wiki/Unicode" title="Unicode">Unicode</a> letters (or <code>\uXXXX</code> Unicode escape sequences) can be used in identifiers.<a href="#cite_note-5">[5]</a> In certain JavaScript implementations, the at sign (@) can be used in an identifier, but this is contrary to the specifications and not supported in newer implementations. [<a href="/wiki/Wikipedia:Citation_needed" title="Wikipedia:Citation needed">citation needed</a>] <div class="mw-heading mw-heading3"><h3 id="Scoping_and_hoisting">Scoping and hoisting</h3>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=7" title="Edit section: Scoping and hoisting">edit</a>]</div> Variables declared with <code>var</code> are <a href="/wiki/Lexical_scoping" class="mw-redirect" title="Lexical scoping">lexically scoped</a> at a <a href="/wiki/Function_scope" class="mw-redirect" title="Function scope">function level</a>, while ones with <code>let</code> or <code>const</code> have a <a href="/wiki/Block_scope" class="mw-redirect" title="Block scope">block level</a> scope. Since declarations are processed before any code is executed, a variable can be assigned to and used prior to being declared in the code.<a href="#cite_note-6">[6]</a> This is referred to as <style data-mw-deduplicate="TemplateStyles:r1238216509">.mw-parser-output .vanchor>:target~.vanchor-text{background-color:#b1d2ff}@media screen{html.skin-theme-clientpref-night .mw-parser-output .vanchor>:target~.vanchor-text{background-color:#0f4dc9}}@media screen and (prefers-color-scheme:dark){html.skin-theme-clientpref-os .mw-parser-output .vanchor>:target~.vanchor-text{background-color:#0f4dc9}}</style>hoisting, and it is equivalent to variables being <a href="/wiki/Forward_declaration" title="Forward declaration">forward declared</a> at the top of the function or block.<a href="#cite_note-7">[7]</a> With <code>var</code>, <code>let</code>, and <code>const</code> statements, only the declaration is hoisted; assignments are not hoisted. Thus a <code class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" style="" dir="ltr">var x = 1</code> statement in the middle of the function is equivalent to a <code class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" style="" dir="ltr">var x</code> declaration statement at the top of the function, and an <code class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" style="" dir="ltr">x = 1</code> assignment statement at that point in the middle of the function. This means that values cannot be accessed before they are declared; <a href="/wiki/Forward_reference" class="mw-redirect" title="Forward reference">forward reference</a> is not possible. With <code>var</code> a variable's value is <code>undefined</code> until it is initialized. Variables declared with <code>let</code> or <code>const</code> cannot be accessed until they have been initialized, so referencing such variables before will cause an error. Function declarations, which declare a variable and assign a function to it, are similar to variable statements, but in addition to hoisting the declaration, they also hoist the assignment – as if the entire statement appeared at the top of the containing function – and thus forward reference is also possible: the location of a function statement within an enclosing function is irrelevant. This is different from a function expression being assigned to a variable in a <code>var</code>, <code>let</code>, or <code>const</code> statement. So, for example, <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>var func = function() { .. } // declaration is hoisted only function func() { .. } // declaration and assignment are hoisted </pre></div> Block scoping can be produced by wrapping the entire block in a function and then executing it – this is known as the <a href="/wiki/Immediately-invoked_function_expression" class="mw-redirect" title="Immediately-invoked function expression">immediately-invoked function expression</a> pattern – or by declaring the variable using the <code>let</code> keyword. <div class="mw-heading mw-heading3"><h3 id="Declaration_and_assignment">Declaration and assignment</h3>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=8" title="Edit section: Declaration and assignment">edit</a>]</div> Variables declared outside a scope are <a href="/wiki/Global_variable" title="Global variable">global</a>. If a variable is declared in a higher scope, it can be accessed by child scopes. When JavaScript tries to resolve an identifier, it looks in the local scope. If this identifier is not found, it looks in the next outer scope, and so on along the scope chain until it reaches the global scope where global variables reside. If it is still not found, JavaScript will raise a <code>ReferenceError</code> exception. When assigning an identifier, JavaScript goes through exactly the same process to retrieve this identifier, except that if it is not found in the global scope, it will create the "variable" in the scope where it was created.<a href="#cite_note-8">[8]</a> As a consequence, a variable never declared will be global, if assigned. Declaring a variable (with the keyword <code>var</code>) in the global scope (i.e. outside of any function body (or block in the case of let/const)), assigning a never declared identifier or adding a property to the global object (usually window) will also create a new global variable. Note that JavaScript's strict mode forbids the assignment of an undeclared variable, which avoids global namespace pollution. <div class="mw-heading mw-heading3"><h3 id="Examples">Examples</h3>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=9" title="Edit section: Examples">edit</a>]</div> Here are some examples of variable declarations and scope: <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>var x1 = 0; // A global variable, because it is not in any function let x2 = 0; // Also global, this time because it is not in any block function f() { var z = 'foxes', r = 'birds'; // 2 local variables m = 'fish'; // global, because it wasn't declared anywhere before function child() { var r = 'monkeys'; // This variable is local and does not affect the "birds" r of the parent function. z = 'penguins'; // Closure: Child function is able to access the variables of the parent function. } twenty = 20; // This variable is declared on the next line, but usable anywhere in the function, even before, as here var twenty; child(); return x1 + x2; // We can use x1 and x2 here, because they are global } f(); console.log(z); // This line will raise a ReferenceError exception, because the value of z is no longer available </pre></div> <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>for (let i = 0; i < 10; i++) console.log(i); console.log(i); // throws a ReferenceError: i is not defined </pre></div> <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>for (const i = 0; i < 10; i++) console.log(i); // throws a TypeError: Assignment to constant variable for (const i of [1,2,3]) console.log(i); //will not raise an exception. i is not reassigned but recreated in every iteration const pi; // throws a SyntaxError: Missing initializer in const declaration </pre></div> <div class="mw-heading mw-heading2"><h2 id="Primitive_data_types">Primitive data types</h2>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=10" title="Edit section: Primitive data types">edit</a>]</div> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1236090951"><div role="note" class="hatnote navigation-not-searchable">Main article: <a href="/wiki/Primitive_data_type" title="Primitive data type">Primitive data type</a></div> The JavaScript language provides six <a href="/wiki/Primitive_data_type" title="Primitive data type">primitive data types</a>: <ul><li>Undefined</li> <li>Number</li> <li>BigInt</li> <li>String</li> <li>Boolean</li> <li>Symbol</li></ul> Some of the primitive data types also provide a set of named values that represent the extents of the type boundaries. These named values are described within the appropriate sections below. <div class="mw-heading mw-heading3"><h3 id="Undefined">Undefined</h3>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=11" title="Edit section: Undefined">edit</a>]</div> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1236090951"><div role="note" class="hatnote navigation-not-searchable">Main article: <a href="/wiki/Undefined_value" title="Undefined value">Undefined value</a></div> The <a href="/wiki/Undefined_value" title="Undefined value">value of "undefined"</a> is assigned to all <a href="/wiki/Uninitialized_variable" title="Uninitialized variable">uninitialized variables</a>, and is also returned when checking for object properties that do not exist. In a Boolean context, the undefined value is considered a false value. Note: undefined is considered a genuine primitive type. Unless explicitly converted, the undefined value may behave unexpectedly in comparison to other types that evaluate to false in a logical context. <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>let test; // variable declared, but not defined, ... // ... set to value of undefined const testObj = {}; console.log(test); // test variable exists, but value not ... // ... defined, displays undefined console.log(testObj.myProp); // testObj exists, property does not, ... // ... displays undefined console.log(undefined == null); // unenforced type during check, displays true console.log(undefined === null); // enforce type during check, displays false </pre></div> Note: There is no built-in language literal for undefined. Thus <code class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" style="" dir="ltr">(x === undefined)</code> is not a foolproof way to check whether a variable is undefined, because in versions before ECMAScript 5, it is legal for someone to write <code class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" style="" dir="ltr">var undefined = "I'm defined now";</code>. A more robust approach is to compare using <code class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" style="" dir="ltr">(typeof x === 'undefined')</code>. Functions like this won't work as expected: <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>function isUndefined(x) { let u; return x === u; } // like this... function isUndefined(x) { return x === void 0; } // ... or that second one function isUndefined(x) { return (typeof x) === "undefined"; } // ... or that third one </pre></div> Here, calling <code>isUndefined(my_var)</code> raises a <style data-mw-deduplicate="TemplateStyles:r886049734">.mw-parser-output .monospaced{font-family:monospace,monospace}</style>ReferenceError if <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">my_var is an unknown identifier, whereas <code class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" style="" dir="ltr">typeof my_var === 'undefined'</code> doesn't. <div class="mw-heading mw-heading3"><h3 id="Number">Number</h3>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=12" title="Edit section: Number">edit</a>]</div> Numbers are represented in binary as <a href="/wiki/IEEE_754" title="IEEE 754">IEEE 754</a> <a href="/wiki/Floating_point" class="mw-redirect" title="Floating point">floating point</a> doubles. Although this format provides an accuracy of nearly 16 <a href="/wiki/Significant_digits" class="mw-redirect" title="Significant digits">significant digits</a>, it cannot always exactly represent real numbers, including fractions. This becomes an issue when comparing or formatting numbers. For example: <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>console.log(0.2 + 0.1 === 0.3); // displays false console.log(0.94 - 0.01); // displays 0.9299999999999999 </pre></div> As a result, a routine such as the <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">toFixed() method should be used to round numbers whenever they are <a rel="nofollow" class="external text" href="http://www.jibbering.com/faq/#formatNumber">formatted for output</a>. Numbers may be specified in any of these notations: <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>345; // an "integer", although there is only one numeric type in JavaScript 34.5; // a floating-point number 3.45e2; // another floating-point, equivalent to 345 0b1011; // a binary integer equal to 11 0o377; // an octal integer equal to 255 0xFF; // a hexadecimal integer equal to 255, digits represented by the ... // ... letters A-F may be upper or lowercase </pre></div> There's also a numeric separator, <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">_ (the underscore), introduced in ES2021: <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>// Note: Wikipedia syntax doesn't support numeric separators yet 1_000_000_000; // Used with big numbers 1_000_000.5; // Support with decimals 1_000e1_000; // Support with exponents // Support with binary, octals and hex 0b0000_0000_0101_1011; 0o0001_3520_0237_1327; 0xFFFF_FFFF_FFFF_FFFE; // But you can't use them next to a non-digit number part, or at the start or end _12; // Variable is not defined (the underscore makes it a variable identifier) 12_; // Syntax error (cannot be at the end of numbers) 12_.0; // Syntax error (doesn't make sense to put a separator next to the decimal point) 12._0; // Syntax error 12e_6; // Syntax error (next to "e", a non-digit. Doesn't make sense to put a separator at the start) 1000____0000; // Syntax error (next to "_", a non-digit. Only 1 separator at a time is allowed </pre></div> The extents <a href="/wiki/Extended_real_number_line" title="Extended real number line">+∞, −∞</a> and <a href="/wiki/NaN" title="NaN">NaN</a> (Not a Number) of the number type may be obtained by two program expressions: <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>Infinity; // positive infinity (negative obtained with -Infinity for instance) NaN; // The Not-A-Number value, also returned as a failure in ... // ... string-to-number conversions </pre></div> Infinity and NaN are numbers: <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>typeof Infinity; // returns "number" typeof NaN; // returns "number" </pre></div> These three special values correspond and behave as the <a href="/wiki/IEEE-754" class="mw-redirect" title="IEEE-754">IEEE-754</a> describes them. The Number constructor (used as a function), or a unary + or -, may be used to perform explicit numeric conversion: <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>const myString = "123.456"; const myNumber1 = Number(myString); const myNumber2 = +myString; </pre></div> When used as a constructor, a numeric wrapper object is created (though it is of little use): <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>const myNumericWrapper = new Number(123.456); </pre></div> However, NaN is not equal to itself: <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>const nan = NaN; console.log(NaN == NaN); // false console.log(NaN === NaN); // false console.log(NaN !== NaN); // true console.log(nan !== nan); // true // You can use the isNaN methods to check for NaN console.log(isNaN("converted to NaN")); // true console.log(isNaN(NaN)); // true console.log(Number.isNaN("not converted")); // false console.log(Number.isNaN(NaN)); // true </pre></div> <div class="mw-heading mw-heading3"><h3 id="BigInt">BigInt</h3>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=13" title="Edit section: BigInt">edit</a>]</div> BigInts can be used for arbitrarily large <a href="/wiki/Integer" title="Integer">integers</a>. Especially whole numbers larger than 253 - 1, which is the largest number JavaScript can reliably represent with the Number primitive and represented by the Number.MAX_SAFE_INTEGER constant. When dividing BigInts, the results are <a href="/wiki/Truncation" title="Truncation">truncated</a>. <div class="mw-heading mw-heading3"><h3 id="String">String</h3>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=14" title="Edit section: String">edit</a>]</div> A <a href="/wiki/String_(computer_science)" title="String (computer science)">string</a> in JavaScript is a sequence of characters. In JavaScript, strings can be created directly (as literals) by placing the series of characters between double (") or single (') quotes. Such strings must be written on a single line, but may include escaped newline characters (such as \n). The JavaScript standard allows the <a href="/wiki/Grave_accent#Use_in_programming" title="Grave accent">backquote</a> character (`, a.k.a. grave accent or backtick) to quote multiline literal strings, as well as template literals, which allow for interpolation of type-coerced evaluated expressions within a string.