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class="sidebar-filter "><label id="sidebar-filter-label" class="sidebar-filter-label" for="sidebar-filter-input"><span class="icon icon-filter"></span><span class="visually-hidden">Filter sidebar</span></label><input id="sidebar-filter-input" autoComplete="off" class="sidebar-filter-input-field false" type="text" placeholder="Filter" value=""/><button type="button" class="button action has-icon clear-sidebar-filter-button"><span class="button-wrap"><span class="icon icon-cancel "></span><span class="visually-hidden">Clear filter input</span></span></button></div></section></header><div class="sidebar-inner-nav"><div class="in-nav-toc"><div class="document-toc-container"><section class="document-toc"><header><h2 class="document-toc-heading">In this article</h2></header><ul class="document-toc-list"><li class="document-toc-item "><a class="document-toc-link" href="#invoking_strict_mode">Invoking strict mode</a></li><li class="document-toc-item "><a class="document-toc-link" href="#changes_in_strict_mode">Changes in strict mode</a></li><li class="document-toc-item "><a class="document-toc-link" href="#transitioning_to_strict_mode">Transitioning to strict mode</a></li><li class="document-toc-item "><a class="document-toc-link" href="#specifications">Specifications</a></li><li class="document-toc-item "><a class="document-toc-link" href="#see_also">See also</a></li></ul></section></div></div><div class="sidebar-body"><ol><li class="section"><a href="/en-US/docs/Web/JavaScript">JavaScript</a></li><li class="section">Tutorials and guides</li><li class="toggle"><details><summary>Beginner's tutorials</summary><ol><li><a href="/en-US/docs/Learn_web_development/Getting_started/Your_first_website/Adding_interactivity">Your first website: Adding interactivity</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Scripting">Dynamic scripting with JavaScript</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Frameworks_libraries">JavaScript frameworks and libraries</a></li></ol></details></li><li class="toggle"><details><summary>JavaScript Guide</summary><ol><li><a href="/en-US/docs/Web/JavaScript/Guide/Introduction">Introduction</a></li><li><a href="/en-US/docs/Web/JavaScript/Guide/Grammar_and_types">Grammar and types</a></li><li><a href="/en-US/docs/Web/JavaScript/Guide/Control_flow_and_error_handling">Control flow and error handling</a></li><li><a href="/en-US/docs/Web/JavaScript/Guide/Loops_and_iteration">Loops and iteration</a></li><li><a href="/en-US/docs/Web/JavaScript/Guide/Functions">Functions</a></li><li><a href="/en-US/docs/Web/JavaScript/Guide/Expressions_and_operators">Expressions and operators</a></li><li><a href="/en-US/docs/Web/JavaScript/Guide/Numbers_and_strings">Numbers and strings</a></li><li><a href="/en-US/docs/Web/JavaScript/Guide/Representing_dates_times">Representing dates & times</a></li><li><a href="/en-US/docs/Web/JavaScript/Guide/Regular_expressions">Regular expressions</a></li><li><a href="/en-US/docs/Web/JavaScript/Guide/Indexed_collections">Indexed collections</a></li><li><a href="/en-US/docs/Web/JavaScript/Guide/Keyed_collections">Keyed collections</a></li><li><a href="/en-US/docs/Web/JavaScript/Guide/Working_with_objects">Working with objects</a></li><li><a href="/en-US/docs/Web/JavaScript/Guide/Using_classes">Using classes</a></li><li><a href="/en-US/docs/Web/JavaScript/Guide/Using_promises">Using promises</a></li><li><a href="/en-US/docs/Web/JavaScript/Guide/Typed_arrays">JavaScript typed arrays</a></li><li><a href="/en-US/docs/Web/JavaScript/Guide/Iterators_and_generators">Iterators and generators</a></li><li><a href="/en-US/docs/Web/JavaScript/Guide/Internationalization">Internationalization</a></li><li><a href="/en-US/docs/Web/JavaScript/Guide/Meta_programming">Meta programming</a></li><li><a href="/en-US/docs/Web/JavaScript/Guide/Modules">JavaScript modules</a></li></ol></details></li><li class="toggle"><details><summary>Intermediate</summary><ol><li><a href="/en-US/docs/Learn_web_development/Extensions/Advanced_JavaScript_objects">Advanced JavaScript objects</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Async_JS">Asynchronous JavaScript</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Client-side_APIs">Client-side web APIs</a></li><li><a href="/en-US/docs/Web/JavaScript/Language_overview">Language overview</a></li><li><a href="/en-US/docs/Web/JavaScript/Data_structures">JavaScript data structures</a></li><li><a href="/en-US/docs/Web/JavaScript/Equality_comparisons_and_sameness">Equality comparisons and sameness</a></li><li><a href="/en-US/docs/Web/JavaScript/Enumerability_and_ownership_of_properties">Enumerability and ownership of properties</a></li><li><a href="/en-US/docs/Web/JavaScript/Closures">Closures</a></li></ol></details></li><li class="toggle"><details><summary>Advanced</summary><ol><li><a href="/en-US/docs/Web/JavaScript/Inheritance_and_the_prototype_chain">Inheritance and the prototype chain</a></li><li><a href="/en-US/docs/Web/JavaScript/Memory_management">Memory Management</a></li><li><a href="/en-US/docs/Web/JavaScript/Event_loop">Concurrency model and Event Loop</a></li></ol></details></li><li class="section"><a href="/en-US/docs/Web/JavaScript/Reference">References</a></li><li class="toggle"><details><summary>Built-in objects</summary><ol><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects">Overview</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/AggregateError">AggregateError</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array">Array</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/ArrayBuffer">ArrayBuffer</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/AsyncFunction">AsyncFunction</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/AsyncGenerator">AsyncGenerator</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/AsyncGeneratorFunction">AsyncGeneratorFunction</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/AsyncIterator">AsyncIterator</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Atomics">Atomics</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/BigInt">BigInt</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/BigInt64Array">BigInt64Array</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/BigUint64Array">BigUint64Array</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Boolean">Boolean</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/DataView">DataView</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date">Date</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/decodeURI">decodeURI()</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/decodeURIComponent">decodeURIComponent()</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/encodeURI">encodeURI()</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/encodeURIComponent">encodeURIComponent()</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error">Error</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/escape">escape()</a><abbr class="icon icon-deprecated" title="Deprecated. Not for use in new websites."> <span class="visually-hidden">Deprecated</span> </abbr></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/eval">eval()</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/EvalError">EvalError</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/FinalizationRegistry">FinalizationRegistry</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Float16Array">Float16Array</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Float32Array">Float32Array</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Float64Array">Float64Array</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function">Function</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Generator">Generator</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/GeneratorFunction">GeneratorFunction</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/globalThis">globalThis</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Infinity">Infinity</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Int16Array">Int16Array</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Int32Array">Int32Array</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Int8Array">Int8Array</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/InternalError">InternalError</a><abbr class="icon icon-nonstandard" title="Non-standard. Check cross-browser support before using."> <span class="visually-hidden">Non-standard</span> </abbr></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl">Intl</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/isFinite">isFinite()</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/isNaN">isNaN()</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Iterator">Iterator</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/JSON">JSON</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map">Map</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math">Math</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/NaN">NaN</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number">Number</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object">Object</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/parseFloat">parseFloat()</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/parseInt">parseInt()</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise">Promise</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy">Proxy</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/RangeError">RangeError</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/ReferenceError">ReferenceError</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Reflect">Reflect</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp">RegExp</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set">Set</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/SharedArrayBuffer">SharedArrayBuffer</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/String">String</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol">Symbol</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/SyntaxError">SyntaxError</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Temporal">Temporal</a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray">TypedArray</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypeError">TypeError</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Uint16Array">Uint16Array</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Uint32Array">Uint32Array</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Uint8Array">Uint8Array</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Uint8ClampedArray">Uint8ClampedArray</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/undefined">undefined</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/unescape">unescape()</a><abbr class="icon icon-deprecated" title="Deprecated. Not for use in new websites."