<a href="#cite_note-9">[9]</a> <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>const greeting = "Hello, World!"; const anotherGreeting = 'Greetings, people of Earth.'; const aMultilineGreeting = `Warm regards, John Doe.` // Template literals type-coerce evaluated expressions and interpolate them into the string. const templateLiteral = `This is what is stored in anotherGreeting: ${anotherGreeting}.`; console.log(templateLiteral); // 'This is what is stored in anotherGreeting: 'Greetings, people of Earth.'' </pre></div> Individual characters within a string can be accessed using the <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">charAt method (provided by <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">String.prototype). This is the preferred way when accessing individual characters within a string, because it also works in non-modern browsers: <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>const h = greeting.charAt(0); </pre></div> In modern browsers, individual characters within a string can be accessed (as strings with only a single character) through the same notation as arrays: <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>const h = greeting[0]; </pre></div> However, JavaScript strings are <a href="/wiki/Immutable_object" title="Immutable object">immutable</a>: <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>greeting[0] = "H"; // Fails. </pre></div> Applying the equality operator ("==") to two strings returns true, if the strings have the same contents, which means: of the same length and containing the same sequence of characters (case is significant for alphabets). Thus: <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>const x = "World"; const compare1 = ("Hello, " + x == "Hello, World"); // Here compare1 contains true. const compare2 = ("Hello, " + x == "hello, World"); // Here compare2 contains ... // ... false since the ... // ... first characters ... // ... of both operands ... // ... are not of the same case. </pre></div> Quotes of the same type cannot be nested unless they are <a href="/wiki/String_literal#Escape_character" title="String literal">escaped</a>. <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>let x = '"Hello, World!" he said.'; // Just fine. x = ""Hello, World!" he said."; // Not good. x = "\"Hello, World!\" he said."; // Works by escaping " with \" </pre></div> The <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">String constructor creates a string object (an object wrapping a string): <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>const greeting = new String("Hello, World!"); </pre></div> These objects have a <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">valueOf method returning the primitive string wrapped within them: <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>const s = new String("Hello !"); typeof s; // Is 'object'. typeof s.valueOf(); // Is 'string'. </pre></div> Equality between two <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">String objects does not behave as with string primitives: <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>const s1 = new String("Hello !"); const s2 = new String("Hello !"); s1 == s2; // Is false, because they are two distinct objects. s1.valueOf() == s2.valueOf(); // Is true. </pre></div> <div class="mw-heading mw-heading3"><h3 id="Boolean">Boolean</h3>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=15" title="Edit section: Boolean">edit</a>]</div> <a href="/wiki/JavaScript" title="JavaScript">JavaScript</a> provides a <a href="/wiki/Boolean_data_type" title="Boolean data type">Boolean data type</a> with <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">true and <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">false literals. The <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734"><a href="/wiki/Typeof" title="Typeof">typeof</a> operator returns the string <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">"boolean" for these <a href="/wiki/Primitive_types" class="mw-redirect" title="Primitive types">primitive types</a>. When used in a logical context, <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">0, <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">-0, <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">null, <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">NaN, <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">undefined, and the empty string (<link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">"") evaluate as <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">false due to automatic <a href="/wiki/Type_conversion" title="Type conversion">type conversion</a>. All other values (the <a href="/wiki/Complement_(set_theory)" title="Complement (set theory)">complement</a> of the previous list) evaluate as <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">true, including the strings <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">"0", <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">"false" and any object. <div class="mw-heading mw-heading3"><h3 id="Type_conversion">Type conversion</h3>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=16" title="Edit section: Type conversion">edit</a>]</div> Automatic type coercion by the equality comparison operators (<code>==</code> and <code>!=</code>) can be avoided by using the type checked comparison operators (<code>===</code> and <code>!==</code>). When type conversion is required, JavaScript converts <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">Boolean, <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">Number, <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">String, or <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">Object operands as follows:<a href="#cite_note-10">[10]</a> <dl><dt>Number and String</dt> <dd>The string is converted to a number value. JavaScript attempts to convert the string numeric literal to a Number type value. First, a mathematical value is derived from the string numeric literal. Next, this value is rounded to nearest Number type value.</dd> <dt>Boolean</dt> <dd>If one of the operands is a Boolean, the Boolean operand is converted to 1 if it is <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">true, or to 0 if it is <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">false.</dd> <dt>Object</dt> <dd>If an object is compared with a number or string, JavaScript attempts to return the default value for the object. An object is converted to a primitive String or Number value, using the <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">.valueOf() or <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">.toString() methods of the object. If this fails, a runtime error is generated.</dd></dl> <div class="mw-heading mw-heading4"><h4 id="Boolean_type_conversion">Boolean type conversion</h4>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=17" title="Edit section: Boolean type conversion">edit</a>]</div> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1236090951"><div role="note" class="hatnote navigation-not-searchable">Further information: <a href="/wiki/Truthy_(computing)" class="mw-redirect" title="Truthy (computing)">Truthy (computing)</a></div> <a href="/wiki/Douglas_Crockford" title="Douglas Crockford">Douglas Crockford</a> advocates the terms "truthy" and "falsy" to describe how values of various types behave when evaluated in a logical context, especially in regard to edge cases.<a href="#cite_note-11">[11]</a> The binary logical operators returned a Boolean value in early versions of JavaScript, but now they return one of the operands instead. The left–operand is returned, if it can be evaluated as : <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">false, in the case of <a href="/wiki/Logical_conjunction" title="Logical conjunction">conjunction</a>: (<code>a && b</code>), or <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">true, in the case of <a href="/wiki/Logical_disjunction" title="Logical disjunction">disjunction</a>: (<code>a || b</code>); otherwise the right–operand is returned. Automatic type coercion by the comparison operators may differ for cases of mixed Boolean and number-compatible operands (including strings that can be evaluated as a number, or objects that can be evaluated as such a string), because the Boolean operand will be compared as a numeric value. This may be unexpected. An expression can be explicitly cast to a Boolean primitive by doubling the logical <a href="/wiki/Negation" title="Negation">negation operator</a>: (<link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">!!), using the <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">Boolean() function, or using the <a href="/wiki/Conditional_(programming)" class="mw-redirect" title="Conditional (programming)">conditional operator</a>: (<code>c ? t : f</code>). <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>// Automatic type coercion console.log(true == 2 ); // false... true → 1 !== 2 ← 2 console.log(false == 2 ); // false... false → 0 !== 2 ← 2 console.log(true == 1 ); // true.... true → 1 === 1 ← 1 console.log(false == 0 ); // true.... false → 0 === 0 ← 0 console.log(true == "2"); // false... true → 1 !== 2 ← "2" console.log(false == "2"); // false... false → 0 !== 2 ← "2" console.log(true == "1"); // true.... true → 1 === 1 ← "1" console.log(false == "0"); // true.... false → 0 === 0 ← "0" console.log(false == "" ); // true.... false → 0 === 0 ← "" console.log(false == NaN); // false... false → 0 !== NaN console.log(NaN == NaN); // false...... NaN is not equivalent to anything, including NaN. // Type checked comparison (no conversion of types and values) console.log(true === 1); // false...... data types do not match // Explicit type coercion console.log(true === !!2); // true.... data types and values match console.log(true === !!0); // false... data types match, but values differ console.log( 1 ? true : false); // true.... only ±0 and NaN are "falsy" numbers console.log("0" ? true : false); // true.... only the empty string is "falsy" console.log(Boolean({})); // true.... all objects are "truthy" </pre></div> The new operator can be used to create an object wrapper for a Boolean primitive. However, the <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">typeof operator does not return <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">boolean for the object wrapper, it returns <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">object. Because all objects evaluate as <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">true, a method such as <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">.valueOf(), or <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">.toString(), must be used to retrieve the wrapped value. For explicit coercion to the Boolean type, Mozilla recommends that the <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">Boolean() function (without <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">new) be used in preference to the Boolean object. <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>const b = new Boolean(false); // Object false {} const t = Boolean(b); // Boolean true const f = Boolean(b.valueOf()); // Boolean false let n = new Boolean(b); // Not recommended n = new Boolean(b.valueOf()); // Preferred if (0 || -0 || "" || null || undefined || b.valueOf() || !new Boolean() || !t) { console.log("Never this"); } else if ([] && {} && b && typeof b === "object" && b.toString() === "false") { console.log("Always this"); } </pre></div> <div class="mw-heading mw-heading3"><h3 id="Symbol">Symbol</h3>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=18" title="Edit section: Symbol">edit</a>]</div> New in ECMAScript6. A Symbol is a unique and immutable identifier. Example: <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>let x = Symbol(1); const y = Symbol(1); x === y; // => false const symbolObject = {}; const normalObject = {}; // since x and y are unique, // they can be used as unique keys in an object symbolObject[x] = 1; symbolObject[y] = 2; symbolObject[x]; // => 1 symbolObject[y]; // => 2 // as compared to normal numeric keys normalObject[1] = 1; normalObject[1] = 2; // overrides the value of 1 normalObject[1]; // => 2 // changing the value of x does not change the key stored in the object x = Symbol(3); symbolObject[x]; // => undefined // changing x back just creates another unique Symbol x = Symbol(1); symbolObject[x]; // => undefined </pre></div> There are also well known symbols. One of which is <code>Symbol.iterator</code>; if something implements <code>Symbol.iterator</code>, it's iterable: <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>const x = [1, 2, 3, 4]; // x is an Array x[Symbol.iterator] === Array.prototype[Symbol.iterator]; // and Arrays are iterable const xIterator = x[Symbol.iterator](); // The [Symbol.iterator] function should provide an iterator for x xIterator.next(); // { value: 1, done: false } xIterator.next(); // { value: 2, done: false } xIterator.next(); // { value: 3, done: false } xIterator.next(); // { value: 4, done: false } xIterator.next(); // { value: undefined, done: true } xIterator.next(); // { value: undefined, done: true } // for..of loops automatically iterate values for (const value of x) { console.log(value); // 1 2 3 4 } // Sets are also iterable: [Symbol.iterator] in Set.prototype; // true for (const value of new Set(['apple', 'orange'])) { console.log(value); // "apple" "orange" } </pre></div> <div class="mw-heading mw-heading2"><h2 id="Native_objects">Native objects</h2>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=19" title="Edit section: Native objects">edit</a>]</div> The JavaScript language provides a handful of native <a href="/wiki/Object_(computer_science)" title="Object (computer science)">objects</a>. JavaScript native objects are considered part of the JavaScript specification. JavaScript environment notwithstanding, this set of objects should always be available. <div class="mw-heading mw-heading3"><h3 id="Array">Array</h3>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=20" title="Edit section: Array">edit</a>]</div> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1236090951"><div role="note" class="hatnote navigation-not-searchable">Main article: <a href="/wiki/Array_data_type" class="mw-redirect" title="Array data type">Array data type</a></div> An <a href="/wiki/Array_data_type" class="mw-redirect" title="Array data type">Array</a> is a JavaScript object prototyped from the <code>Array</code> constructor specifically designed to store data values indexed by integer keys. Arrays, unlike the basic Object type, are prototyped with methods and properties to aid the programmer in routine tasks (for example, <code>join</code>, <code>slice</code>, and <code>push</code>). As in the <a href="/wiki/Category:C_programming_language_family" title="Category:C programming language family">C family</a>, arrays use a zero-based indexing scheme: A value that is inserted into an empty array by means of the <code>push</code> method occupies the 0th index of the array. <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>const myArray = []; // Point the variable myArray to a newly ... // ... created, empty Array myArray.push("hello World"); // Fill the next empty index, in this case 0 console.log(myArray[0]); // Equivalent to console.log("hello World"); </pre></div> Arrays have a <code>length</code> property that is guaranteed to always be larger than the largest integer index used in the array. It is automatically updated, if one creates a property with an even larger index. Writing a smaller number to the <code>length</code> property will remove larger indices. Elements of <code>Array</code>s may be accessed using normal object property access notation: <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>myArray[1]; // the 2nd item in myArray myArray["1"]; </pre></div> The above two are equivalent. It's not possible to use the "dot"-notation or strings with alternative representations of the number: <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>myArray.1; // syntax error myArray["01"]; // not the same as myArray[1] </pre></div> Declaration of an array can use either an <code>Array</code> literal or the <code>Array</code> constructor: <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>let myArray; // Array literals myArray = [1, 2]; // length of 2 myArray = [1, 2,]; // same array - You can also have an extra comma at the end // It's also possible to not fill in parts of the array myArray = [0, 1, /* hole */, /* hole */, 4, 5]; // length of 6 myArray = [0, 1, /* hole */, /* hole */, 4, 5,]; // same array myArray = [0, 1, /* hole */, /* hole */, 4, 5, /* hole */,]; // length of 7 // With the constructor myArray = new Array(0, 1, 2, 3, 4, 5); // length of 6 myArray = new Array(365); // an empty array with length 365 </pre></div> <a href="/wiki/Array_data_structure" class="mw-redirect" title="Array data structure">Arrays</a> are implemented so that only the defined elements use memory; they are "<a href="/wiki/Sparse_array" class="mw-redirect" title="Sparse array">sparse arrays</a>". Setting <code class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" style="" dir="ltr">myArray[10] = 'someThing'</code> and <code class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" style="" dir="ltr">myArray[57] = 'somethingOther'</code> only uses space for these two elements, just like any other object. The <code>length</code> of the array will still be reported as 58. The maximum length of an array is 4,294,967,295 which corresponds to 32-bit binary number (11111111111111111111111111111111)2. One can use the object declaration literal to create objects that behave much like associative arrays in other languages: <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>const dog = {color: "brown", size: "large"}; dog["color"]; // results in "brown" dog.color; // also results in "brown" </pre></div> One can use the object and array declaration literals to quickly create arrays that are associative, multidimensional, or both. (Technically, JavaScript does not support multidimensional arrays, but one can mimic them with arrays-of-arrays.) <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>const cats = [{color: "brown", size: "large"}, {color: "black", size: "small"}]; cats[0]["size"]; // results in "large" const dogs = {rover: {color: "brown", size: "large"}, spot: {color: "black", size: "small"}}; dogs["spot"]["size"]; // results in "small" dogs.rover.color; // results in "brown" </pre></div> <div class="mw-heading mw-heading3"><h3 id="Date">Date</h3>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=21" title="Edit section: Date">edit</a>]</div> A <code>Date</code> object stores a signed millisecond count with zero representing 1970-01-01 00:00:00 UT and a range of ±108 days. There are several ways of providing arguments to the <code>Date</code> constructor. Note that months are zero-based. <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>new Date(); // create a new Date instance representing the current time/date. new Date(2010, 2, 1); // create a new Date instance representing 2010-Mar-01 00:00:00 new Date(2010, 2, 1, 14, 25, 30); // create a new Date instance representing 2010-Mar-01 14:25:30 new Date("2010-3-1 14:25:30"); // create a new Date instance from a String. </pre></div> Methods to extract fields are provided, as well as a useful <code>toString</code>: <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>const d = new Date(2010, 2, 1, 14, 25, 30); // 2010-Mar-01 14:25:30; // Displays '2010-3-1 14:25:30': console.log(d.getFullYear() + '-' + (d.getMonth() + 1) + '-' + d.getDate() + ' ' + d.getHours() + ':' + d.getMinutes() + ':' + d.getSeconds()); // Built-in toString returns something like 'Mon Mar 01 2010 14:25:30 GMT-0500 (EST)': console.log(d); </pre></div> <div class="mw-heading mw-heading3"><h3 id="Error">Error</h3>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=22" title="Edit section: Error">edit</a>]</div> Custom error messages can be created using the <code>Error</code> class: <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>throw new Error("Something went wrong."); </pre></div> These can be caught by try...catch...finally blocks as described in the section on <a href="#Exception_handling">exception handling</a>. <div class="mw-heading mw-heading3"><h3 id="Math">Math</h3>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=23" title="Edit section: Math">edit</a>]</div> The <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">Math object contains various math-related constants (for example, π) and functions (for example, cosine). (Note that the <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">Math object has no constructor, unlike <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">Array or <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">Date. All its methods are "static", that is "class" methods.) All the trigonometric functions use angles expressed in <a href="/wiki/Radian" title="Radian">radians</a>, not <a href="/wiki/Degree_(angle)" title="Degree (angle)">degrees</a> or <a href="/wiki/Grad_(angle)" class="mw-redirect" title="Grad (angle)">grads</a>. <table class="wikitable" border="1"> <caption>Some of the constants contained in the Math object </caption> <tbody><tr> <th>Property</th> <th>Returned value rounded to 5 digits</th> <th>Description </th></tr> <tr> <td><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">Math.E</td> <td>2.7183</td> <td><a href="/wiki/E_(mathematical_constant)" title="E (mathematical constant)">e</a>: Natural logarithm base </td></tr> <tr> <td><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">Math.LN2</td> <td>0.69315</td> <td><a href="/wiki/Natural_logarithm" title="Natural logarithm">Natural logarithm</a> of 2 </td></tr> <tr> <td><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">Math.LN10</td> <td>2.3026</td> <td>Natural logarithm of 10 </td></tr> <tr> <td><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">Math.LOG2E</td> <td>1.4427</td> <td><a href="/wiki/Logarithm" title="Logarithm">Logarithm</a> to the base 2 of e </td></tr> <tr> <td><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">Math.LOG10E</td> <td>0.43429</td> <td>Logarithm to the base 10 of e </td></tr> <tr> <td><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">Math.PI</td> <td>3.14159</td> <td><a href="/wiki/Pi" title="Pi">π</a>: circumference/diameter of a circle </td></tr> <tr> <td><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">Math.SQRT1_2</td> <td>0.70711</td> <td><a href="/wiki/Square_root" title="Square root">Square root</a> of ½ </td></tr> <tr> <td><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">Math.SQRT2</td> <td>1.4142</td> <td><a href="/wiki/Square_root_of_2" title="Square root of 2">Square root of 2</a> </td></tr></tbody></table> <table class="wikitable" border="1"> <caption>Methods of the Math object </caption> <tbody><tr> <th>Example</th> <th>Returned value rounded to 5 digits</th> <th>Description </th></tr> <tr> <td><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">Math.abs(-2.3)</td> <td>2.3</td> <td><a href="/wiki/Absolute_value" title="Absolute value">Absolute value</a> </td></tr> <tr> <td><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">Math.acos(Math.SQRT1_2)</td> <td>0.78540 <a href="/wiki/Rad_(unit)" class="mw-redirect" title="Rad (unit)">rad</a> = 45°</td> <td><a href="/wiki/Arccosine" class="mw-redirect" title="Arccosine">Arccosine</a> </td></tr> <tr> <td><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">Math.asin(Math.SQRT1_2)</td> <td>0.78540 rad = 45°</td> <td><a href="/wiki/Arcsine" class="mw-redirect" title="Arcsine">Arcsine</a> </td></tr> <tr> <td><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">Math.atan(1)</td> <td>0.78540 rad = 45°</td> <td>Half circle <a href="/wiki/Arctangent" class="mw-redirect" title="Arctangent">arctangent</a> (⁠<math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle -\pi /2}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mo>−</mo> <mi>π</mi> <mrow class="MJX-TeXAtom-ORD"> <mo>/</mo> </mrow> <mn>2</mn> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle -\pi /2}</annotation> </semantics> </math><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/4644f935f59f6d1bac8c2ef618246eee6c1f07b3" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:5.465ex; height:2.843ex;" alt="{\displaystyle -\pi /2}">⁠ to ⁠<math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle +\pi /2}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mo>+</mo> <mi>π</mi> <mrow class="MJX-TeXAtom-ORD"> <mo>/</mo> </mrow> <mn>2</mn> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle +\pi /2}</annotation> </semantics> </math><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/b7fac17a374c4e9eae65df5546ec64b79b0e16ca" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:5.465ex; height:2.843ex;" alt="{\displaystyle +\pi /2}">⁠) </td></tr> <tr> <td><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">Math.atan2(-3.7, -3.7)</td> <td>−2.3562 rad = −135°</td> <td>Whole circle arctangent (⁠<math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle -\pi }"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mo>−</mo> <mi>π</mi> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle -\pi }</annotation> </semantics> </math><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/f2359073fe90a84a705e02f0c1e63b32df850a60" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.505ex; width:3.14ex; height:2.176ex;" alt="{\displaystyle -\pi }">⁠ to ⁠<math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle +\pi }"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mo>+</mo> <mi>π</mi> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle +\pi }</annotation> </semantics> </math><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/cbae717a544919265768ad63c73919176113fb12" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.505ex; width:3.14ex; height:2.176ex;" alt="{\displaystyle +\pi }">⁠) </td></tr> <tr> <td><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">Math.ceil(1.1)</td> <td>2</td> <td>Ceiling: <a href="/wiki/Rounding" title="Rounding">round</a> up to smallest integer ≥ argument </td></tr> <tr> <td><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">Math.cos(Math.PI/4)</td> <td>0.70711</td> <td><a href="/wiki/Trigonometric_functions" title="Trigonometric functions">Cosine</a> </td></tr> <tr> <td><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">Math.exp(1)</td> <td>2.7183</td> <td><a href="/wiki/Exponential_function" title="Exponential function">Exponential function</a>: e raised to this power </td></tr> <tr> <td><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">Math.floor(1.9)</td> <td>1</td> <td>Floor: round down to largest integer ≤ argument </td></tr> <tr> <td><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">Math.log(Math.E)</td> <td>1</td> <td>Natural logarithm, base e </td></tr> <tr> <td><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">Math.max(1, -2)</td> <td>1</td> <td>Maximum: <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">(x > y) ? x : y </td></tr> <tr> <td><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">Math.min(1, -2)</td> <td>−2</td> <td>Minimum: <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">(x < y) ? x : y </td></tr> <tr> <td><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">Math.pow(-3, 2)</td> <td>9</td> <td><a href="/wiki/Exponentiation" title="Exponentiation">Exponentiation</a> (raised to the power of): <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">Math.pow(x, y) gives xy </td></tr> <tr> <td><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">Math.random()</td> <td>e.g. 0.17068</td> <td><a href="/wiki/Pseudorandom" class="mw-redirect" title="Pseudorandom">Pseudorandom</a> number between 0 (inclusive) and 1 (exclusive) </td></tr> <tr> <td><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">Math.round(1.5)</td> <td>2</td> <td>Round to the nearest integer; half fractions are rounded up (e.g. 1.5 rounds to 2) </td></tr> <tr> <td><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">Math.sin(Math.PI/4)</td> <td>0.70711</td> <td><a href="/wiki/Sine" class="mw-redirect" title="Sine">Sine</a> </td></tr> <tr> <td><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">Math.sqrt(49)</td> <td>7</td> <td>Square root </td></tr> <tr> <td><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">Math.tan(Math.PI/4)</td> <td>1</td> <td><a href="/wiki/Trigonometric_functions" title="Trigonometric functions">Tangent</a> </td></tr></tbody></table> <div class="mw-heading mw-heading3"><h3 id="Regular_expression">Regular expression</h3>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=24" title="Edit section: Regular expression">edit</a>]</div> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1236090951"><div role="note" class="hatnote navigation-not-searchable">Main article: <a href="/wiki/Regular_expression" title="Regular expression">Regular expression</a></div> <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>/expression/.test(string); // returns Boolean "string".search(/expression/); // returns position Number "string".replace(/expression/, replacement); // Here are some examples if (/Tom/.test("My name is Tom")) console.log("Hello Tom!"); console.log("My name is Tom".search(/Tom/)); // == 11 (letters before Tom) console.log("My name is Tom".replace(/Tom/, "John")); // == "My name is John" </pre></div> <div class="mw-heading mw-heading4"><h4 id="Character_classes">Character classes</h4>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=25" title="Edit section: Character classes">edit</a>]</div> <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>// \d - digit // \D - non digit // \s - space // \S - non space // \w - word char // \W - non word // [ ] - one of // [^] - one not of // - - range if (/\d/.test('0')) console.log('Digit'); if (/[0-9]/.test('6')) console.log('Digit'); if (/[13579]/.test('1')) console.log('Odd number'); if (/\S\S\s\S\S\S\S/.test('My name')) console.log('Format OK'); if (/\w\w\w/.test('Tom')) console.log('Hello Tom'); if (/[a-zA-Z]/.test('B')) console.log('Letter'); </pre></div> <div class="mw-heading mw-heading4"><h4 id="Character_matching">Character matching</h4>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=26" title="Edit section: Character matching">edit</a>]</div> <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>// A...Z a...z 0...9 - alphanumeric // \u0000...\uFFFF - Unicode hexadecimal // \x00...