> <span class="visually-hidden">Deprecated</span> </abbr></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/URIError">URIError</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/WeakMap">WeakMap</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/WeakRef">WeakRef</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/WeakSet">WeakSet</a></li></ol></details></li><li class="toggle"><details><summary>Expressions & operators</summary><ol><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators">Overview</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Addition">Addition (+)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Addition_assignment">Addition assignment (+=)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Assignment">Assignment (=)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/async_function">async function expression</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/async_function*">async function* expression</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/await">await</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Bitwise_AND">Bitwise AND (&)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Bitwise_AND_assignment">Bitwise AND assignment (&=)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Bitwise_NOT">Bitwise NOT (~)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Bitwise_OR">Bitwise OR (|)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Bitwise_OR_assignment">Bitwise OR assignment (|=)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Bitwise_XOR">Bitwise XOR (^)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Bitwise_XOR_assignment">Bitwise XOR assignment (^=)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/class">class expression</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Comma_operator">Comma operator (,)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Conditional_operator">Conditional (ternary) operator</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Decrement">Decrement (--)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/delete">delete</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Destructuring_assignment">Destructuring assignment</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Division">Division (/)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Division_assignment">Division assignment (/=)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Equality">Equality (==)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Exponentiation">Exponentiation (**)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Exponentiation_assignment">Exponentiation assignment (**=)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/function">function expression</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/function*">function* expression</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Greater_than">Greater than (>)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Greater_than_or_equal">Greater than or equal (>=)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Grouping">Grouping operator ( )</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/import.meta">import.meta</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/import">import()</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/in">in</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Increment">Increment (++)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Inequality">Inequality (!=)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/instanceof">instanceof</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Left_shift">Left shift (<<)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Left_shift_assignment">Left shift assignment (<<=)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Less_than">Less than (<)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Less_than_or_equal">Less than or equal (<=)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Logical_AND">Logical AND (&&)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Logical_AND_assignment">Logical AND assignment (&&=)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Logical_NOT">Logical NOT (!)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Logical_OR">Logical OR (||)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Logical_OR_assignment">Logical OR assignment (||=)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Multiplication">Multiplication (*)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Multiplication_assignment">Multiplication assignment (*=)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/new">new</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/new.target">new.target</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/null">null</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Nullish_coalescing_assignment">Nullish coalescing assignment (??=)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Nullish_coalescing">Nullish coalescing operator (??)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Object_initializer">Object initializer</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Operator_precedence">Operator precedence</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Optional_chaining">Optional chaining (?.)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Property_accessors">Property accessors</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Remainder">Remainder (%)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Remainder_assignment">Remainder assignment (%=)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Right_shift">Right shift (>>)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Right_shift_assignment">Right shift assignment (>>=)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Spread_syntax">Spread syntax (...)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Strict_equality">Strict equality (===)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Strict_inequality">Strict inequality (!==)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Subtraction">Subtraction (-)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Subtraction_assignment">Subtraction assignment (-=)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/super">super</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/this">this</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/typeof">typeof</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Unary_negation">Unary negation (-)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Unary_plus">Unary plus (+)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Unsigned_right_shift">Unsigned right shift (>>>)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Unsigned_right_shift_assignment">Unsigned right shift assignment (>>>=)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/void">void operator</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/yield">yield</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/yield*">yield*</a></li></ol></details></li><li class="toggle"><details><summary>Statements & declarations</summary><ol><li><a href="/en-US/docs/Web/JavaScript/Reference/Statements">Overview</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Statements/async_function">async function</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Statements/async_function*">async function*</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Statements/block">Block statement</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Statements/break">break</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Statements/class">class</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Statements/const">const</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Statements/continue">continue</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Statements/debugger">debugger</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Statements/do...while">do...while</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Statements/Empty">Empty statement</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Statements/export">export</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Statements/Expression_statement">Expression statement</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Statements/for">for</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Statements/for-await...of">for await...of</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Statements/for...in">for...in</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Statements/for...of">for...of</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Statements/function">function</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Statements/function*">function*</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Statements/if...else">if...else</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Statements/import">import</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Statements/label">Labeled statement</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Statements/let">let</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Statements/return">return</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Statements/switch">switch</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Statements/throw">throw</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Statements/try...catch">try...catch</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Statements/var">var</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Statements/while">while</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Statements/with">with</a><abbr class="icon icon-deprecated" title="Deprecated. Not for use in new websites."> <span class="visually-hidden">Deprecated</span> </abbr></li></ol></details></li><li class="toggle"><details><summary>Functions</summary><ol><li><a href="/en-US/docs/Web/JavaScript/Reference/Functions">Overview</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions">Arrow function expressions</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Functions/Default_parameters">Default parameters</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Functions/get">get</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Functions/Method_definitions">Method definitions</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Functions/rest_parameters">Rest parameters</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Functions/set">set</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Functions/arguments">The arguments object</a></li></ol></details></li><li class="toggle"><details><summary>Classes</summary><ol><li><a href="/en-US/docs/Web/JavaScript/Reference/Classes">Overview</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Classes/constructor">constructor</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Classes/extends">extends</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Classes/Private_properties">Private properties</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Classes/Public_class_fields">Public class fields</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Classes/static">static</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Classes/Static_initialization_blocks">Static initialization blocks</a></li></ol></details></li><li class="toggle"><details><summary>Regular expressions</summary><ol><li><a href="/en-US/docs/Web/JavaScript/Reference/Regular_expressions">Overview</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Backreference">Backreference: \1, \2</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Capturing_group">Capturing group: (...)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Character_class_escape">Character class escape: \d, \D, \w, \W, \s, \S</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Character_class">Character class: [...], [^...]</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Character_escape">Character escape: \n, \u{...}</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Disjunction">Disjunction: |</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Input_boundary_assertion">Input boundary assertion: ^, $</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Literal_character">Literal character: a, b</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Lookahead_assertion">Lookahead assertion: (?=...), (?!...)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Lookbehind_assertion">Lookbehind assertion: (?<=...), (?<!...)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Modifier">Modifier: (?ims-ims:...)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Named_backreference">Named backreference: \k<name></a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Named_capturing_group">Named capturing group: (?<name>...)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Non-capturing_group">Non-capturing group: (?:...)</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Quantifier">Quantifier: *, +, ?, {n}, {n,}, {n,m}</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Unicode_character_class_escape">Unicode character class escape: \p{...}, \P{...}</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Wildcard">Wildcard: .