\xFF - ASCII hexadecimal // \t - tab // \n - new line // \r - CR // . - any character // | - OR if (/T.m/.test('Tom')) console.log ('Hi Tom, Tam or Tim'); if (/A|B/.test("A")) console.log ('A or B'); </pre></div> <div class="mw-heading mw-heading4"><h4 id="Repeaters">Repeaters</h4>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=27" title="Edit section: Repeaters">edit</a>]</div> <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>// ? - 0 or 1 match // * - 0 or more // + - 1 or more // {n} - exactly n // {n,} - n or more // {0,n} - n or less // {n,m} - range n to m if (/ab?c/.test("ac")) console.log("OK"); // match: "ac", "abc" if (/ab*c/.test("ac")) console.log("OK"); // match: "ac", "abc", "abbc", "abbbc" etc. if (/ab+c/.test("abc")) console.log("OK"); // match: "abc", "abbc", "abbbc" etc. if (/ab{3}c/.test("abbbc")) console.log("OK"); // match: "abbbc" if (/ab{3,}c/.test("abbbc")) console.log("OK"); // match: "abbbc", "abbbbc", "abbbbbc" etc. if (/ab{1,3}c/.test("abc")) console.log("OK"); // match: "abc", "abbc", "abbbc" </pre></div> <div class="mw-heading mw-heading4"><h4 id="Anchors">Anchors</h4>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=28" title="Edit section: Anchors">edit</a>]</div> <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>// ^ - string starts with // $ - string ends with if (/^My/.test("My name is Tom")) console.log ("Hi!"); if (/Tom$/.test("My name is Tom")) console.log ("Hi Tom!"); </pre></div> <div class="mw-heading mw-heading4"><h4 id="Subexpression">Subexpression</h4>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=29" title="Edit section: Subexpression">edit</a>]</div> <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>// ( ) - groups characters if (/water(mark)?/.test("watermark")) console.log("Here is water!"); // match: "water", "watermark", if (/(Tom)|(John)/.test("John")) console.log("Hi Tom or John!"); </pre></div> <div class="mw-heading mw-heading4"><h4 id="Flags">Flags</h4>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=30" title="Edit section: Flags">edit</a>]</div> <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>// /g - global // /i - ignore upper/lower case // /m - allow matches to span multiple lines console.log("hi tom!".replace(/Tom/i, "John")); // == "hi John!" console.log("ratatam".replace(/ta/, "tu")); // == "ratutam" console.log("ratatam".replace(/ta/g, "tu")); // == "ratutum" </pre></div> <div class="mw-heading mw-heading4"><h4 id="Advanced_methods">Advanced methods</h4>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=31" title="Edit section: Advanced methods">edit</a>]</div> <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>my_array = my_string.split(my_delimiter); // example my_array = "dog,cat,cow".split(","); // my_array==["dog","cat","cow"]; my_array = my_string.match(my_expression); // example my_array = "We start at 11:30, 12:15 and 16:45".match(/\d\d:\d\d/g); // my_array==["11:30","12:15","16:45"]; </pre></div> <div class="mw-heading mw-heading4"><h4 id="Capturing_groups">Capturing groups</h4>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=32" title="Edit section: Capturing groups">edit</a>]</div> <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>const myRe = /(\d{4}-\d{2}-\d{2}) (\d{2}:\d{2}:\d{2})/; const results = myRe.exec("The date and time are 2009-09-08 09:37:08."); if (results) { console.log("Matched: " + results[0]); // Entire match const my_date = results[1]; // First group == "2009-09-08" const my_time = results[2]; // Second group == "09:37:08" console.log(`It is ${my_time} on ${my_date}`); } else console.log("Did not find a valid date!"); </pre></div> <div class="mw-heading mw-heading3"><h3 id="Function">Function</h3>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=33" title="Edit section: Function">edit</a>]</div> Every function in JavaScript is an instance of the <code>Function</code> constructor: <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>// x, y is the argument. 'return x + y' is the function body, which is the last in the argument list. const add = new Function('x', 'y', 'return x + y'); add(1, 2); // => 3 </pre></div> The add function above may also be defined using a function expression: <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>const add = function(x, y) { return x + y; }; add(1, 2); // => 3 </pre></div> In ES6, arrow function syntax was added, allowing functions that return a value to be more concise. They also retain the <code>this</code> of the global object instead of inheriting it from where it was called / what it was called on, unlike the <code>function() {}</code> expression. <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>const add = (x, y) => {return x + y;}; // values can also be implicitly returned (i.e. no return statement is needed) const addImplicit = (x, y) => x + y; add(1, 2); // => 3 addImplicit(1, 2) // => 3 </pre></div> For functions that need to be hoisted, there is a separate expression: <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>function add(x, y) { return x + y; } add(1, 2); // => 3 </pre></div> Hoisting allows you to use the function before it is "declared": <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>add(1, 2); // => 3, not a ReferenceError function add(x, y) { return x + y; } </pre></div> A function instance has properties and methods. <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>function subtract(x, y) { return x - y; } console.log(subtract.length); // => 2, arity of the function (number of arguments) console.log(subtract.toString()); /* "function subtract(x, y) { return x - y; }" */ </pre></div> <div class="mw-heading mw-heading2"><h2 id="Operators">Operators</h2>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=34" title="Edit section: Operators">edit</a>]</div> The '+' operator is <a href="/wiki/Operator_overloading" title="Operator overloading">overloaded</a>: it is used for string concatenation and arithmetic addition. This may cause problems when inadvertently mixing strings and numbers. As a unary operator, it can convert a numeric string to a number. <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>// Concatenate 2 strings console.log('He' + 'llo'); // displays Hello // Add two numbers console.log(2 + 6); // displays 8 // Adding a number and a string results in concatenation (from left to right) console.log(2 + '2'); // displays 22 console.log('$' + 3 + 4); // displays $34, but $7 may have been expected console.log('$' + (3 + 4)); // displays $7 console.log(3 + 4 + '7'); // displays 77, numbers stay numbers until a string is added // Convert a string to a number using the unary plus console.log(+'2' === 2); // displays true console.log(+'Hello'); // displays NaN </pre></div> Similarly, the '*' operator is overloaded: it can convert a string into a number. <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>console.log(2 + '6'*1); // displays 8 console.log(3*'7'); // 21 console.log('3'*'7'); // 21 console.log('hello'*'world'); // displays NaN </pre></div> <div class="mw-heading mw-heading3"><h3 id="Arithmetic">Arithmetic</h3>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=35" title="Edit section: Arithmetic">edit</a>]</div> JavaScript supports the following binary arithmetic operators: <table class="wikitable"> <tbody><tr> <td align="center"><code>+</code></td> <td>addition </td></tr> <tr> <td align="center"><code>-</code></td> <td>subtraction </td></tr> <tr> <td align="center"><code>*</code></td> <td>multiplication </td></tr> <tr> <td align="center"><code>/</code></td> <td>division (returns a floating-point value) </td></tr> <tr> <td align="center"><code>%</code></td> <td>modulo (returns the remainder) </td></tr> <tr> <td align="center"><code>**</code></td> <td>exponentiation </td></tr></tbody></table> JavaScript supports the following unary arithmetic operators: <table class="wikitable"> <tbody><tr> <td align="center"><code>+</code></td> <td>unary conversion of string to number </td></tr> <tr> <td align="center"><code>-</code></td> <td>unary negation (reverses the sign) </td></tr> <tr> <td align="center"><code>++</code></td> <td>increment (can be prefix or postfix) </td></tr> <tr> <td align="center"><code>--</code></td> <td>decrement (can be prefix or postfix) </td></tr></tbody></table> <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>let x = 1; console.log(++x); // x becomes 2; displays 2 console.log(x++); // displays 2; x becomes 3 console.log(x); // x is 3; displays 3 console.log(x--); // displays 3; x becomes 2 console.log(x); // displays 2; x is 2 console.log(--x); // x becomes 1; displays 1 </pre></div> The modulo operator displays the remainder after division by the modulus. If negative numbers are involved, the returned value depends on the operand. <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>const x = 17; console.log(x%5); // displays 2 console.log(x%6); // displays 5 console.log(-x%5); // displays -2 console.log(-x%-5); // displays -2 console.log(x%-5); // displays 2 </pre></div> To always return a non-negative number, re-add the modulus and apply the modulo operator again: <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>const x = 17; console.log((-x%5+5)%5); // displays 3 </pre></div> You could also do: <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>const x = 17; console.log(Math.abs(-x%5)); // also 3 </pre></div> <div class="mw-heading mw-heading3"><h3 id="Assignment">Assignment</h3>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=36" title="Edit section: Assignment">edit</a>]</div> <table class="wikitable"> <tbody><tr> <td align="center"><code>=</code></td> <td>assign </td></tr> <tr> <td align="center"><code>+=</code></td> <td>add and assign </td></tr> <tr> <td align="center"><code>-=</code></td> <td>subtract and assign </td></tr> <tr> <td align="center"><code>*=</code></td> <td>multiply and assign </td></tr> <tr> <td align="center"><code>/=</code></td> <td>divide and assign </td></tr> <tr> <td align="center"><code>%=</code></td> <td>modulo and assign </td></tr> <tr> <td align="center"><code>**=</code></td> <td>exponentiation and assign </td></tr></tbody></table> <a href="/wiki/Assignment_(computer_science)" title="Assignment (computer science)">Assignment</a> of <a href="/wiki/Primitive_type" class="mw-redirect" title="Primitive type">primitive types</a> <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>let x = 9; x += 1; console.log(x); // displays: 10 x *= 30; console.log(x); // displays: 300 x /= 6; console.log(x); // displays: 50 x -= 3; console.log(x); // displays: 47 x %= 7; console.log(x); // displays: 5 </pre></div> Assignment of object types <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>/** * To learn JavaScript objects... */ const object_1 = {a: 1}; // assign reference of newly created object to object_1 let object_2 = {a: 0}; let object_3 = object_2; // object_3 references the same object as object_2 does object_3.a = 2; message(); // displays 1 2 2 object_2 = object_1; // object_2 now references the same object as object_1 // object_3 still references what object_2 referenced before message(); // displays 1 1 2 object_2.a = 7; // modifies object_1 message(); // displays 7 7 2 object_3.a = 5; // object_3 doesn't change object_2 message(); // displays 7 7 5 object_3 = object_2; object_3.a=4; // object_3 changes object_1 and object_2 message(); // displays 4 4 4 /** * Prints the console.log message */ function message() { console.log(object_1.a + " " + object_2.a + " " + object_3.a); } </pre></div> <div class="mw-heading mw-heading4"><h4 id="Destructuring_assignment">Destructuring assignment</h4>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=37" title="Edit section: Destructuring assignment">edit</a>]</div> In Mozilla's JavaScript, since version 1.7, destructuring assignment allows the assignment of parts of data structures to several variables at once. The left hand side of an assignment is a pattern that resembles an arbitrarily nested object/array literal containing l-lvalues at its leaves that are to receive the substructures of the assigned value. <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>let a, b, c, d, e; [a, b, c] = [3, 4, 5]; console.log(`${a},${b},${c}`); // displays: 3,4,5 e = {foo: 5, bar: 6, baz: ['Baz', 'Content']}; const arr = []; ({baz: [arr[0], arr[3]], foo: a, bar: b} = e); console.log(`${a},${b},${arr}`); // displays: 5,6,Baz,,,Content [a, b] = [b, a]; // swap contents of a and b console.log(a + ',' + b); // displays: 6,5 [a, b, c] = [3, 4, 5]; // permutations [a, b, c] = [b, c, a]; console.log(`${a},${b},${c}`); // displays: 4,5,3 </pre></div> <div class="mw-heading mw-heading4"><h4 id="Spread/rest_operator">Spread/rest operator</h4>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=38" title="Edit section: Spread/rest operator">edit</a>]</div> The ECMAScript 2015 standard introduced the "<code>...</code>" array operator, for the related concepts of "spread syntax"<a href="#cite_note-12">[12]</a> and "rest parameters".<a href="#cite_note-13">[13]</a> Object spreading was added in ECMAScript 2018. Spread syntax provides another way to destructure arrays and objects. For arrays, it indicates that the elements should be used as the parameters in a function call or the items in an array literal. For objects, it can be used for merging objects together or overriding properties. In other words, "<code>...</code>" transforms "<code>[...foo]</code>" into "<code>[foo[0], foo[1], foo[2]]</code>", and "<code>this.bar(...foo);</code>" into "<code>this.bar(foo[0], foo[1], foo[2]);</code>", and "<code>{ ...bar }</code>" into <code>{ prop: bar.prop, prop2: bar.prop2 }</code>. <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr mw-highlight-lines" dir="ltr"><pre>const a = [1, 2, 3, 4]; // It can be used multiple times in the same expression const b = [...a, ...a]; // b = [1, 2, 3, 4, 1, 2, 3, 4]; // It can be combined with non-spread items. const c = [5, 6, ...a, 7, 9]; // c = [5, 6, 1, 2, 3, 4, 7, 9]; // For comparison, doing this without the spread operator // creates a nested array. const d = [a, a]; // d = [[1, 2, 3, 4], [1, 2, 3, 4]] // It works the same with function calls function foo(arg1, arg2, arg3) { console.log(`${arg1}:${arg2}:${arg3}`); } // You can use it even if it passes more parameters than the function will use foo(...a); // "1:2:3" → foo(a[0], a[1], a[2], a[3]); // You can mix it with non-spread parameters foo(5, ...a, 6); // "5:1:2" → foo(5, a[0], a[1], a[2], a[3], 6); // For comparison, doing this without the spread operator // assigns the array to arg1, and nothing to the other parameters. foo(a); // "1,2,3,4:undefined:undefined" const bar = { a: 1, b: 2, c: 3 }; // This would copy the object const copy = { ...bar }; // copy = { a: 1, b: 2, c: 3 }; // "b" would be overridden here const override = { ...bar, b: 4 }; // override = { a: 1, c: 3, b: 4 } </pre></div> When <code>...</code> is used in a function declaration, it indicates a rest parameter. The rest parameter must be the final named parameter in the function's parameter list. It will be assigned an <code>Array</code> containing any arguments passed to the function in excess of the other named parameters. In other words, it gets "the rest" of the arguments passed to the function (hence the name). <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>function foo(a, b, ...c) { console.log(c.length); } foo(1, 2, 3, 4, 5); // "3" → c = [3, 4, 5] foo('a', 'b'); // "0" → c = [] </pre></div> Rest parameters are similar to Javascript's <code>arguments</code> object, which is an array-like object that contains all of the parameters (named and unnamed) in the current function call. Unlike <code>arguments</code>, however, rest parameters are true <code>Array</code> objects, so methods such as <code>.slice()</code> and <code>.sort()</code> can be used on them directly. <div class="mw-heading mw-heading3"><h3 id="Comparison">Comparison</h3>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=39" title="Edit section: Comparison">edit</a>]</div> <table class="wikitable"> <tbody><tr> <td align="center"><code>==</code></td> <td>equal </td></tr> <tr> <td align="center"><code>!=</code></td> <td>not equal </td></tr> <tr> <td align="center"><code>></code></td> <td>greater than </td></tr> <tr> <td align="center"><code>>=</code></td> <td>greater than or equal to </td></tr> <tr> <td align="center"><code><</code></td> <td>less than </td></tr> <tr> <td align="center"><code><=</code></td> <td>less than or equal to </td></tr> <tr> <td align="center"><code>===</code></td> <td>identical (equal and of same type) </td></tr> <tr> <td align="center"><code>!==</code></td> <td>not identical </td></tr></tbody></table> Variables referencing objects are equal or identical only if they reference the same object: <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>const obj1 = {a: 1}; const obj2 = {a: 1}; const obj3 = obj1; console.log(obj1 == obj2); //false console.log(obj3 == obj1); //true console.log(obj3 === obj1); //true </pre></div> See also <a href="#String">String</a>. <div class="mw-heading mw-heading3"><h3 id="Logical">Logical</h3>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=40" title="Edit section: Logical">edit</a>]</div> JavaScript provides four logical operators: <ul><li>unary <a href="/wiki/Negation" title="Negation">negation</a> (<code>NOT = !a</code>)</li> <li>binary <a href="/wiki/Logical_disjunction" title="Logical disjunction">disjunction</a> (<code>OR = a || b</code>) and <a href="/wiki/Logical_conjunction" title="Logical conjunction">conjunction</a> (<code>AND = a && b</code>)</li> <li>ternary <a href="/wiki/Conditional_(programming)" class="mw-redirect" title="Conditional (programming)">conditional</a> (<code>c ? t : f</code>)</li></ul> In the context of a logical operation, any expression evaluates to true except the following: <ul><li>Strings: <code>""</code>, <code>''</code>,</li> <li>Numbers: <code>0</code>, <code>-0</code>, <code>NaN</code>,</li> <li>Special: <code>null</code>, <code>undefined</code>,</li> <li>Boolean: <code>false</code>.</li></ul> The Boolean function can be used to explicitly convert to a primitive of type <code>Boolean</code>: <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>// Only empty strings return false console.log(Boolean("") === false); console.log(Boolean("false") === true); console.log(Boolean("0") === true); // Only zero and NaN return false console.log(Boolean(NaN) === false); console.log(Boolean(0) === false); console.log(Boolean(-0) === false); // equivalent to -1*0 console.log(Boolean(-2) === true); // All objects return true console.log(Boolean(this) === true); console.log(Boolean({}) === true); console.log(Boolean([]) === true); // These types return false console.log(Boolean(null) === false); console.log(Boolean(undefined) === false); // equivalent to Boolean() </pre></div> The NOT operator evaluates its operand as a Boolean and returns the negation. Using the operator twice in a row, as a <a href="/wiki/Double_negative" title="Double negative">double negative</a>, explicitly converts an expression to a primitive of type Boolean: <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>console.log( !0 === Boolean(!0)); console.log(Boolean(!0) === !!1); console.log(!!1 === Boolean(1)); console.log(!!0 === Boolean(0)); console.log(Boolean(0) === !1); console.log(!1 === Boolean(!1)); console.log(!"" === Boolean(!"")); console.log(Boolean(!"") === !!"s"); console.log(!!"s" === Boolean("s")); console.log(!!"" === Boolean("")); console.log(Boolean("") === !"s"); console.log(!"s" === Boolean(!"s")); </pre></div> The ternary operator can also be used for explicit conversion: <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>console.log([] == false); console.log([] ? true : false); // “truthy”, but the comparison uses [].toString() console.log([0] == false); console.log([0]? true : false); // [0].toString() == "0" console.log("0" == false); console.log("0"? true : false); // "0" → 0 ... (0 == 0) ... 0 ← false console.log([1] == true); console.log([1]? true : false); // [1].toString() == "1" console.log("1" == true); console.log("1"? true : false); // "1" → 1 ... (1 == 1) ... 1 ← true console.log([2] != true); console.log([2]? true : false); // [2].toString() == "2" console.log("2" != true); console.log("2"? true : false); // "2" → 2 ... (2 != 1) ... 1 ← true </pre></div> Expressions that use features such as post–incrementation (<code>i++</code>) have an anticipated <a href="/wiki/Side_effect_(computer_science)" title="Side effect (computer science)">side effect</a>. JavaScript provides <a href="/wiki/Short-circuit_evaluation" title="Short-circuit evaluation">short-circuit evaluation</a> of expressions; the right operand is only executed if the left operand does not suffice to determine the value of the expression. <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>console.log(a || b); // When a is true, there is no reason to evaluate b. console.log(a && b); // When a is false, there is no reason to evaluate b. console.log(c ? t : f); // When c is true, there is no reason to evaluate f. </pre></div> In early versions of JavaScript and <a href="/wiki/JScript" title="JScript">JScript</a>, the binary logical operators returned a Boolean value (like most C-derived programming languages). However, all contemporary implementations return one of their operands instead: <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>console.log(a || b); // if a is true, return a, otherwise return b console.log(a && b); // if a is false, return a, otherwise return b </pre></div> Programmers who are more familiar with the behavior in C might find this feature surprising, but it allows for a more concise expression of patterns like <a href="/wiki/Null_coalescing_operator" title="Null coalescing operator">null coalescing</a>: <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>const s = t || "(default)"; // assigns t, or the default value, if t is null, empty, etc. </pre></div> <div class="mw-heading mw-heading3"><h3 id="Logical_assignment">Logical assignment</h3>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=41" title="Edit section: Logical assignment">edit</a>]</div> <table class="wikitable"> <tbody><tr> <td align="center"><code>??=</code> </td> <td>Nullish assignment </td></tr> <tr> <td align="center"><code>||=</code> </td> <td>Logical Or assignment </td></tr> <tr> <td align="center"><code>&&=</code> </td> <td>Logical And assignment </td></tr></tbody></table> <div class="mw-heading mw-heading3"><h3 id="Bitwise">Bitwise</h3>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=42" title="Edit section: Bitwise">edit</a>]</div> JavaScript supports the following binary <a href="/wiki/Bitwise_operation" title="Bitwise operation">bitwise operators</a>: <table class="wikitable"> <tbody><tr> <td align="center"><code>&</code></td> <td>AND </td></tr> <tr> <td align="center"><code>|</code></td> <td>OR </td></tr> <tr> <td align="center"><code>^</code></td> <td>XOR </td></tr> <tr> <td align="center"><code>!</code></td> <td>NOT </td></tr> <tr> <td align="center"><code><<</code></td> <td>shift left (zero fill at right) </td></tr> <tr> <td align="center"><code>>></code></td> <td>shift right (sign-propagating); copies of the leftmost bit (sign bit) are shifted in from the left </td></tr> <tr> <td align="center"><code>>>></code></td> <td>shift right (zero fill at left). For positive numbers, <code>>></code> and <code>>>></code> yield the same result. </td></tr></tbody></table> Examples: <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>const x = 11 & 6; console.log(x); // 2 </pre></div> JavaScript supports the following unary bitwise operator: <table class="wikitable"> <tbody><tr> <td align="center"><code>~</code></td> <td>NOT (inverts the bits) </td></tr></tbody></table> <div class="mw-heading mw-heading3"><h3 id="Bitwise_Assignment">Bitwise Assignment</h3>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=43" title="Edit section: Bitwise Assignment">edit</a>]</div> JavaScript supports the following binary assignment operators: <table class="wikitable"> <tbody><tr> <td align="center"><code>&=</code></td> <td>and </td></tr> <tr> <td align="center"><code>|=</code></td> <td>or </td></tr> <tr> <td align="center"><code>^=</code></td> <td>xor </td></tr> <tr> <td align="center"><code><<=</code></td> <td>shift left (zero fill at right) </td></tr> <tr> <td align="center"><code>>>=</code></td> <td>shift right (sign-propagating); copies of the leftmost bit (sign bit) are shifted in from the left </td></tr> <tr> <td align="center"><code>>>>=</code></td> <td>shift right (zero fill at left). For positive numbers, <code>>>=</code> and <code>>>>=</code> yield the same result. </td></tr></tbody></table> Examples: <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>let x=7; console.log(x); // 7 x<<=3; console.log(x); // 7->14->28->56 </pre></div> <div class="mw-heading mw-heading3"><h3 id="String_2">String</h3>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=44" title="Edit section: String">edit</a>]</div> <table class="wikitable"> <tbody><tr> <td align="center"><code>=</code></td> <td>assignment </td></tr> <tr> <td align="center"><code>+</code></td> <td>concatenation </td></tr> <tr> <td align="center"><code>+=</code></td> <td>concatenate and assign </td></tr></tbody></table> Examples: <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>let str = "ab" + "cd"; // "abcd" str += "e"; // "abcde" const str2 = "2" + 2; // "22", not "4" or 4. </pre></div> <div class="mw-heading mw-heading3"><h3 id="??">??</h3>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=45" title="Edit section: ??">edit</a>]</div> <div class="excerpt-block"><style data-mw-deduplicate="TemplateStyles:r1066933788">.mw-parser-output .excerpt-hat .mw-editsection-like{font-style:normal}</style><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1236090951"><div role="note" class="hatnote navigation-not-searchable dablink excerpt-hat selfref">This section is an excerpt from <a href="/wiki/Null_coalescing_operator#JavaScript" title="Null coalescing operator">Null coalescing operator § JavaScript</a>.[<a class="external text" href="https://en.wikipedia.org/w/index.php?title=Null_coalescing_operator&action=edit">edit</a>]</div><div class="excerpt"> <a href="/wiki/JavaScript" title="JavaScript">JavaScript</a>'s nearest operator is <code>??</code>, the "nullish coalescing operator", which was added to the standard in <a href="/wiki/ECMAScript" title="ECMAScript">ECMAScript</a>'s 11th edition.<a href="#cite_note-14">[14]</a> In earlier versions, it could be used via a <a href="/wiki/Babel_(compiler)" class="mw-redirect" title="Babel (compiler)">Babel</a> plugin, and in <a href="/wiki/TypeScript" title="TypeScript">TypeScript</a>. It evaluates its left-hand operand and, if the result value is not "nullish" (<code>null</code> or <code>undefined</code>), takes that value as its result; otherwise, it evaluates the right-hand operand and takes the resulting value as its result. In the following example, <code>a</code> will be assigned the value of <code>b</code> if the value of <code>b</code> is not <code>null</code> or <code>undefined</code>, otherwise it will be assigned 3. <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>const a = b ?? 3; </pre></div> Before the nullish coalescing operator, programmers would use the logical OR operator (<code>||</code>). But where <code>??</code> looks specifically for <code>null</code> or <code>undefined</code>, the <code>||</code> operator looks for any <a class="mw-selflink-fragment" href="#Boolean">falsy</a> value: <code>null</code>, <code>undefined</code>, <code>""</code>, <code>0</code>, <code>NaN</code>, and of course, <code>false</code>. In the following example, <code>a</code> will be assigned the value of <code>b</code> if the value of <code>b</code> is <a class="mw-selflink-fragment" href="#Boolean">truthy</a>, otherwise it will be assigned 3. <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>const a = b || 3; </pre></div></div></div> <div class="mw-heading mw-heading2"><h2 id="Control_structures">Control structures</h2>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=46" title="Edit section: Control structures">edit</a>]</div> <div class="mw-heading mw-heading3"><h3 id="Compound_statements">Compound statements</h3>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=47" title="Edit section: Compound statements">edit</a>]</div> A pair of curly brackets <code>{ }</code> and an enclosed sequence of statements constitute a compound statement, which can be used wherever a statement can be used. <div class="mw-heading mw-heading3"><h3 id="If_..._else">If ... else</h3>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=48" title="Edit section: If ... else">edit</a>]</div> <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>if (expr) { //statements; } else if (expr2) { //statements; } else { //statements; } </pre></div> <div class="mw-heading mw-heading3"><h3 id="Conditional_(ternary)_operator">Conditional (ternary) operator</h3>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=49" title="Edit section: Conditional (ternary) operator">edit</a>]</div> The conditional operator creates an expression that evaluates as one of two expressions depending on a condition. This is similar to the if statement that selects one of two statements to execute depending on a condition. I.e., the conditional operator is to expressions what if is to statements. <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>const result = condition ? expression : alternative; </pre></div> is the same as: <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>if (condition) { const result = expression; } else { const result = alternative; } </pre></div> Unlike the if statement, the conditional operator cannot omit its "else-branch". <div class="mw-heading mw-heading3"><h3 id="Switch_statement">Switch statement</h3>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=50" title="Edit section: Switch statement">edit</a>]</div> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1236090951"><div role="note" class="hatnote navigation-not-searchable">Main article: <a href="/wiki/Switch_statement" title="Switch statement">Switch statement</a></div> The syntax of the JavaScript <a href="/wiki/Control_flow#Choice" title="Control flow">switch statement</a> is as follows: <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre> switch (expr) { case SOMEVALUE: // statements; break; case ANOTHERVALUE: // statements for when ANOTHERVALUE || ORNAOTHERONE // no break statement, falling through to the following case case ORANOTHERONE: // statements specific to ORANOTHERONE (i.e. !ANOTHERVALUE && ORANOTHER); break; //The buck stops here. case YETANOTHER: // statements; break; default: // statements; break; } </pre></div> <ul><li><code>break;</code> is optional; however, it is usually needed, since otherwise code execution will continue to the body of the next case block. This fall through behavior can be used when the same set of statements apply in several cases, effectively creating a <a href="/wiki/Disjunction" class="mw-redirect" title="Disjunction">disjunction</a> between those cases.</li> <li>Add a break statement to the end of the last case as a precautionary measure, in case additional cases are added later.</li> <li>String literal values can also be used for the case values.</li> <li>Expressions can be used instead of values.</li> <li>The default case (optional) is executed when the expression does not match any other specified cases.</li> <li>Braces are required.