</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Word_boundary_assertion">Word boundary assertion: \b, \B</a></li></ol></details></li><li class="toggle"><details><summary>Errors</summary><ol><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors">Overview</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Promise_any_all_rejected">AggregateError: No Promise in Promise.any was resolved</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Property_access_denied">Error: Permission denied to access property "x"</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Too_much_recursion">InternalError: too much recursion</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Not_a_valid_code_point">RangeError: argument is not a valid code point</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/BigInt_division_by_zero">RangeError: BigInt division by zero</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/BigInt_negative_exponent">RangeError: BigInt negative exponent</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Form_must_be_one_of">RangeError: form must be one of 'NFC', 'NFD', 'NFKC', or 'NFKD'</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Invalid_array_length">RangeError: invalid array length</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Invalid_date">RangeError: invalid date</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Precision_range">RangeError: precision is out of range</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Bad_radix">RangeError: radix must be an integer</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Resulting_string_too_large">RangeError: repeat count must be less than infinity</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Negative_repetition_count">RangeError: repeat count must be non-negative</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Cant_be_converted_to_BigInt_because_it_isnt_an_integer">RangeError: x can't be converted to BigInt because it isn't an integer</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Not_defined">ReferenceError: "x" is not defined</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Undeclared_var">ReferenceError: assignment to undeclared variable "x"</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Cant_access_lexical_declaration_before_init">ReferenceError: can't access lexical declaration 'X' before initialization</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Deprecated_caller_or_arguments_usage">ReferenceError: deprecated caller or arguments usage</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Super_not_called">ReferenceError: must call super constructor before using 'this' in derived class constructor</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Super_called_twice">ReferenceError: super() called twice in derived class constructor</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Bad_strict_arguments_eval">SyntaxError: 'arguments'/'eval' can't be defined or assigned to in strict mode code</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Deprecated_octal_literal">SyntaxError: "0"-prefixed octal literals are deprecated</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Strict_non_simple_params">SyntaxError: "use strict" not allowed in function with non-simple parameters</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Reserved_identifier">SyntaxError: "x" is a reserved identifier</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Regex_backslash_at_end_of_pattern">SyntaxError: \ at end of pattern</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Invalid_for-of_initializer">SyntaxError: a declaration in the head of a for-of loop can't have an initializer</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Delete_in_strict_mode">SyntaxError: applying the 'delete' operator to an unqualified name is deprecated</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Arguments_not_allowed">SyntaxError: arguments is not valid in fields</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Bad_await">SyntaxError: await is only valid in async functions, async generators and modules</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/await_yield_in_parameter">SyntaxError: await/yield expression can't be used in parameter</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Cant_use_nullish_coalescing_unparenthesized">SyntaxError: cannot use `??` unparenthesized within `||` and `&&` expressions</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Regex_character_class_escape_in_class_range">SyntaxError: character class escape cannot be used in class range in regular expression</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Bad_continue">SyntaxError: continue must be inside loop</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Regex_duplicate_capture_group_name">SyntaxError: duplicate capture group name in regular expression</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Duplicate_parameter">SyntaxError: duplicate formal argument x</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Invalid_for-in_initializer">SyntaxError: for-in loop head declarations may not have initializers</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Unnamed_function_statement">SyntaxError: function statement requires a name</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Function_label">SyntaxError: functions cannot be labelled</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Either_be_both_static_or_non-static">SyntaxError: getter and setter for private name #x should either be both static or non-static</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Getter_no_arguments">SyntaxError: getter functions must have no arguments</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Identifier_after_number">SyntaxError: identifier starts immediately after numeric literal</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Illegal_character">SyntaxError: illegal character</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/import_decl_module_top_level">SyntaxError: import declarations may only appear at top level of a module</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Regex_incomplete_quantifier">SyntaxError: incomplete quantifier in regular expression</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Invalid_assignment_left-hand_side">SyntaxError: invalid assignment left-hand side</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Invalid_BigInt_syntax">SyntaxError: invalid BigInt syntax</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Regex_invalid_capture_group_name">SyntaxError: invalid capture group name in regular expression</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Regex_invalid_char_in_class">SyntaxError: invalid character in class in regular expression</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Regex_invalid_class_set_operation">SyntaxError: invalid class set operation in regular expression</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Regex_invalid_decimal_escape">SyntaxError: invalid decimal escape in regular expression</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Regex_invalid_identity_escape">SyntaxError: invalid identity escape in regular expression</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Regex_invalid_named_capture_reference">SyntaxError: invalid named capture reference in regular expression</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Regex_invalid_property_name">SyntaxError: invalid property name in regular expression</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Regex_invalid_range_in_character_class">SyntaxError: invalid range in character class</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Regex_invalid_group">SyntaxError: invalid regexp group</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Bad_regexp_flag">SyntaxError: invalid regular expression flag "x"</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Regex_invalid_unicode_escape">SyntaxError: invalid unicode escape in regular expression</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/JSON_bad_parse">SyntaxError: JSON.parse: bad parsing</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Label_not_found">SyntaxError: label not found</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Missing_colon_after_property_id">SyntaxError: missing : after property id</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Missing_parenthesis_after_argument_list">SyntaxError: missing ) after argument list</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Missing_parenthesis_after_condition">SyntaxError: missing ) after condition</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Missing_bracket_after_list">SyntaxError: missing ] after element list</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Missing_curly_after_function_body">SyntaxError: missing } after function body</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Missing_curly_after_property_list">SyntaxError: missing } after property list</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Missing_initializer_in_const">SyntaxError: missing = in const declaration</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Missing_formal_parameter">SyntaxError: missing formal parameter</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Missing_name_after_dot_operator">SyntaxError: missing name after . operator</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/No_variable_name">SyntaxError: missing variable name</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Regex_negated_char_class_with_strings">SyntaxError: negated character class with strings in regular expression</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Bad_new_optional">SyntaxError: new keyword cannot be used with an optional chain</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Regex_nothing_to_repeat">SyntaxError: nothing to repeat</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Regex_numbers_out_of_order_in_quantifier">SyntaxError: numbers out of order in {} quantifier.</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Deprecated_octal_escape_sequence">SyntaxError: octal escape sequences can't be used in untagged template literals or in strict mode code</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Parameter_after_rest_parameter">SyntaxError: parameter after rest parameter</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Cant_delete_private_fields">SyntaxError: private fields can't be deleted</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Duplicate_proto">SyntaxError: property name __proto__ appears more than once in object literal</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Regex_raw_bracket">SyntaxError: raw bracket is not allowed in regular expression with unicode flag</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Redeclared_parameter">SyntaxError: redeclaration of formal parameter "x"</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Undeclared_private_field_or_method">SyntaxError: reference to undeclared private field or method #x</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Rest_with_default">SyntaxError: rest parameter may not have a default</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Bad_return">SyntaxError: return not in function</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Setter_one_argument">SyntaxError: setter functions must have one argument</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/String_literal_EOL">SyntaxError: string literal contains an unescaped line break</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Bad_super_call">SyntaxError: super() is only valid in derived class constructors</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Bad_optional_template">SyntaxError: tagged template cannot be used with optional chain</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Hash_outside_class">SyntaxError: Unexpected '#' used outside of class body</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Unexpected_token">SyntaxError: Unexpected token</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Bad_break">SyntaxError: unlabeled break must