</li></ul> <div class="mw-heading mw-heading3"><h3 id="For_loop">For loop</h3>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=51" title="Edit section: For loop">edit</a>]</div> The syntax of the JavaScript <a href="/wiki/For_loop" title="For loop">for loop</a> is as follows: <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre> for (initial; condition; loop statement) { /* statements will be executed every time the for{} loop cycles, while the condition is satisfied */ } </pre></div> or <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre> for (initial; condition; loop statement(iteration)) // one statement </pre></div> <div class="mw-heading mw-heading3"><h3 id="For_..._in_loop">For ... in loop</h3>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=52" title="Edit section: For ... in loop">edit</a>]</div> The syntax of the JavaScript <code><a href="/wiki/Foreach_loop" title="Foreach loop">for ... in loop</a></code> is as follows: <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>for (var property_name in some_object) { // statements using some_object[property_name]; } </pre></div> <ul><li>Iterates through all enumerable properties of an object.</li> <li>Iterates through all used indices of array including all user-defined properties of array object, if any. Thus it may be better to use a traditional for loop with a numeric index when iterating over arrays.</li> <li>There are differences between the various Web browsers with regard to which properties will be reflected with the for...in loop statement. In theory, this is controlled by an internal state property defined by the ECMAscript standard called "DontEnum", but in practice, each browser returns a slightly different set of properties during introspection. It is useful to test for a given property using <code class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" style="" dir="ltr">if (some_object.hasOwnProperty(property_name)) { ...</code>}. Thus, adding a method to the array prototype with <code class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" style="" dir="ltr">Array.prototype.newMethod = function() {...</code>} may cause <code>for ... in</code> loops to loop over the method's name.</li></ul> <div class="mw-heading mw-heading3"><h3 id="While_loop">While loop</h3>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=53" title="Edit section: While loop">edit</a>]</div> The syntax of the JavaScript <a href="/wiki/While_loop" title="While loop">while loop</a> is as follows: <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>while (condition) { statement1; statement2; statement3; ... } </pre></div> <div class="mw-heading mw-heading3"><h3 id="Do_..._while_loop">Do ... while loop</h3>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=54" title="Edit section: Do ... while loop">edit</a>]</div> The syntax of the JavaScript <code><a href="/wiki/Do_while_loop" title="Do while loop">do ... while loop</a></code> is as follows: <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>do { statement1; statement2; statement3; ... } while (condition); </pre></div> <div class="mw-heading mw-heading3"><h3 id="With">With</h3>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=55" title="Edit section: With">edit</a>]</div> The with statement adds all of the given object's properties and methods into the following block's scope, letting them be referenced as if they were local variables. <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>with (document) { const a = getElementById('a'); const b = getElementById('b'); const c = getElementById('c'); }; </pre></div> <ul><li>Note the absence of <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">document. before each <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">getElementById() invocation.</li></ul> The semantics are similar to the with statement of <a href="/wiki/Pascal_(programming_language)" title="Pascal (programming language)">Pascal</a>. Because the availability of with statements hinders program performance and is believed to reduce code clarity (since any given variable could actually be a property from an enclosing <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">with), this statement is not allowed in strict mode. <div class="mw-heading mw-heading3"><h3 id="Labels">Labels</h3>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=56" title="Edit section: Labels">edit</a>]</div> JavaScript supports nested labels in most implementations. Loops or blocks can be labelled for the break statement, and loops for <code>continue</code>. Although <code><a href="/wiki/Goto" title="Goto">goto</a></code> is a reserved word,<a href="#cite_note-15">[15]</a> <code>goto</code> is not implemented in JavaScript. <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>loop1: for (let a = 0; a < 10; ++a) { if (a === 4) break loop1; // Stops after the 4th attempt console.log('a = ' + a); loop2: for (let b = 0; b < 10; ++b) { if (b === 3) continue loop2; // Number 3 is skipped if (b === 6) continue loop1; // Continues the first loop, 'finished' is not shown console.log('b = ' + b); } //end of loop2 console.log('finished'); } //end of loop1 block1: { console.log('Hello'); // Displays 'Hello' break block1; console.log('World'); // Will never get here } goto block1; // Parse error. </pre></div> <div class="mw-heading mw-heading2"><h2 id="Functions">Functions</h2>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=57" title="Edit section: Functions">edit</a>]</div> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1236090951"><div role="note" class="hatnote navigation-not-searchable">Main article: <a href="/wiki/Function_(computer_programming)" title="Function (computer programming)">Function (computer programming)</a></div> A <a href="/wiki/Function_(computer_programming)" title="Function (computer programming)">function</a> is a block with a (possibly empty) parameter list that is normally given a name. A function may use local variables. If you exit the function without a return statement, the value <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">undefined is returned. <div class="mw-highlight mw-highlight-lang-javascript+genshitext mw-content-ltr" dir="ltr"><pre>function gcd(number1, number2) { if (isNaN(number1*number2)) throw TypeError("Non-Numeric arguments not allowed."); number1 = Math.round(number1); number2 = Math.round(number2); let difference = number1 - number2; if (difference === 0) return number1; return difference > 0 ? gcd(number2, difference) : gcd(number1, -difference); } console.log(gcd(60, 40)); // 20 //In the absence of parentheses following the identifier 'gcd' on the RHS of the assignment below, //'gcd' returns a reference to the function itself without invoking it. let mygcd = gcd; // mygcd and gcd reference the same function. console.log(mygcd(60, 40)); // 20 </pre></div> Functions are <a href="/wiki/First_class_object" class="mw-redirect" title="First class object">first class objects</a> and may be assigned to other variables. The number of arguments given when calling a function may not necessarily correspond to the number of arguments in the function definition; a named argument in the definition that does not have a matching argument in the call will have the value <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">undefined (that can be implicitly cast to false). Within the function, the arguments may also be accessed through the <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">arguments object; this provides access to all arguments using indices (e.g. <code class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" style="" dir="ltr">arguments[0], arguments[1], ... arguments[n]</code>), including those beyond the number of named arguments. (While the arguments list has a <code>.length</code> property, it is not an instance of <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">Array; it does not have methods such as <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">.slice(), <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">.sort(), etc.) <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>function add7(x, y) { if (!y) { y = 7; } console.log(x + y + arguments.length); }; add7(3); // 11 add7(3, 4); // 9 </pre></div> Primitive values (number, boolean, string) are passed by value. For objects, it is the reference to the object that is passed. <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>const obj1 = {a : 1}; const obj2 = {b : 2}; function foo(p) { p = obj2; // Ignores actual parameter p.b = arguments[1]; } foo(obj1, 3); // Does not affect obj1 at all. 3 is additional parameter console.log(`${obj1.a} ${obj2.b}`); // writes 1 3 </pre></div> Functions can be declared inside other functions, and access the outer function's local variables. Furthermore, they implement full <a href="/wiki/Closure_(computer_science)" class="mw-redirect" title="Closure (computer science)">closures</a> by remembering the outer function's local variables even after the outer function has exited. <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>let t = "Top"; let bar, baz; function foo() { let f = "foo var"; bar = function() { console.log(f) }; baz = function(x) { f = x; }; } foo(); baz("baz arg"); bar(); // "baz arg" (not "foo var") even though foo() has exited. console.log(t); // Top </pre></div> <div class="mw-heading mw-heading3"><h3 id="Async/await">Async/await</h3>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=58" title="Edit section: Async/await">edit</a>]</div> <div class="excerpt-block"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1066933788"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1236090951"><div role="note" class="hatnote navigation-not-searchable dablink excerpt-hat selfref">This section is an excerpt from <a href="/wiki/Async/await#In_JavaScript" title="Async/await">Async/await § In JavaScript</a>.[<a class="external text" href="https://en.wikipedia.org/w/index.php?title=Async/await&action=edit">edit</a>]</div><div class="excerpt"> The await operator in JavaScript can only be used from inside an async function or at the top level of a <a href="/wiki/Modular_programming" title="Modular programming">module</a>. If the parameter is a <a href="/wiki/Futures_and_promises" title="Futures and promises">promise</a>, execution of the async function will resume when the promise is resolved (unless the promise is rejected, in which case an error will be thrown that can be handled with normal JavaScript <a href="/wiki/Exception_handling" title="Exception handling">exception handling</a>). If the parameter is not a promise, the parameter itself will be returned immediately.<a href="#cite_note-16">[16]</a> Many libraries provide promise objects that can also be used with await, as long as they match the specification for native JavaScript promises. However, promises from the <a href="/wiki/JQuery" title="JQuery">jQuery</a> library were not Promises/A+ compatible until jQuery 3.0.<a href="#cite_note-17">[17]</a> Here's an example (modified from this<a href="#cite_note-Async/await_JavaScript-async-await-18">[18]</a> article): <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>async function createNewDoc() { let response = await db.post({}); // post a new doc return db.get(response.id); // find by id } async function main() { try { let doc = await createNewDoc(); console.log(doc); } catch (err) { console.log(err); } } main(); </pre></div> <a href="/wiki/Node.js" title="Node.js">Node.js</a> version 8 includes a utility that enables using the standard library callback-based methods as promises.<a href="#cite_note-19">[19]</a></div></div> <div class="mw-heading mw-heading2"><h2 id="Objects">Objects</h2>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=59" title="Edit section: Objects">edit</a>]</div> For convenience, types are normally subdivided into primitives and objects. Objects are entities that have an identity (they are only equal to themselves) and that map property names to values ("slots" in <a href="/wiki/Prototype-based_programming" title="Prototype-based programming">prototype-based programming</a> terminology). Objects may be thought of as <a href="/wiki/Associative_arrays" class="mw-redirect" title="Associative arrays">associative arrays</a> or hashes, and are often implemented using these data structures. However, objects have additional features, such as a <a href="/wiki/Prototype-based_programming" title="Prototype-based programming">prototype chain</a>, which ordinary associative arrays do not have. JavaScript has several kinds of built-in objects, namely <code>Array</code>, <code>Boolean</code>, <code>Date</code>, <code>Function</code>, <code>Math</code>, <code>Number</code>, <code>Object</code>, <code>RegExp</code> and <code>String</code>. Other objects are "host objects", defined not by the language, but by the runtime environment. For example, in a browser, typical host objects belong to the DOM (window, form, links, etc.). <div class="mw-heading mw-heading3"><h3 id="Creating_objects">Creating objects</h3>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=60" title="Edit section: Creating objects">edit</a>]</div> Objects can be created using a constructor or an object literal. The constructor can use either a built-in Object function or a custom function. It is a convention that constructor functions are given a name that starts with a capital letter: <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>// Constructor const anObject = new Object(); // Object literal const objectA = {}; const objectA2 = {}; // A != A2, {}s create new objects as copies. const objectB = {index1: 'value 1', index2: 'value 2'}; // Custom constructor (see below) </pre></div> Object literals and array literals allow one to easily create flexible data structures: <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>const myStructure = { name: { first: "Mel", last: "Smith" }, age: 33, hobbies: ["chess", "jogging"] }; </pre></div> This is the basis for <a href="/wiki/JSON" title="JSON">JSON</a>, which is a simple notation that uses JavaScript-like syntax for data exchange. <div class="mw-heading mw-heading3"><h3 id="Methods">Methods</h3>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=61" title="Edit section: Methods">edit</a>]</div> A <a href="/wiki/Method_(computer_programming)" title="Method (computer programming)">method</a> is simply a function that has been assigned to a property name of an object. Unlike many object-oriented languages, there is no distinction between a function definition and a method definition in object-related JavaScript. Rather, the distinction occurs during function calling; a function can be called as a method. When called as a method, the standard local variable <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">this is just automatically set to the object instance to the left of the "<link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">.". (There are also <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">call and <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">apply methods that can set <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">this explicitly—some packages such as <a href="/wiki/JQuery" title="JQuery">jQuery</a> do unusual things with <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">this.) In the example below, Foo is being used as a constructor. There is nothing special about a constructor - it is just a plain function that initialises an object. When used with the <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">new keyword, as is the norm, <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">this is set to a newly created blank object. Note that in the example below, Foo is simply assigning values to slots, some of which are functions. Thus it can assign different functions to different instances. There is no prototyping in this example. <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>function px() { return this.prefix + "X"; } function Foo(yz) { this.prefix = "a-"; if (yz > 0) { this.pyz = function() { return this.prefix + "Y"; }; } else { this.pyz = function() { return this.prefix + "Z"; }; } this.m1 = px; return this; } const foo1 = new Foo(1); const foo2 = new Foo(0); foo2.prefix = "b-"; console.log("foo1/2 " + foo1.pyz() + foo2.pyz()); // foo1/2 a-Y b-Z foo1.m3 = px; // Assigns the function itself, not its evaluated result, i.e. not px() const baz = {"prefix": "c-"}; baz.m4 = px; // No need for a constructor to make an object. console.log("m1/m3/m4 " + foo1.m1() + foo1.m3() + baz.m4()); // m1/m3/m4 a-X a-X c-X foo1.m2(); // Throws an exception, because foo1.m2 doesn't exist. </pre></div> <div class="mw-heading mw-heading3"><h3 id="Constructors">Constructors</h3>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=62" title="Edit section: Constructors">edit</a>]</div> <a href="/wiki/Constructor_function" class="mw-redirect" title="Constructor function">Constructor functions</a> simply assign values to slots of a newly created object. The values may be data or other functions. Example: Manipulating an object: <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>function MyObject(attributeA, attributeB) { this.attributeA = attributeA; this.attributeB = attributeB; } MyObject.staticC = "blue"; // On MyObject Function, not object console.log(MyObject.staticC); // blue const object = new MyObject('red', 1000); console.log(object.attributeA); // red console.log(object.attributeB); // 1000 console.log(object.staticC); // undefined object.attributeC = new Date(); // add a new property delete object.attributeB; // remove a property of object console.log(object.attributeB); // undefined </pre></div> The constructor itself is referenced in the object's prototype's constructor slot. So, <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>function Foo() {} // Use of 'new' sets prototype slots (for example, // x = new Foo() would set x's prototype to Foo.prototype, // and Foo.prototype has a constructor slot pointing back to Foo). const x = new Foo(); // The above is almost equivalent to const y = {}; y.constructor = Foo; y.constructor(); // Except x.constructor == y.constructor; // true x instanceof Foo; // true y instanceof Foo; // false // y's prototype is Object.prototype, not // Foo.prototype, since it was initialised with // {} instead of new Foo. // Even though Foo is set to y's constructor slot, // this is ignored by instanceof - only y's prototype's // constructor slot is considered. </pre></div> Functions are objects themselves, which can be used to produce an effect similar to "static properties" (using C++/Java terminology) as shown below. (The function object also has a special <code>prototype</code> property, as discussed in the "Inheritance" section below.) Object deletion is rarely used as the scripting engine will <a href="/wiki/Garbage_collection_(computer_science)" title="Garbage collection (computer science)">garbage collect</a> objects that are no longer being referenced. <div class="mw-heading mw-heading3"><h3 id="Inheritance">Inheritance</h3>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=63" title="Edit section: Inheritance">edit</a>]</div> JavaScript supports inheritance hierarchies through prototyping in the manner of <a href="/wiki/Self_(programming_language)" title="Self (programming language)">Self</a>. In the following example, the <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">Derived class inherits from the <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">Base class. When <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">d is created as <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">Derived, the reference to the base instance of <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">Base is copied to <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">d.base. Derive does not contain a value for <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">aBaseFunction, so it is retrieved from <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">aBaseFunction when <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">aBaseFunction is accessed. This is made clear by changing the value of <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">base.aBaseFunction, which is reflected in the value of <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">d.aBaseFunction. Some implementations allow the prototype to be accessed or set explicitly using the <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">__proto__ slot as shown below. <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>function Base() { this.anOverride = function() { console.log("Base::anOverride()"); }; this.aBaseFunction = function() { console.log("Base::aBaseFunction()"); }; } function Derived() { this.anOverride = function() { console.log("Derived::anOverride()"); }; } const base = new Base(); Derived.prototype = base; // Must be before new Derived() Derived.prototype.constructor = Derived; // Required to make `instanceof` work const d = new Derived(); // Copies Derived.prototype to d instance's hidden prototype slot. d instanceof Derived; // true d instanceof Base; // true base.aBaseFunction = function() { console.log("Base::aNEWBaseFunction()"); }; d.anOverride(); // Derived::anOverride() d.aBaseFunction(); // Base::aNEWBaseFunction() console.log(d.aBaseFunction == Derived.prototype.aBaseFunction); // true console.log(d.__proto__ == base); // true in Mozilla-based implementations and false in many others. </pre></div> The following shows clearly how references to prototypes are copied on instance creation, but that changes to a prototype can affect all instances that refer to it. <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>function m1() { return "One"; } function m2() { return "Two"; } function m3() { return "Three"; } function Base() {} Base.prototype.m = m2; const bar = new Base(); console.log("bar.m " + bar.m()); // bar.m Two function Top() { this.m = m3; } const t = new Top(); const foo = new Base(); Base.prototype = t; // No effect on foo, the *reference* to t is copied. console.log("foo.m " + foo.m()); // foo.m Two const baz = new Base(); console.log("baz.m " + baz.m()); // baz.m Three t.m = m1; // Does affect baz, and any other derived classes. console.log("baz.m1 " + baz.m()); // baz.m1 One </pre></div> In practice many variations of these themes are used, and it can be both powerful and confusing. <div class="mw-heading mw-heading2"><h2 id="Exception_handling">Exception handling</h2>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=64" title="Edit section: Exception handling">edit</a>]</div> JavaScript includes a <code>try ... catch ... finally</code> <a href="/wiki/Exception_handling" title="Exception handling">exception handling</a> statement to handle run-time errors. The <code>try ... catch ... finally</code> statement catches <a href="/wiki/Exception_handling" title="Exception handling">exceptions</a> resulting from an error or a throw statement. Its syntax is as follows: <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>try { // Statements in which exceptions might be thrown } catch(errorValue) { // Statements that execute in the event of an exception } finally { // Statements that execute afterward either way } </pre></div> Initially, the statements within the try block execute. If an exception is thrown, the script's control flow immediately transfers to the statements in the catch block, with the exception available as the error argument. Otherwise the catch block is skipped. The catch block can <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">throw(errorValue), if it does not want to handle a specific error. In any case the statements in the finally block are always executed. This can be used to free resources, although memory is automatically garbage collected. Either the catch or the finally clause may be omitted. The catch argument is required. The Mozilla implementation allows for multiple catch statements, as an extension to the ECMAScript standard. They follow a syntax similar to that used in <a href="/wiki/Java_(programming_language)" title="Java (programming language)">Java</a>: <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>try { statement; } catch (e if e == "InvalidNameException") { statement; } catch (e if e == "InvalidIdException") { statement; } catch (e if e == "InvalidEmailException") { statement; } catch (e) { statement; } </pre></div> In a browser, the <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886049734">onerror event is more commonly used to trap exceptions. <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>onerror = function (errorValue, url, lineNr) {...; return true;}; </pre></div> <div class="mw-heading mw-heading2"><h2 id="Native_functions_and_methods">Native functions and methods</h2>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=65" title="Edit section: Native functions and methods">edit</a>]</div> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1236090951">(Not related to Web browsers.) <div class="mw-heading mw-heading3"><h3 id="eval_(expression)">eval (expression)</h3>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=66" title="Edit section: eval (expression)">edit</a>]</div> Evaluates the first parameter as an expression, which can include assignment statements. Variables local to functions can be referenced by the expression. However, <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">eval</code> represents a major security risk, as it allows a bad actor to execute arbitrary code, so its use is discouraged.<a href="#cite_note-deve_eval-20">[20]</a> <div class="mw-highlight mw-highlight-lang-nodejsrepl mw-content-ltr" dir="ltr"><pre>> (function foo() { ... var x = 7; ... console.log("val " + eval("x + 2")); ... })(); val 9 undefined </pre></div> <div class="mw-heading mw-heading2"><h2 id="See_also">See also</h2>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=67" title="Edit section: See also">edit</a>]</div> <ul><li><a href="/wiki/Comparison_of_JavaScript-based_source_code_editors" title="Comparison of JavaScript-based source code editors">Comparison of JavaScript-based source code editors</a></li> <li><a href="/wiki/JavaScript" title="JavaScript">JavaScript</a></li></ul> <div class="mw-heading mw-heading2"><h2 id="References">References</h2>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=68" title="Edit section: References">edit</a>]</div> <style data-mw-deduplicate="TemplateStyles:r1239543626">.mw-parser-output .reflist{margin-bottom:0.5em;list-style-type:decimal}@media screen{.mw-parser-output .reflist{font-size:90%}}.mw-parser-output .reflist .references{font-size:100%;margin-bottom:0;list-style-type:inherit}.mw-parser-output .reflist-columns-2{column-width:30em}.mw-parser-output .reflist-columns-3{column-width:25em}.mw-parser-output .reflist-columns{margin-top:0.3em}.mw-parser-output .reflist-columns ol{margin-top:0}.mw-parser-output .reflist-columns li{page-break-inside:avoid;break-inside:avoid-column}.mw-parser-output .reflist-upper-alpha{list-style-type:upper-alpha}.mw-parser-output .reflist-upper-roman{list-style-type:upper-roman}.mw-parser-output .reflist-lower-alpha{list-style-type:lower-alpha}.mw-parser-output .reflist-lower-greek{list-style-type:lower-greek}.mw-parser-output .reflist-lower-roman{list-style-type:lower-roman}</style><div class="reflist"> <div class="mw-references-wrap mw-references-columns"><ol class="references"> <li id="cite_note-1"><a href="#cite_ref-1">^</a> <a rel="nofollow" class="external text" href="http://hepunx.rl.ac.uk/~adye/jsspec11/intro.htm#1006028">JavaScript 1.1 specification</a> </li> <li id="cite_note-2"><a href="#cite_ref-2">^</a> <style data-mw-deduplicate="TemplateStyles:r1238218222">.mw-parser-output cite.citation{font-style:inherit;word-wrap:break-word}.mw-parser-output .citation q{quotes:"\"""\"""'""'"}.mw-parser-output .citation:target{background-color:rgba(0,127,255,0.133)}.mw-parser-output .id-lock-free.id-lock-free a{background:url("//upload.wikimedia.org/wikipedia/commons/6/65/Lock-green.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-limited.id-lock-limited a,.mw-parser-output .id-lock-registration.id-lock-registration a{background:url("//upload.wikimedia.org/wikipedia/commons/d/d6/Lock-gray-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-subscription.id-lock-subscription a{background:url("//upload.wikimedia.org/wikipedia/commons/a/aa/Lock-red-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .cs1-ws-icon a{background:url("//upload.wikimedia.org/wikipedia/commons/4/4c/Wikisource-logo.svg")right 0.1em center/12px no-repeat}body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-free a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-limited a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-registration a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-subscription a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .cs1-ws-icon a{background-size:contain;padding:0 1em 0 0}.mw-parser-output .cs1-code{color:inherit;background:inherit;border:none;padding:inherit}.mw-parser-output .cs1-hidden-error{display:none;color:var(--color-error,#d33)}.mw-parser-output .cs1-visible-error{color:var(--color-error,#d33)}.mw-parser-output .cs1-maint{display:none;color:#085;margin-left:0.3em}.mw-parser-output .cs1-kern-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right{padding-right:0.2em}.mw-parser-output .citation .mw-selflink{font-weight:inherit}@media screen{.mw-parser-output .cs1-format{font-size:95%}html.skin-theme-clientpref-night .mw-parser-output .cs1-maint{color:#18911f}}@media screen and (prefers-color-scheme:dark){html.skin-theme-clientpref-os .mw-parser-output .cs1-maint{color:#18911f}}</style><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://web.archive.org/web/20220210041253/http://speakingjs.com/es5/ch01.html">"Chapter 1. Basic JavaScript"</a>. speakingjs.com. Archived from <a rel="nofollow" class="external text" href="http://speakingjs.com/es5/ch01.html">the original</a> on 10 February 2022. Retrieved 22 September 2020.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=unknown&rft.jtitle=speakingjs.com&rft.atitle=Chapter+1.+Basic+JavaScript&rft_id=http%3A%2F%2Fspeakingjs.com%2Fes5%2Fch01.html&rfr_id=info%3Asid%2Fen.wikipedia.org%3AJavaScript+syntax" class="Z3988"> </li> <li id="cite_note-3"><a href="#cite_ref-3">^</a> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFFlanagan2006" class="citation book cs1">Flanagan, David (2006). <a rel="nofollow" class="external text" href="https://archive.org/details/javascript00libg_297">JavaScript: The definitive Guide</a>. p. <a rel="nofollow" class="external text" href="https://archive.org/details/javascript00libg_297/page/n14">16</a>. <a href="/wiki/ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <a href="/wiki/Special:BookSources/978-0-596-10199-2" title="Special:BookSources/978-0-596-10199-2"><bdi>978-0-596-10199-2</bdi></a>. <q>Omitting semicolons is not a good programming practice; you should get into the habit of inserting them.</q></cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=book&rft.btitle=JavaScript%3A+The+definitive+Guide&rft.pages=16&rft.date=2006&rft.isbn=978-0-596-10199-2&rft.aulast=Flanagan&rft.aufirst=David&rft_id=https%3A%2F%2Farchive.org%2Fdetails%2Fjavascript00libg_297&rfr_id=info%3Asid%2Fen.wikipedia.org%3AJavaScript+syntax" class="Z3988"> </li> <li id="cite_note-4"><a href="#cite_ref-4">^</a> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://developer.mozilla.org/en-US/docs/Learn/JavaScript/First_steps/Variables">"Storing the information you need — Variables - Learn web development | MDN"</a>. developer.mozilla.org. 9 May 2023. Retrieved 23 June 2023.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=unknown&rft.jtitle=developer.mozilla.org&rft.atitle=Storing+the+information+you+need+%E2%80%94+Variables+-+Learn+web+development+%7C+MDN&rft.date=2023-05-09&rft_id=https%3A%2F%2Fdeveloper.mozilla.org%2Fen-US%2Fdocs%2FLearn%2FJavaScript%2FFirst_steps%2FVariables&rfr_id=info%3Asid%2Fen.wikipedia.org%3AJavaScript+syntax" class="Z3988"> </li> <li id="cite_note-5"><a href="#cite_ref-5">^</a> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://web.archive.org/web/20110629131728/https://developer.mozilla.org/en/JavaScript/Guide/Values%2C_Variables%2C_and_Literals%26revision%3D22#Variables">"Values, Variables, and Literals - MDC"</a>. Mozilla Developer Network. 