be inside loop or switch</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Unparenthesized_unary_expr_lhs_exponentiation">SyntaxError: unparenthesized unary expression can't appear on the left-hand side of '**'</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Bad_super_prop">SyntaxError: use of super property/member accesses only valid within methods or eval code within methods</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Deprecated_source_map_pragma">SyntaxError: Using //@ to indicate sourceURL pragmas is deprecated. Use //# instead</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/is_not_iterable">TypeError: 'x' is not iterable</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Unexpected_type">TypeError: "x" is (not) "y"</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Not_a_constructor">TypeError: "x" is not a constructor</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Not_a_function">TypeError: "x" is not a function</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/No_non-null_object">TypeError: "x" is not a non-null object</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Read-only">TypeError: "x" is read-only</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/BigInt_not_serializable">TypeError: BigInt value can't be serialized in JSON</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Builtin_ctor_no_new">TypeError: calling a builtin X constructor without new is forbidden</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Get_set_missing_private">TypeError: can't access/set private field or method: object is not the right class</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Cant_assign_to_property">TypeError: can't assign to property "x" on "y": not an object</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Cant_convert_BigInt_to_number">TypeError: can't convert BigInt to number</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Cant_convert_x_to_BigInt">TypeError: can't convert x to BigInt</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Cant_define_property_object_not_extensible">TypeError: can't define property "x": "obj" is not extensible</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Non_configurable_array_element">TypeError: can't delete non-configurable array element</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Cant_redefine_property">TypeError: can't redefine non-configurable property "x"</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Cant_set_prototype">TypeError: can't set prototype of this object</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Cyclic_prototype">TypeError: can't set prototype: it would cause a prototype chain cycle</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/in_operator_no_object">TypeError: cannot use 'in' operator to search for 'x' in 'y'</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Class_ctor_no_new">TypeError: class constructors must be invoked with 'new'</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Cyclic_object_value">TypeError: cyclic object value</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Invalid_derived_return">TypeError: derived class constructor returned invalid value x</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Private_setter_only">TypeError: getting private setter-only property</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Private_double_initialization">TypeError: Initializing an object twice is an error with private fields/methods</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/invalid_right_hand_side_instanceof_operand">TypeError: invalid 'instanceof' operand 'x'</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Array_sort_argument">TypeError: invalid Array.prototype.sort argument</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Invalid_const_assignment">TypeError: invalid assignment to const "x"</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Constructor_cant_be_used_directly">TypeError: Iterator/AsyncIterator constructor can't be used directly</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Requires_global_RegExp">TypeError: matchAll/replaceAll must be called with a global RegExp</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/More_arguments_needed">TypeError: More arguments needed</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/No_properties">TypeError: null/undefined has no properties</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Cant_delete">TypeError: property "x" is non-configurable and can't be deleted</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Reduce_of_empty_array_with_no_initial_value">TypeError: Reduce of empty array with no initial value</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Getter_only">TypeError: setting getter-only property "x"</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Key_not_weakly_held">TypeError: WeakSet key/WeakMap value 'x' must be an object or an unregistered symbol</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Called_on_incompatible_type">TypeError: X.prototype.y called on incompatible type</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Malformed_URI">URIError: malformed URI sequence</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Already_has_pragma">Warning: -file- is being assigned a //# sourceMappingURL, but already has one</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Errors/Stmt_after_return">Warning: unreachable code after return statement</a></li></ol></details></li><li class="toggle"><details open=""><summary>Misc</summary><ol><li><a href="/en-US/docs/Web/JavaScript/JavaScript_technologies_overview">JavaScript technologies overview</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Lexical_grammar">Lexical grammar</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Iteration_protocols">Iteration protocols</a></li><li><em><a href="/en-US/docs/Web/JavaScript/Reference/Strict_mode" aria-current="page">Strict mode</a></em></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Template_literals">Template literals</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Trailing_commas">Trailing commas</a></li><li><a href="/en-US/docs/Web/JavaScript/Reference/Deprecated_and_obsolete_features">Deprecated features</a></li></ol></details></li></ol></div></div><section class="place side"></section></nav></aside><div class="toc-container"><aside class="toc"><nav><div class="document-toc-container"><section class="document-toc"><header><h2 class="document-toc-heading">In this article</h2></header><ul class="document-toc-list"><li class="document-toc-item "><a class="document-toc-link" href="#invoking_strict_mode">Invoking strict mode</a></li><li class="document-toc-item "><a class="document-toc-link" href="#changes_in_strict_mode">Changes in strict mode</a></li><li class="document-toc-item "><a class="document-toc-link" href="#transitioning_to_strict_mode">Transitioning to strict mode</a></li><li class="document-toc-item "><a class="document-toc-link" href="#specifications">Specifications</a></li><li class="document-toc-item "><a class="document-toc-link" href="#see_also">See also</a></li></ul></section></div></nav></aside><section class="place side"></section></div></div><main id="content" class="main-content "><article class="main-page-content" lang="en-US"><header><h1>Strict mode</h1></header><div class="section-content"><div class="notecard note"> <p><strong>Note:</strong> Sometimes you'll see the default, non-strict mode referred to as <em><a href="/en-US/docs/Glossary/Sloppy_mode">sloppy mode</a></em>. This isn't an official term, but be aware of it, just in case.</p> </div> <p>JavaScript's strict mode is a way to <em>opt in</em> to a restricted variant of JavaScript, thereby implicitly opting-out of "<a href="/en-US/docs/Glossary/Sloppy_mode">sloppy mode</a>". Strict mode isn't just a subset: it <em>intentionally</em> has different semantics from normal code. Strict mode code and non-strict mode code can coexist, so scripts can opt into strict mode incrementally.</p> <p>Strict mode makes several changes to normal JavaScript semantics:</p> <ol> <li>Eliminates some JavaScript silent errors by changing them to throw errors.</li> <li>Fixes mistakes that make it difficult for JavaScript engines to perform optimizations: strict mode code can sometimes be made to run faster than identical code that's not strict mode.</li> <li>Prohibits some syntax likely to be defined in future versions of ECMAScript.</li> </ol></div><section aria-labelledby="invoking_strict_mode"><h2 id="invoking_strict_mode"><a href="#invoking_strict_mode">Invoking strict mode</a></h2><div class="section-content"><p>Strict mode applies to <em>entire scripts</em> or to <em>individual functions</em>. It doesn't apply to <a href="/en-US/docs/Web/JavaScript/Reference/Statements/block">block statements</a> enclosed in <code>{}</code> braces; attempting to apply it to such contexts does nothing. <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/eval"><code>eval</code></a> code, <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/Function"><code>Function</code></a> code, <a href="/en-US/docs/Web/HTML/Attributes#event_handler_attributes">event handler</a> attributes, strings passed to <a href="/en-US/docs/Web/API/Window/setTimeout" title="setTimeout()"><code>setTimeout()</code></a>, and related functions are either function bodies or entire scripts, and invoking strict mode in them works as expected.</p></div></section><section aria-labelledby="strict_mode_for_scripts"><h3 id="strict_mode_for_scripts"><a href="#strict_mode_for_scripts">Strict mode for scripts</a></h3><div class="section-content"><p>To invoke strict mode for an entire script, put the <em>exact</em> statement <code>"use strict";</code> (or <code>'use strict';</code>) before any other statements.</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>// Whole-script strict mode syntax "use strict"; const v = "Hi! I'm a strict mode script!"; </code></pre></div></div></section><section aria-labelledby="strict_mode_for_functions"><h3 id="strict_mode_for_functions"><a href="#strict_mode_for_functions">Strict mode for functions</a></h3><div class="section-content"><p>Likewise, to invoke strict mode for a function, put the <em>exact</em> statement <code>"use strict";</code> (or <code>'use strict';</code>) in the function's body before any other statements.</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>function myStrictFunction() { // Function-level strict mode syntax "use strict"; function nested() { return "And so am I!"; } return `Hi! I'm a strict mode function! ${nested()}`; } function myNotStrictFunction() { return "I'm not strict."; } </code></pre></div> <p>The <code>"use strict"</code> directive can only be applied to the body of functions with simple parameters. Using <code>"use strict"</code> in functions with <a href="/en-US/docs/Web/JavaScript/Reference/Functions/rest_parameters">rest</a>, <a href="/en-US/docs/Web/JavaScript/Reference/Functions/Default_parameters">default</a>, or <a href="/en-US/docs/Web/JavaScript/Reference/Operators/Destructuring_assignment">destructured</a> parameters is a <a href="/en-US/docs/Web/JavaScript/Reference/Errors/Strict_non_simple_params">syntax error</a>.</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js example-bad notranslate"><code>function sum(a = 1, b = 2) { // SyntaxError: "use strict" not allowed in function with default parameter "use strict"; return a + b; } </code></pre></div></div></section><section aria-labelledby="strict_mode_for_modules"><h3 id="strict_mode_for_modules"><a href="#strict_mode_for_modules">Strict mode for modules</a></h3><div class="section-content"><p>The entire contents of <a href="/en-US/docs/Web/JavaScript/Guide/Modules">JavaScript modules</a> are automatically in strict mode, with no statement needed to initiate it.</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>function myStrictFunction() { // because this is a module, I'm strict by default } export default myStrictFunction; </code></pre></div></div></section><section aria-labelledby="strict_mode_for_classes"><h3 id="strict_mode_for_classes"><a href="#strict_mode_for_classes">Strict mode for classes</a></h3><div class="section-content"><p>All parts of a <a href="/en-US/docs/Web/JavaScript/Reference/Classes">class</a>'s body are strict mode code, including both <a href="/en-US/docs/Web/JavaScript/Reference/Statements/class">class declarations</a> and <a href="/en-US/docs/Web/JavaScript/Reference/Operators/class">class expressions</a>.</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>class C1 { // All code here is evaluated in strict mode test() { delete Object.prototype; } } new C1().test(); // TypeError, because test() is in strict mode const C2 = class { // All code here is evaluated in strict mode }; // Code here may not be in strict mode delete Object.prototype; // Will not throw error </code></pre></div></div></section><section aria-labelledby="changes_in_strict_mode"><h2 id="changes_in_strict_mode"><a href="#changes_in_strict_mode">Changes in strict mode</a></h2><div class="section-content"><p>Strict mode changes both syntax and runtime behavior. Changes generally fall into these categories:</p> <ul> <li>changes converting mistakes into errors (as syntax errors or at runtime)</li> <li>changes simplifying how variable references are resolved</li> <li>changes simplifying <code>eval</code> and <code>arguments</code></li> <li>changes making it easier to write "secure" JavaScript</li> <li>changes anticipating future ECMAScript evolution.</li> </ul></div></section><section aria-labelledby="converting_mistakes_into_errors"><h3 id="converting_mistakes_into_errors"><a href="#converting_mistakes_into_errors">Converting mistakes into errors</a></h3><div class="section-content"><p>Strict mode changes some previously-accepted mistakes into errors. JavaScript was designed to be easy for novice developers, and sometimes it gives operations which should be errors non-error semantics. Sometimes this fixes the immediate problem, but sometimes this creates worse problems in the future. Strict mode treats these mistakes as errors so that they're discovered and promptly fixed.</p> <h4 id="assigning_to_undeclared_variables">Assigning to undeclared variables</h4> <p>Strict mode makes it impossible to accidentally create global variables. In sloppy mode, mistyping a variable in an assignment creates a new property on the global object and continues to "work". Assignments which would accidentally create global variables throw an error in strict mode:</p>  <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>"use strict"; let mistypeVariable; // Assuming no global variable mistypeVarible exists // this line throws a ReferenceError due to the // misspelling of "mistypeVariable" (lack of an "a") mistypeVarible = 17; </code></pre></div> <h4 id="failing_to_assign_to_object_properties">Failing to assign to object properties</h4> <p>Strict mode makes assignments which would otherwise silently fail to throw an exception. There are three ways to fail a property assignment:</p> <ul> <li>assignment to a non-writable data property</li> <li>assignment to a getter-only accessor property</li> <li>assignment to a new property on a <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/isExtensible">non-extensible</a> object</li> </ul> <p>For example, <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/NaN"><code>NaN</code></a> is a non-writable global variable. In sloppy mode, assigning to <code>NaN</code> does nothing; the developer receives no failure feedback. In strict mode, assigning to <code>NaN</code> throws an exception.</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>"use strict"; // Assignment to a non-writable global undefined = 5; // TypeError Infinity = 5; // TypeError // Assignment to a non-writable property const obj1 = {}; Object.defineProperty(obj1, "x", { value: 42, writable: false }); obj1.x = 9; // TypeError // Assignment to a getter-only property const obj2 = { get x() { return 17; }, }; obj2.x = 5; // TypeError // Assignment to a new property on a non-extensible object const fixed = {}; Object.preventExtensions(fixed); fixed.newProp = "ohai"; // TypeError </code></pre></div> <h4 id="failing_to_delete_object_properties">Failing to delete object properties</h4> <p>Attempts to <a href="/en-US/docs/Web/JavaScript/Reference/Operators/delete">delete</a> a non-configurable or otherwise undeletable (e.g. it's intercepted by a proxy's <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy/Proxy/deleteProperty"><code>deleteProperty</code></a> handler which returns <code>false</code>) property throw in strict mode (where before the attempt would have no effect):</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>"use strict"; delete Object.prototype; // TypeError delete [].length; // TypeError </code></pre></div> <p>Strict mode also forbids deleting plain names. <code>delete name</code> in strict mode is a syntax error:</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js example-bad notranslate"><code>"use strict"; var x; delete x; // syntax error </code></pre></div> <p>If the name is a configurable global property, prefix it with <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/globalThis"><code>globalThis</code></a> to delete it.</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js example-good notranslate"><code>"use strict"; delete globalThis.x; </code></pre></div> <h4 id="duplicate_parameter_names">Duplicate parameter names</h4> <p>Strict mode requires that function parameter names be unique. In sloppy mode, the last duplicated argument hides previous identically-named arguments. Those previous arguments remain available through <a href="/en-US/docs/Web/JavaScript/Reference/Functions/arguments"><code>arguments</code></a>, so they're not completely inaccessible. Still, this hiding makes little sense and is probably undesirable (it might hide a typo, for example), so in strict mode, duplicate argument names are a syntax error:</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js example-bad notranslate"><code>function sum(a, a, c) { // syntax error "use strict"; return a + a + c; // wrong if this code ran } </code></pre></div> <p>It is also a syntax error in non-strict mode to have duplicate parameter names, if the function has a default parameter, rest parameter, or destructured parameter.</p> <h4 id="legacy_octal_literals">Legacy octal literals</h4> <p>Strict mode <a href="/en-US/docs/Web/JavaScript/Reference/Errors/Deprecated_octal_literal">forbids a <code>0</code>-prefixed octal literal</a>. In sloppy mode, a number beginning with a <code>0</code>, such as <code>0644</code>, is interpreted as an octal number (<code>0644 === 420</code>), if all digits are smaller than 8. Novice developers sometimes believe a leading-zero prefix has no semantic meaning, so they might use it as an alignment device — but this changes the number's meaning! A leading-zero syntax for the octal is rarely useful and can be mistakenly used, so strict mode makes it a syntax error:</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js example-bad notranslate"><code>"use strict"; const sum = 015 + // syntax error 197 + 142; </code></pre></div> <p>The standardized way to denote octal literals is via the <code>0o</code> prefix. For example:</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js example-good notranslate"><code>const sumWithOctal = 0o10 + 8; console.log(sumWithOctal); // 16 </code></pre></div> <p>Octal escape sequences, such as <code>"\45"</code>, which is equal to <code>"%"</code>, can be used to represent characters by extended-<a href="/en-US/docs/Glossary/ASCII">ASCII</a> character code numbers in octal. In strict mode, this is a <a href="/en-US/docs/Web/JavaScript/Reference/Errors/Deprecated_octal_escape_sequence">syntax error</a>. More formally, it's disallowed to have <code>\</code> followed by any decimal digit other than <code>0</code>, or <code>\0</code> followed by a decimal digit; for example <code>\9</code> and <code>\07</code>.</p> <h4 id="setting_properties_on_primitive_values">Setting properties on primitive values</h4> <p>Strict mode forbids setting properties on <a href="/en-US/docs/Glossary/Primitive">primitive</a> values. Accessing a property on a primitive implicitly creates a wrapper object that's unobservable, so in sloppy mode, setting properties is ignored (no-op). In strict mode, a <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypeError"><code>TypeError</code></a> is thrown.</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>"use strict"; false.true = ""; // TypeError (14).sailing = "home"; // TypeError "with".you = "far away"; // TypeError </code></pre></div> <h4 id="duplicate_property_names">Duplicate property names</h4> <p>Duplicate property names used to be considered a <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/SyntaxError"><code>SyntaxError</code></a> in strict mode. With the introduction of <a href="/en-US/docs/Web/JavaScript/Reference/Operators/Object_initializer#computed_property_names">computed property names</a>, making duplication possible at runtime, this restriction was removed in ES2015.</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>"use strict"; const o = { p: 1, p: 2 }; // syntax error prior to ECMAScript 2015 </code></pre></div> <div class="notecard note"> <p><strong>Note:</strong> Making code that used to error become non-errors is always considered backwards-compatible. This is a good part of the language being strict about throwing errors: it leaves room for future semantic changes.</p> </div></div></section><section aria-labelledby="simplifying_scope_management"><h3 id="simplifying_scope_management"><a href="#simplifying_scope_management">Simplifying scope management</a></h3><div class="section-content"><p>Strict mode simplifies how variable names map to particular variable definitions in the code. Many compiler optimizations rely on the ability to say that variable <em>X</em> is stored in <em>that</em> location: this is critical to fully optimizing JavaScript code. JavaScript sometimes makes this basic mapping of name to variable definition in the code impossible to perform until runtime. Strict mode removes most cases where this happens, so the compiler can better optimize strict mode code.</p> <h4 id="removal_of_the_with_statement">Removal of the with statement</h4> <p>Strict mode prohibits <a href="/en-US/docs/Web/JavaScript/Reference/Statements/with"><code>with</code></a>. The problem with <code>with</code> is that any name inside the block might map either to a property of the object passed to it, or to a variable in surrounding (or even global) scope, at runtime; it's impossible to know which beforehand. Strict mode makes <code>with</code> a syntax error, so there's no chance for a name in a <code>with</code> to refer to an unknown location at runtime:</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js example-bad notranslate"><code>"use strict"; const x = 17; with (obj) { // Syntax error // If this weren't strict mode, would this be const x, or // would it instead be obj.x? It's impossible in general // to say without running the code, so the name can't be // optimized. x; } </code></pre></div> <p>The alternative of assigning the object to a short name variable, then accessing the corresponding property on that variable, stands ready to replace <code>with</code>.