16 September 2010. Archived from <a rel="nofollow" class="external text" href="https://developer.mozilla.org/en/JavaScript/Guide/Values,_Variables,_and_Literals&revision=22#Variables">the original</a> on 29 June 2011. Retrieved 1 February 2020.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=Values%2C+Variables%2C+and+Literals+-+MDC&rft.pub=Mozilla+Developer+Network&rft.date=2010-09-16&rft_id=https%3A%2F%2Fdeveloper.mozilla.org%2Fen%2FJavaScript%2FGuide%2FValues%2C_Variables%2C_and_Literals%26revision%3D22%23Variables&rfr_id=info%3Asid%2Fen.wikipedia.org%3AJavaScript+syntax" class="Z3988"> </li> <li id="cite_note-6"><a href="#cite_ref-6">^</a> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://www.w3schools.com/js/js_hoisting.asp">"JavaScript Hoisting"</a>. <a href="/wiki/W3Schools" title="W3Schools">W3Schools</a>. <q>In JavaScript, a variable can be declared after it has been used. In other words; a variable can be used before it has been declared.</q></cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=JavaScript+Hoisting&rft.pub=W3Schools&rft_id=https%3A%2F%2Fwww.w3schools.com%2Fjs%2Fjs_hoisting.asp&rfr_id=info%3Asid%2Fen.wikipedia.org%3AJavaScript+syntax" class="Z3988"> </li> <li id="cite_note-7"><a href="#cite_ref-7">^</a> "<a rel="nofollow" class="external text" href="http://www.adequatelygood.com/JavaScript-Scoping-and-Hoisting.html">JavaScript Scoping and Hoisting</a>", <a rel="nofollow" class="external text" href="http://www.adequatelygood.com/about.html">Ben Cherry</a>, <a rel="nofollow" class="external text" href="http://www.adequatelygood.com/">Adequately Good</a>, 2010-02-08 </li> <li id="cite_note-8"><a href="#cite_ref-8">^</a> ECMA-262 5e edition clarified this behavior with the Declarative Environment Record and Object Environment Record. With this formalism, the global object is the Object Environment Record of the global Lexical Environment (the global scope). </li> <li id="cite_note-9"><a href="#cite_ref-9">^</a> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Template_literals">"Template literals"</a>. MDN Web Docs. 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Retrieved 5 March 2011.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=Comparison+Operators+-+MDC+Doc+Center&rft.pub=Mozilla&rft.date=2010-08-05&rft_id=https%3A%2F%2Fdeveloper.mozilla.org%2Fen%2FJavaScript%2FReference%2FOperators%2FComparison_Operators&rfr_id=info%3Asid%2Fen.wikipedia.org%3AJavaScript+syntax" class="Z3988"> </li> <li id="cite_note-11"><a href="#cite_ref-11">^</a> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="http://javascript.crockford.com/style2.html">"The Elements of JavaScript Style"</a>. Douglas Crockford. Retrieved 5 March 2011.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=The+Elements+of+JavaScript+Style&rft.pub=Douglas+Crockford&rft_id=http%3A%2F%2Fjavascript.crockford.com%2Fstyle2.html&rfr_id=info%3Asid%2Fen.wikipedia.org%3AJavaScript+syntax" class="Z3988"> </li> <li id="cite_note-12"><a href="#cite_ref-12">^</a> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Spread_syntax">"Spread syntax (...) - JavaScript | MDN"</a>. developer.mozilla.org. 25 September 2023.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=unknown&rft.jtitle=developer.mozilla.org&rft.atitle=Spread+syntax+%28...%29+-+JavaScript+%26%23124%3B+MDN&rft.date=2023-09-25&rft_id=https%3A%2F%2Fdeveloper.mozilla.org%2Fen-US%2Fdocs%2FWeb%2FJavaScript%2FReference%2FOperators%2FSpread_syntax&rfr_id=info%3Asid%2Fen.wikipedia.org%3AJavaScript+syntax" class="Z3988"> </li> <li id="cite_note-13"><a href="#cite_ref-13">^</a> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/rest_parameters">"rest parameters"</a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=rest+parameters&rft_id=https%3A%2F%2Fdeveloper.mozilla.org%2Fen-US%2Fdocs%2FWeb%2FJavaScript%2FReference%2FFunctions%2Frest_parameters&rfr_id=info%3Asid%2Fen.wikipedia.org%3AJavaScript+syntax" class="Z3988"> </li> <li id="cite_note-14"><a href="#cite_ref-14">^</a> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="http://www.ecma-international.org/ecma-262/11.0/index.html">"ECMAScript 2020 Language Specification"</a>. <a href="/wiki/Ecma_International" title="Ecma International">Ecma International</a>. June 2020.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=ECMAScript+2020+Language+Specification&rft.pub=Ecma+International&rft.date=2020-06&rft_id=http%3A%2F%2Fwww.ecma-international.org%2Fecma-262%2F11.0%2Findex.html&rfr_id=info%3Asid%2Fen.wikipedia.org%3AJavaScript+syntax" class="Z3988"> </li> <li id="cite_note-15"><a href="#cite_ref-15">^</a> ECMA-262, Edition 3, 7.5.3 Future Reserved Words </li> <li id="cite_note-16"><a href="#cite_ref-16">^</a> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/await">"await - JavaScript (MDN)"</a>. Retrieved 2 May 2017.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=await+-+JavaScript+%28MDN%29&rft_id=https%3A%2F%2Fdeveloper.mozilla.org%2Fen-US%2Fdocs%2FWeb%2FJavaScript%2FReference%2FOperators%2Fawait&rfr_id=info%3Asid%2Fen.wikipedia.org%3AJavaScript+syntax" class="Z3988"> </li> <li id="cite_note-17"><a href="#cite_ref-17">^</a> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://jquery.com/upgrade-guide/3.0/#breaking-change-and-feature-jquery-deferred-is-now-promises-a-compatible">"jQuery Core 3.0 Upgrade Guide"</a>. Retrieved 2 May 2017.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=jQuery+Core+3.0+Upgrade+Guide&rft_id=https%3A%2F%2Fjquery.com%2Fupgrade-guide%2F3.0%2F%23breaking-change-and-feature-jquery-deferred-is-now-promises-a-compatible&rfr_id=info%3Asid%2Fen.wikipedia.org%3AJavaScript+syntax" class="Z3988"> </li> <li id="cite_note-Async/await_JavaScript-async-await-18"><a href="#cite_ref-Async/await_JavaScript-async-await_18-0">^</a> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="http://pouchdb.com/2015/03/05/taming-the-async-beast-with-es7.html">"Taming the asynchronous beast with ES7"</a>. Retrieved 12 November 2015.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=Taming+the+asynchronous+beast+with+ES7&rft_id=http%3A%2F%2Fpouchdb.com%2F2015%2F03%2F05%2Ftaming-the-async-beast-with-es7.html&rfr_id=info%3Asid%2Fen.wikipedia.org%3AJavaScript+syntax" class="Z3988"> </li> <li id="cite_note-19"><a href="#cite_ref-19">^</a> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFFoundation2017" class="citation web cs1">Foundation, Node.js (30 May 2017). <a rel="nofollow" class="external text" href="https://nodejs.org/en/blog/release/v8.0.0/#improved-support-for-promises">"Node v8.0.0 (Current) - Node.js"</a>. Node.js.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=unknown&rft.jtitle=Node.js&rft.atitle=Node+v8.0.0+%28Current%29+-+Node.js&rft.date=2017-05-30&rft.aulast=Foundation&rft.aufirst=Node.js&rft_id=https%3A%2F%2Fnodejs.org%2Fen%2Fblog%2Frelease%2Fv8.0.0%2F%23improved-support-for-promises&rfr_id=info%3Asid%2Fen.wikipedia.org%3AJavaScript+syntax" class="Z3988"> </li> <li id="cite_note-deve_eval-20"><a href="#cite_ref-deve_eval_20-0">^</a> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/eval#Never_use_eval!">"eval()"</a>. MDN Web Docs. Retrieved 29 January 2020.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=unknown&rft.jtitle=MDN+Web+Docs&rft.atitle=eval%28%29&rft_id=https%3A%2F%2Fdeveloper.mozilla.org%2Fen-US%2Fdocs%2FWeb%2FJavaScript%2FReference%2FGlobal_Objects%2Feval%23Never_use_eval%21&rfr_id=info%3Asid%2Fen.wikipedia.org%3AJavaScript+syntax" class="Z3988"> </li> </ol></div></div> <div class="mw-heading mw-heading2"><h2 id="Further_reading">Further reading</h2>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=69" title="Edit section: Further reading">edit</a>]</div> <ul><li>Danny Goodman: JavaScript Bible, Wiley, John & Sons, <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><a href="/wiki/ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <a href="/wiki/Special:BookSources/0-7645-3342-8" title="Special:BookSources/0-7645-3342-8">0-7645-3342-8</a>.</li> <li>David Flanagan, Paula Ferguson: JavaScript: The Definitive Guide, O'Reilly & Associates, <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><a href="/wiki/ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <a href="/wiki/Special:BookSources/0-596-10199-6" title="Special:BookSources/0-596-10199-6">0-596-10199-6</a>.</li> <li>Thomas A. Powell, Fritz Schneider: JavaScript: The Complete Reference, McGraw-Hill Companies, <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><a href="/wiki/ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <a href="/wiki/Special:BookSources/0-07-219127-9" title="Special:BookSources/0-07-219127-9">0-07-219127-9</a>.</li> <li>Axel Rauschmayer: Speaking JavaScript: An In-Depth Guide for Programmers, 460 pages, O'Reilly Media, 25 February 2014, <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><a href="/wiki/ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <a href="/wiki/Special:BookSources/978-1449365035" title="Special:BookSources/978-1449365035">978-1449365035</a>. (<a rel="nofollow" class="external text" href="http://speakingjs.com/">free online edition</a>)</li> <li>Emily Vander Veer: JavaScript For Dummies, 4th Edition, Wiley, <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><a href="/wiki/ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <a href="/wiki/Special:BookSources/0-7645-7659-3" title="Special:BookSources/0-7645-7659-3">0-7645-7659-3</a>.</li></ul> <div class="mw-heading mw-heading2"><h2 id="External_links">External links</h2>[<a href="/w/index.php?title=JavaScript_syntax&action=edit&section=70" title="Edit section: External links">edit</a>]</div> <style data-mw-deduplicate="TemplateStyles:r1235681985">.mw-parser-output .side-box{margin:4px 0;box-sizing:border-box;border:1px solid #aaa;font-size:88%;line-height:1.25em;background-color:var(--background-color-interactive-subtle,#f8f9fa);display:flow-root}.mw-parser-output .side-box-abovebelow,.mw-parser-output .side-box-text{padding:0.25em 0.9em}.mw-parser-output .side-box-image{padding:2px 0 2px 0.9em;text-align:center}.mw-parser-output .side-box-imageright{padding:2px 0.9em 2px 0;text-align:center}@media(min-width:500px){.mw-parser-output .side-box-flex{display:flex;align-items:center}.mw-parser-output .side-box-text{flex:1;min-width:0}}@media(min-width:720px){.mw-parser-output .side-box{width:238px}.mw-parser-output .side-box-right{clear:right;float:right;margin-left:1em}.mw-parser-output .side-box-left{margin-right:1em}}</style><style data-mw-deduplicate="TemplateStyles:r1237033735">@media print{body.ns-0 .mw-parser-output .sistersitebox{display:none!important}}@media screen{html.skin-theme-clientpref-night .mw-parser-output .sistersitebox img[src*="Wiktionary-logo-en-v2.svg"]{background-color:white}}@media screen and (prefers-color-scheme:dark){html.skin-theme-clientpref-os .mw-parser-output .sistersitebox img[src*="Wiktionary-logo-en-v2.svg"]{background-color:white}}</style><div class="side-box side-box-right plainlinks sistersitebox"><style data-mw-deduplicate="TemplateStyles:r1126788409">.mw-parser-output .plainlist ol,.mw-parser-output .plainlist ul{line-height:inherit;list-style:none;margin:0;padding:0}.mw-parser-output .plainlist ol li,.mw-parser-output .plainlist ul li{margin-bottom:0}</style> <div class="side-box-flex"> <div class="side-box-image"><img alt="" src="//upload.wikimedia.org/wikipedia/commons/thumb/d/df/Wikibooks-logo-en-noslogan.svg/40px-Wikibooks-logo-en-noslogan.svg.png" decoding="async" width="40" height="40" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/d/df/Wikibooks-logo-en-noslogan.svg/60px-Wikibooks-logo-en-noslogan.svg.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/d/df/Wikibooks-logo-en-noslogan.svg/80px-Wikibooks-logo-en-noslogan.svg.png 2x" data-file-width="400" data-file-height="400" /></div> <div class="side-box-text plainlist">Wikibooks has a book on the topic of: <a href="https://en.wikibooks.org/wiki/JavaScript" class="extiw" title="wikibooks:JavaScript">JavaScript</a></div></div> </div> <ul><li><a rel="nofollow" class="external text" href="https://developer.mozilla.org/en/docs/A_re-introduction_to_JavaScript">A re-introduction to JavaScript - Mozilla Developer Center</a></li> <li><a rel="nofollow" class="external text" href="https://codetopology.com/category/scripts/javascript-tutorials/">JavaScript Loops</a></li> <li>ECMAScript standard references: <a rel="nofollow" class="external text" href="http://www.ecma-international.org/publications/standards/Ecma-262.htm">ECMA-262</a></li> <li><a rel="nofollow" class="external text" href="https://web.archive.org/web/20120527095306/http://javalessons.com/cgi-bin/fun/java-tutorials-main.cgi?sub=javascript&code=script">Interactive JavaScript Lessons - example-based</a></li> <li><a rel="nofollow" class="external text" href="http://javascript.about.com/">JavaScript on About.com: lessons and explanation</a> <a rel="nofollow" class="external text" href="https://web.archive.org/web/20170225135310/http://javascript.about.com/">Archived</a> 25 February 2017 at the <a href="/wiki/Wayback_Machine" title="Wayback Machine">Wayback Machine</a></li> <li><a rel="nofollow" class="external text" href="https://web.archive.org/web/20150110202627/http://wisentechnologies.com/it-courses/JavaScript-Training-in-Chennai.aspx">JavaScript Training</a></li> <li>Mozilla Developer Center Core References for JavaScript versions <a rel="nofollow" class="external text" href="https://developer.mozilla.org/en/docs/Core_JavaScript_1.5_Reference">1.5</a>, <a rel="nofollow" class="external text" href="https://web.archive.org/web/20070210000908/http://research.nihonsoft.org/javascript/CoreReferenceJS14/index.html">1.4</a>, <a rel="nofollow" class="external text" href="https://web.archive.org/web/20070210000504/http://research.nihonsoft.org/javascript/ClientReferenceJS13/index.html">1.3</a> and <a rel="nofollow" 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editor)">Vim</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="/wiki/JavaScript_engine" title="JavaScript engine">Engines</a></th><td class="navbox-list-with-group navbox-list navbox-even hlist" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/List_of_ECMAScript_engines" title="List of ECMAScript engines">List of ECMAScript engines</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="/wiki/Software_framework" title="Software framework">Frameworks</a></th><td class="navbox-list-with-group navbox-list navbox-odd hlist" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Comparison_of_JavaScript-based_web_frameworks" title="Comparison of JavaScript-based web frameworks">Comparison of JavaScript frameworks</a></li> <li><a href="/wiki/List_of_JavaScript_libraries" title="List of JavaScript libraries">List of JavaScript libraries</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Related <a href="/wiki/Technology" title="Technology">technologies</a></th><td class="navbox-list-with-group navbox-list navbox-even hlist" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Ajax_(programming)" title="Ajax (programming)">Ajax</a></li> <li><a href="/wiki/AssemblyScript" title="AssemblyScript">AssemblyScript</a></li> <li><a href="/wiki/Asm.js" title="Asm.js">asm.js</a></li> <li><a href="/wiki/Cascading_Style_Sheets" class="mw-redirect" title="Cascading Style Sheets">Cascading Style Sheets</a></li> <li><a href="/wiki/Document_Object_Model" title="Document Object Model">Document Object Model</a></li> <li><a href="/wiki/HTML" title="HTML">HTML</a> <ul><li><a href="/wiki/HTML5" title="HTML5">HTML5</a></li></ul></li> <li><a href="/wiki/JSON" title="JSON">JSON</a></li> <li><a href="/wiki/WebAssembly" title="WebAssembly">WebAssembly</a></li> <li><a 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