</p> <h4 id="non-leaking_eval">Non-leaking eval</h4> <p>In strict mode, <a href="https://whereswalden.com/2011/01/10/new-es5-strict-mode-support-new-vars-created-by-strict-mode-eval-code-are-local-to-that-code-only/" class="external" target="_blank"><code>eval</code> does not introduce new variables into the surrounding scope</a>. In sloppy mode, <code>eval("var x;")</code> introduces a variable <code>x</code> into the surrounding function or the global scope. This means that, in general, in a function containing a call to <code>eval</code>, every name not referring to an argument or local variable must be mapped to a particular definition at runtime (because that <code>eval</code> might have introduced a new variable that would hide the outer variable). In strict mode, <code>eval</code> creates variables only for the code being evaluated, so <code>eval</code> can't affect whether a name refers to an outer variable or some local variable:</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>var x = 17; var evalX = eval("'use strict'; var x = 42; x;"); console.assert(x === 17); console.assert(evalX === 42); </code></pre></div> <p>Whether the string passed to <code>eval()</code> is evaluated in strict mode depends on how <code>eval()</code> is invoked (<a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/eval#direct_and_indirect_eval">direct eval or indirect eval</a>).</p> <h4 id="block-scoped_function_declarations">Block-scoped function declarations</h4> <p>The JavaScript language specification, since its start, had not allowed function declarations nested in block statements. However, it was so intuitive that most browsers implemented it as an extension grammar. Unfortunately, the implementations' semantics diverged, and it became impossible for the language specification to reconcile all implementations. Therefore, <a href="/en-US/docs/Web/JavaScript/Reference/Statements/function#block-level_function_declaration">block-scoped function declarations</a> are only explicitly specified in strict mode (whereas they were once disallowed in strict mode), while sloppy mode behavior remains divergent among browsers.</p></div></section><section aria-labelledby="making_eval_and_arguments_simpler"><h3 id="making_eval_and_arguments_simpler"><a href="#making_eval_and_arguments_simpler">Making eval and arguments simpler</a></h3><div class="section-content"><p>Strict mode makes <a href="/en-US/docs/Web/JavaScript/Reference/Functions/arguments"><code>arguments</code></a> and <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/eval"><code>eval</code></a> less bizarrely magical. Both involve a considerable amount of magical behavior in sloppy mode: <code>eval</code> to add or remove bindings and to change binding values, and <code>arguments</code> syncing named arguments with its indexed properties. Strict mode makes great strides toward treating <code>eval</code> and <code>arguments</code> as keywords.</p> <h4 id="preventing_binding_or_assigning_eval_and_arguments">Preventing binding or assigning eval and arguments</h4> <p>The names <code>eval</code> and <code>arguments</code> can't be bound or assigned in language syntax. All these attempts to do so are syntax errors:</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js example-bad notranslate"><code>"use strict"; eval = 17; arguments++; ++eval; const obj = { set p(arguments) {} }; let eval; try { } catch (arguments) {} function x(eval) {} function arguments() {} const y = function eval() {}; const f = new Function("arguments", "'use strict'; return 17;"); </code></pre></div> <h4 id="no_syncing_between_parameters_and_arguments_indices">No syncing between parameters and arguments indices</h4> <p>Strict mode code doesn't sync indices of the <code>arguments</code> object with each parameter binding. In a sloppy mode function whose first argument is <code>arg</code>, setting <code>arg</code> also sets <code>arguments[0]</code>, and vice versa (unless no arguments were provided or <code>arguments[0]</code> is deleted). <code>arguments</code> objects for strict mode functions store the original arguments when the function was invoked. <code>arguments[i]</code> does not track the value of the corresponding named argument, nor does a named argument track the value in the corresponding <code>arguments[i]</code>.</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>function f(a) { "use strict"; a = 42; return [a, arguments[0]]; } const pair = f(17); console.assert(pair[0] === 42); console.assert(pair[1] === 17); </code></pre></div></div></section><section aria-labelledby="securing_javascript"><h3 id="securing_javascript"><a href="#securing_javascript">"Securing" JavaScript</a></h3><div class="section-content"><p>Strict mode makes it easier to write "secure" JavaScript. Some websites now provide ways for users to write JavaScript which will be run by the website <em>on behalf of other users</em>. JavaScript in browsers can access the user's private information, so such JavaScript must be partially transformed before it is run, to censor access to forbidden functionality. JavaScript's flexibility makes it effectively impossible to do this without many runtime checks. Certain language functions are so pervasive that performing runtime checks has a considerable performance cost. A few strict mode tweaks, plus requiring that user-submitted JavaScript be strict mode code and that it be invoked in a certain manner, substantially reduce the need for those runtime checks.</p> <h4 id="no_this_substitution">No this substitution</h4> <p>The value passed as <code>this</code> to a function in strict mode is not forced into being an object (a.k.a. "boxed"). For a sloppy mode function, <code>this</code> is always an object: either the provided object, if called with an object-valued <code>this</code>; or the boxed value of <code>this</code>, if called with a primitive as <code>this</code>; or the global object, if called with <code>undefined</code> or <code>null</code> as <code>this</code>. (Use <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/call"><code>call</code></a>, <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/apply"><code>apply</code></a>, or <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/bind"><code>bind</code></a> to specify a particular <code>this</code>.) Not only is automatic boxing a performance cost, but exposing the global object in browsers is a security hazard because the global object provides access to functionality that "secure" JavaScript environments must restrict. Thus for a strict mode function, the specified <code>this</code> is not boxed into an object, and if unspecified, <code>this</code> is <code>undefined</code> instead of <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/globalThis"><code>globalThis</code></a>:</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>"use strict"; function fun() { return this; } console.assert(fun() === undefined); console.assert(fun.call(2) === 2); console.assert(fun.apply(null) === null); console.assert(fun.call(undefined) === undefined); console.assert(fun.bind(true)() === true); </code></pre></div> <h4 id="removal_of_stack-walking_properties">Removal of stack-walking properties</h4> <p>In strict mode it's no longer possible to "walk" the JavaScript stack. Many implementations used to implement some extension features that make it possible to detect the upstream caller of a function. When a function <code>fun</code> is in the middle of being called, <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/caller"><code>fun.caller</code></a> is the function that most recently called <code>fun</code>, and <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/arguments"><code>fun.arguments</code></a> is the <code>arguments</code> for that invocation of <code>fun</code>. Both extensions are problematic for "secure" JavaScript because they allow "secured" code to access "privileged" functions and their (potentially unsecured) arguments. If <code>fun</code> is in strict mode, both <code>fun.caller</code> and <code>fun.arguments</code> are non-deletable properties which throw when set or retrieved:</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>function restricted() { "use strict"; restricted.caller; // throws a TypeError restricted.arguments; // throws a TypeError } function privilegedInvoker() { return restricted(); } privilegedInvoker(); </code></pre></div> <p>Similarly, <a href="/en-US/docs/Web/JavaScript/Reference/Functions/arguments/callee"><code>arguments.callee</code></a> is no longer supported. In sloppy mode, <code>arguments.callee</code> refers to the enclosing function. This use case is weak: name the enclosing function! Moreover, <code>arguments.callee</code> substantially hinders optimizations like inlining functions, because it must be made possible to provide a reference to the un-inlined function if <code>arguments.callee</code> is accessed. <code>arguments.callee</code> for strict mode functions is a non-deletable property which throws an error when set or retrieved:</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>"use strict"; const f = function () { return arguments.callee; }; f(); // throws a TypeError </code></pre></div></div></section><section aria-labelledby="future-proofing_javascript"><h3 id="future-proofing_javascript"><a href="#future-proofing_javascript">Future-proofing JavaScript</a></h3><div class="section-content"><h4 id="extra_reserved_words">Extra reserved words</h4> <p><a href="/en-US/docs/Web/JavaScript/Reference/Lexical_grammar#reserved_words">Reserved words</a> are identifiers that can't be used as variable names. Strict mode reserves some more names than sloppy mode, some of which are already used in the language, and some of which are reserved for the future to make future syntax extensions easier to implement.</p> <ul> <li><code>implements</code></li> <li><code>interface</code></li> <li><a href="/en-US/docs/Web/JavaScript/Reference/Statements/let"><code>let</code></a></li> <li><code>package</code></li> <li><code>private</code></li> <li><code>protected</code></li> <li><code>public</code></li> <li><a href="/en-US/docs/Web/JavaScript/Reference/Classes/static"><code>static</code></a></li> <li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/yield"><code>yield</code></a></li> </ul></div></section><section aria-labelledby="transitioning_to_strict_mode"><h2 id="transitioning_to_strict_mode"><a href="#transitioning_to_strict_mode">Transitioning to strict mode</a></h2><div class="section-content"><p>Strict mode has been designed so that the transition to it can be made gradually. It is possible to change each file individually and even to transition code to strict mode down to the function granularity.</p> <p>You can migrate a codebase to strict mode by first adding <code>"use strict"</code> to a piece of source code, and then fixing all execution errors, while watching out for semantic differences.</p></div></section><section aria-labelledby="syntax_errors"><h3 id="syntax_errors"><a href="#syntax_errors">Syntax errors</a></h3><div class="section-content"><p>When adding <code>'use strict';</code>, the following cases will throw a <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/SyntaxError"><code>SyntaxError</code></a> before the script is executing:</p> <ul> <li>Octal syntax <code>const n = 023;</code></li> <li><a href="/en-US/docs/Web/JavaScript/Reference/Statements/with"><code>with</code></a> statement</li> <li>Using <a href="/en-US/docs/Web/JavaScript/Reference/Operators/delete"><code>delete</code></a> on a variable name <code>delete myVariable</code>;</li> <li>Using <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/eval"><code>eval</code></a> or <a href="/en-US/docs/Web/JavaScript/Reference/Functions/arguments"><code>arguments</code></a> as variable or function argument name</li> <li>Using one of the newly <a href="/en-US/docs/Web/JavaScript/Reference/Lexical_grammar#reserved_words">reserved keywords</a> (in prevision for future language features): <code>implements</code>, <code>interface</code>, <code>let</code>, <code>package</code>, <code>private</code>, <code>protected</code>, <code>public</code>, <code>static</code>, and <code>yield</code></li> <li>Declaring two function parameters with the same name <code>function f(a, b, b) {}</code></li> <li>Declaring the same property name twice in an object literal <code>{a: 1, b: 3, a: 7}</code>. This constraint was later removed (<a href="https://bugzil.la/1041128" class="external" target="_blank">bug 1041128</a>).</li> </ul> <p>These errors are good, because they reveal plain errors or bad practices. They occur before the code is running, so they are easily discoverable as long as the code gets parsed by the runtime.</p></div></section><section aria-labelledby="new_runtime_errors"><h3 id="new_runtime_errors"><a href="#new_runtime_errors">New runtime errors</a></h3><div class="section-content"><p>JavaScript used to silently fail in contexts where what was done should be an error. Strict mode throws in such cases. If your code base contains such cases, testing will be necessary to be sure nothing is broken. You can screen for such errors at the function granularity level.</p> <ul> <li>Assigning to an undeclared variable throws a <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/ReferenceError"><code>ReferenceError</code></a>. This used to set a property on the global object, which is rarely the expected effect. If you really want to set a value to the global object, explicitly assign it as a property on <code>globalThis</code>.</li> <li>Failing to assign to an object's property (e.g. it's read-only) throws a <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypeError"><code>TypeError</code></a>. In sloppy mode, this would silently fail.</li> <li>Deleting a non-deletable property throws a <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypeError"><code>TypeError</code></a>. In sloppy mode, this would silently fail.</li> <li>Accessing <a href="/en-US/docs/Web/JavaScript/Reference/Functions/arguments/callee"><code>arguments.callee</code></a>, <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/caller"><code>strictFunction.caller</code></a>, or <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/arguments"><code>strictFunction.arguments</code></a> throws a <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypeError"><code>TypeError</code></a> if the function is in strict mode. If you are using <code>arguments.callee</code> to call the function recursively, you can use a named function expression instead.</li> </ul></div></section><section aria-labelledby="semantic_differences"><h3 id="semantic_differences"><a href="#semantic_differences">Semantic differences</a></h3><div class="section-content"><p>These differences are very subtle differences. It's possible that a test suite doesn't catch this kind of subtle difference. Careful review of your code base will probably be necessary to be sure these differences don't affect the semantics of your code. Fortunately, this careful review can be done gradually down the function granularity.</p> <dl> <dt id="this"><a href="#this"><code>this</code></a></dt> <dd> <p>In sloppy mode, function calls like <code>f()</code> would pass the global object as the <code>this</code> value. In strict mode, it is now <code>undefined</code>. When a function was called with <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/call"><code>call</code></a> or <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/apply"><code>apply</code></a>, if the value was a primitive value, this one was boxed into an object (or the global object for <code>undefined</code> and <code>null</code>). In strict mode, the value is passed directly without conversion or replacement.</p> </dd> <dt id="arguments"><a href="#arguments"><code>arguments</code></a></dt> <dd> <p>In sloppy mode, modifying a value in the <code>arguments</code> object modifies the corresponding named argument. This made optimizations complicated for JavaScript engine and made code harder to read/understand. In strict mode, the <code>arguments</code> object is created and initialized with the same values than the named arguments, but changes to either the <code>arguments</code> object or the named arguments aren't reflected in one another.</p> </dd> <dt id="eval"><a href="#eval"><code>eval</code></a></dt> <dd> <p>In strict mode code, <code>eval</code> doesn't create a new variable in the scope from which it was called. Also, of course, in strict mode, the string is evaluated with strict mode rules. Thorough testing will need to be performed to make sure nothing breaks. Not using eval if you don't really need it may be another pragmatic solution.</p> </dd> <dt id="block-scoped_function_declarations_2"><a href="#block-scoped_function_declarations_2">Block-scoped function declarations</a></dt> <dd> <p>In sloppy mode, a function declaration inside a block may be visible outside the block and even callable. In strict mode, a function declaration inside a block is only visible inside the block.</p> </dd> </dl></div></section><h2 id="specifications"><a href="#specifications">Specifications</a></h2><table class="standard-table"><thead><tr><th scope="col">Specification</th></tr></thead><tbody><tr><td><a href="https://tc39.es/ecma262/multipage/strict-mode-of-ecmascript.html">ECMAScript® 2025 Language Specification <br/></a></td></tr></tbody></table><section aria-labelledby="see_also"><h2 id="see_also"><a href="#see_also">See also</a></h2><div class="section-content"><ul> <li><a href="/en-US/docs/Web/JavaScript/Guide/Modules">JavaScript modules</a> guide</li> <li><a href="/en-US/docs/Web/JavaScript/Reference/Lexical_grammar">Lexical grammar</a></li> </ul></div></section></article><aside class="article-footer"><div class="article-footer-inner"><div class="svg-container"><svg xmlns="http://www.w3.org/2000/svg" width="162" height="162" viewBox="0 0 162 162" fill="none" role="none"><mask id="b" 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Content available under <a href="/en-US/docs/MDN/Writing_guidelines/Attrib_copyright_license">a Creative Commons license</a>.</p></div></div></footer></div><script type="application/json" id="hydration">{"url":"/en-US/docs/Web/JavaScript/Reference/Strict_mode","doc":{"body":[{"type":"prose","value":{"id":null,"title":null,"isH3":false,"content":"<div class=\"notecard note\">\n<p><strong>Note:</strong>\nSometimes you'll see the default, non-strict mode referred to as <em><a href=\"/en-US/docs/Glossary/Sloppy_mode\">sloppy mode</a></em>. This isn't an official term, but be aware of it, just in case.</p>\n</div>\n<p>JavaScript's strict mode is a way to <em>opt in</em> to a restricted variant of JavaScript, thereby implicitly opting-out of \"<a href=\"/en-US/docs/Glossary/Sloppy_mode\">sloppy mode</a>\". Strict mode isn't just a subset: it <em>intentionally</em> has different semantics from normal code. Strict mode code and non-strict mode code can coexist, so scripts can opt into strict mode incrementally.</p>\n<p>Strict mode makes several changes to normal JavaScript semantics:</p>\n<ol>\n<li>Eliminates some JavaScript silent errors by changing them to throw errors.</li>\n<li>Fixes mistakes that make it difficult for JavaScript engines to perform optimizations: strict mode code can sometimes be made to run faster than identical code that's not strict mode.</li>\n<li>Prohibits some syntax likely to be defined in future versions of ECMAScript.</li>\n</ol>"}},{"type":"prose","value":{"id":"invoking_strict_mode","title":"Invoking strict mode","isH3":false,"content":"<p>Strict mode applies to <em>entire scripts</em> or to <em>individual functions</em>. It doesn't apply to <a href=\"/en-US/docs/Web/JavaScript/Reference/Statements/block\">block statements</a> enclosed in <code>{}</code> braces; attempting to apply it to such contexts does nothing. <a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/eval\"><code>eval</code></a> code, <a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/Function\"><code>Function</code></a> code, <a href=\"/en-US/docs/Web/HTML/Attributes#event_handler_attributes\">event handler</a> attributes, strings passed to <a href=\"/en-US/docs/Web/API/Window/setTimeout\" title=\"setTimeout()\"><code>setTimeout()</code></a>, and related functions are either function bodies or entire scripts, and invoking strict mode in them works as expected.</p>"}},{"type":"prose","value":{"id":"strict_mode_for_scripts","title":"Strict mode for scripts","isH3":true,"content":"<p>To invoke strict mode for an entire script, put the <em>exact</em> statement <code>\"use strict\";</code> (or <code>'use strict';</code>) before any other statements.</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js notranslate\"><code>// Whole-script strict mode syntax\n\"use strict\";\nconst v = \"Hi! I'm a strict mode script!\";\n</code></pre></div>"}},{"type":"prose","value":{"id":"strict_mode_for_functions","title":"Strict mode for functions","isH3":true,"content":"<p>Likewise, to invoke strict mode for a function, put the <em>exact</em> statement <code>\"use strict\";</code> (or <code>'use strict';</code>) in the function's body before any other statements.</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js notranslate\"><code>function myStrictFunction() {\n // Function-level strict mode syntax\n \"use strict\";\n function nested() {\n return \"And so am I!\";\n }\n return `Hi! I'm a strict mode function! ${nested()}`;\n}\nfunction myNotStrictFunction() {\n return \"I'm not strict.\";\n}\n</code></pre></div>\n<p>The <code>\"use strict\"</code> directive can only be applied to the body of functions with simple parameters. Using <code>\"use strict\"</code> in functions with <a href=\"/en-US/docs/Web/JavaScript/Reference/Functions/rest_parameters\">rest</a>, <a href=\"/en-US/docs/Web/JavaScript/Reference/Functions/Default_parameters\">default</a>, or <a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Destructuring_assignment\">destructured</a> parameters is a <a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Strict_non_simple_params\">syntax error</a>.</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js example-bad notranslate\"><code>function sum(a = 1, b = 2) {\n // SyntaxError: \"use strict\" not allowed in function with default parameter\n \"use strict\";\n return a + b;\n}\n</code></pre></div>"}},{"type":"prose","value":{"id":"strict_mode_for_modules","title":"Strict mode for modules","isH3":true,"content":"<p>The entire contents of <a href=\"/en-US/docs/Web/JavaScript/Guide/Modules\">JavaScript modules</a> are automatically in strict mode, with no statement needed to initiate it.</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js notranslate\"><code>function myStrictFunction() {\n // because this is a module, I'm strict by default\n}\nexport default myStrictFunction;\n</code></pre></div>"}},{"type":"prose","value":{"id":"strict_mode_for_classes","title":"Strict mode for classes","isH3":true,"content":"<p>All parts of a <a href=\"/en-US/docs/Web/JavaScript/Reference/Classes\">class</a>'s body are strict mode code, including both <a href=\"/en-US/docs/Web/JavaScript/Reference/Statements/class\">class declarations</a> and <a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/class\">class expressions</a>.</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js notranslate\"><code>class C1 {\n // All code here is evaluated in strict mode\n test() {\n delete Object.prototype;\n }\n}\nnew C1().test(); // TypeError, because test() is in strict mode\n\nconst C2 = class {\n // All code here is evaluated in strict mode\n};\n\n// Code here may not be in strict mode\ndelete Object.prototype; // Will not throw error\n</code></pre></div>"}},{"type":"prose","value":{"id":"changes_in_strict_mode","title":"Changes in strict mode","isH3":false,"content":"<p>Strict mode changes both syntax and runtime behavior. Changes generally fall into these categories:</p>\n<ul>\n<li>changes converting mistakes into errors (as syntax errors or at runtime)</li>\n<li>changes simplifying how variable references are resolved</li>\n<li>changes simplifying <code>eval</code> and <code>arguments</code></li>\n<li>changes making it easier to write \"secure\" JavaScript</li>\n<li>changes anticipating future ECMAScript evolution.</li>\n</ul>"}},{"type":"prose","value":{"id":"converting_mistakes_into_errors","title":"Converting mistakes into errors","isH3":true,"content":"<p>Strict mode changes some previously-accepted mistakes into errors. JavaScript was designed to be easy for novice developers, and sometimes it gives operations which should be errors non-error semantics. Sometimes this fixes the immediate problem, but sometimes this creates worse problems in the future. Strict mode treats these mistakes as errors so that they're discovered and promptly fixed.</p>\n<h4 id=\"assigning_to_undeclared_variables\">Assigning to undeclared variables</h4>\n<p>Strict mode makes it impossible to accidentally create global variables. In sloppy mode, mistyping a variable in an assignment creates a new property on the global object and continues to \"work\". Assignments which would accidentally create global variables throw an error in strict mode:</p>\n\u003c!-- cSpell:ignore mistypeVarible -->\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js notranslate\"><code>\"use strict\";\nlet mistypeVariable;\n\n// Assuming no global variable mistypeVarible exists\n// this line throws a ReferenceError due to the\n// misspelling of \"mistypeVariable\" (lack of an \"a\")\nmistypeVarible = 17;\n</code></pre></div>\n<h4 id=\"failing_to_assign_to_object_properties\">Failing to assign to object properties</h4>\n<p>Strict mode makes assignments which would otherwise silently fail to throw an exception. There are three ways to fail a property assignment:</p>\n<ul>\n<li>assignment to a non-writable data property</li>\n<li>assignment to a getter-only accessor property</li>\n<li>assignment to a new property on a <a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/isExtensible\">non-extensible</a> object</li>\n</ul>\n<p>For example, <a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/NaN\"><code>NaN</code></a> is a non-writable global variable. In sloppy mode, assigning to <code>NaN</code> does nothing; the developer receives no failure feedback. In strict mode, assigning to <code>NaN</code> throws an exception.</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js notranslate\"><code>\"use strict\";\n\n// Assignment to a non-writable global\nundefined = 5; // TypeError\nInfinity = 5; // TypeError\n\n// Assignment to a non-writable property\nconst obj1 = {};\nObject.defineProperty(obj1, \"x\", { value: 42, writable: false });\nobj1.x = 9; // TypeError\n\n// Assignment to a getter-only property\nconst obj2 = {\n get x() {\n return 17;\n },\n};\nobj2.x = 5; // TypeError\n\n// Assignment to a new property on a non-extensible object\nconst fixed = {};\nObject.preventExtensions(fixed);\nfixed.newProp = \"ohai\"; // TypeError\n</code></pre></div>\n<h4 id=\"failing_to_delete_object_properties\">Failing to delete object properties</h4>\n<p>Attempts to <a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/delete\">delete</a> a non-configurable or otherwise undeletable (e.g. it's intercepted by a proxy's <a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy/Proxy/deleteProperty\"><code>deleteProperty</code></a> handler which returns <code>false</code>) property throw in strict mode (where before the attempt would have no effect):</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js notranslate\"><code>\"use strict\";\ndelete Object.prototype; // TypeError\ndelete [].length; // TypeError\n</code></pre></div>\n<p>Strict mode also forbids deleting plain names. <code>delete name</code> in strict mode is a syntax error:</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js example-bad notranslate\"><code>\"use strict\";\n\nvar x;\ndelete x; // syntax error\n</code></pre></div>\n<p>If the name is a configurable global property, prefix it with <a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/globalThis\"><code>globalThis</code></a> to delete it.</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js example-good notranslate\"><code>\"use strict\";\n\ndelete globalThis.x;\n</code></pre></div>\n<h4 id=\"duplicate_parameter_names\">Duplicate parameter names</h4>\n<p>Strict mode requires that function parameter names be unique. In sloppy mode, the last duplicated argument hides previous identically-named arguments. Those previous arguments remain available through <a href=\"/en-US/docs/Web/JavaScript/Reference/Functions/arguments\"><code>arguments</code></a>, so they're not completely inaccessible. Still, this hiding makes little sense and is probably undesirable (it might hide a typo, for example), so in strict mode, duplicate argument names are a syntax error:</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js example-bad notranslate\"><code>function sum(a, a, c) {\n // syntax error\n \"use strict\";\n return a + a + c; // wrong if this code ran\n}\n</code></pre></div>\n<p>It is also a syntax error in non-strict mode to have duplicate parameter names, if the function has a default parameter, rest parameter, or destructured parameter.</p>\n<h4 id=\"legacy_octal_literals\">Legacy octal literals</h4>\n<p>Strict mode <a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Deprecated_octal_literal\">forbids a <code>0</code>-prefixed octal literal</a>. In sloppy mode, a number beginning with a <code>0</code>, such as <code>0644</code>, is interpreted as an octal number (<code>0644 === 420</code>), if all digits are smaller than 8. Novice developers sometimes believe a leading-zero prefix has no semantic meaning, so they might use it as an alignment device — but this changes the number's meaning! A leading-zero syntax for the octal is rarely useful and can be mistakenly used, so strict mode makes it a syntax error:</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js example-bad notranslate\"><code>\"use strict\";\nconst sum =\n 015 + // syntax error\n 197 +\n 142;\n</code></pre></div>\n<p>The standardized way to denote octal literals is via the <code>0o</code> prefix. For example:</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js example-good notranslate\"><code>const sumWithOctal = 0o10 + 8;\nconsole.log(sumWithOctal); // 16\n</code></pre></div>\n<p>Octal escape sequences, such as <code>\"\\45\"</code>, which is equal to <code>\"%\"</code>, can be used to represent characters by extended-<a href=\"/en-US/docs/Glossary/ASCII\">ASCII</a> character code numbers in octal. In strict mode, this is a <a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Deprecated_octal_escape_sequence\">syntax error</a>. More formally, it's disallowed to have <code>\\</code> followed by any decimal digit other than <code>0</code>, or <code>\\0</code> followed by a decimal digit; for example <code>\\9</code> and <code>\\07</code>.</p>\n<h4 id=\"setting_properties_on_primitive_values\">Setting properties on primitive values</h4>\n<p>Strict mode forbids setting properties on <a href=\"/en-US/docs/Glossary/Primitive\">primitive</a> values. Accessing a property on a primitive implicitly creates a wrapper object that's unobservable, so in sloppy mode, setting properties is ignored (no-op). In strict mode, a <a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypeError\"><code>TypeError</code></a> is thrown.</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js notranslate\"><code>\"use strict\";\n\nfalse.true = \"\"; // TypeError\n(14).sailing = \"home\"; // TypeError\n\"with\".you = \"far away\"; // TypeError\n</code></pre></div>\n<h4 id=\"duplicate_property_names\">Duplicate property names</h4>\n<p>Duplicate property names used to be considered a <a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/SyntaxError\"><code>SyntaxError</code></a> in strict mode. With the introduction of <a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Object_initializer#computed_property_names\">computed property names</a>, making duplication possible at runtime, this restriction was removed in ES2015.</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js notranslate\"><code>\"use strict\";\nconst o = { p: 1, p: 2 }; // syntax error prior to ECMAScript 2015\n</code></pre></div>\n<div class=\"notecard note\">\n<p><strong>Note:</strong>\nMaking code that used to error become non-errors is always considered backwards-compatible. This is a good part of the language being strict about throwing errors: it leaves room for future semantic changes.</p>\n</div>"}},{"type":"prose","value":{"id":"simplifying_scope_management","title":"Simplifying scope management","isH3":true,"content":"<p>Strict mode simplifies how variable names map to particular variable definitions in the code. Many compiler optimizations rely on the ability to say that variable <em>X</em> is stored in <em>that</em> location: this is critical to fully optimizing JavaScript code. JavaScript sometimes makes this basic mapping of name to variable definition in the code impossible to perform until runtime. Strict mode removes most cases where this happens, so the compiler can better optimize strict mode code.</p>\n<h4 id=\"removal_of_the_with_statement\">Removal of the with statement</h4>\n<p>Strict mode prohibits <a href=\"/en-US/docs/Web/JavaScript/Reference/Statements/with\"><code>with</code></a>. The problem with <code>with</code> is that any name inside the block might map either to a property of the object passed to it, or to a variable in surrounding (or even global) scope, at runtime; it's impossible to know which beforehand. Strict mode makes <code>with</code> a syntax error, so there's no chance for a name in a <code>with</code> to refer to an unknown location at runtime:</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js example-bad notranslate\"><code>\"use strict\";\nconst x = 17;\nwith (obj) {\n // Syntax error\n // If this weren't strict mode, would this be const x, or\n // would it instead be obj.x? It's impossible in general\n // to say without running the code, so the name can't be\n // optimized.\n x;\n}\n</code></pre></div>\n<p>The alternative of assigning the object to a short name variable, then accessing the corresponding property on that variable, stands ready to replace <code>with</code>.</p>\n<h4 id=\"non-leaking_eval\">Non-leaking eval</h4>\n<p>In strict mode, <a href=\"https://whereswalden.com/2011/01/10/new-es5-strict-mode-support-new-vars-created-by-strict-mode-eval-code-are-local-to-that-code-only/\" class=\"external\" target=\"_blank\"><code>eval</code> does not introduce new variables into the surrounding scope</a>. In sloppy mode, <code>eval(\"var x;\")</code> introduces a variable <code>x</code> into the surrounding function or the global scope. This means that, in general, in a function containing a call to <code>eval</code>, every name not referring to an argument or local variable must be mapped to a particular definition at runtime (because that <code>eval</code> might have introduced a new variable that would hide the outer variable). In strict mode, <code>eval</code> creates variables only for the code being evaluated, so <code>eval</code> can't affect whether a name refers to an outer variable or some local variable:</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js notranslate\"><code>var x = 17;\nvar evalX = eval(\"'use strict'; var x = 42; x;\");\nconsole.assert(x === 17);\nconsole.assert(evalX === 42);\n</code></pre></div>\n<p>Whether the string passed to <code>eval()</code> is evaluated in strict mode depends on how <code>eval()</code> is invoked (<a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/eval#direct_and_indirect_eval\">direct eval or indirect eval</a>).</p>\n<h4 id=\"block-scoped_function_declarations\">Block-scoped function declarations</h4>\n<p>The JavaScript language specification, since its start, had not allowed function declarations nested in block statements. However, it was so intuitive that most browsers implemented it as an extension grammar. Unfortunately, the implementations' semantics diverged, and it became impossible for the language specification to reconcile all implementations. Therefore, <a href=\"/en-US/docs/Web/JavaScript/Reference/Statements/function#block-level_function_declaration\">block-scoped function declarations</a> are only explicitly specified in strict mode (whereas they were once disallowed in strict mode), while sloppy mode behavior remains divergent among browsers.</p>"}},{"type":"prose","value":{"id":"making_eval_and_arguments_simpler","title":"Making eval and arguments simpler","isH3":true,"content":"<p>Strict mode makes <a href=\"/en-US/docs/Web/JavaScript/Reference/Functions/arguments\"><code>arguments</code></a> and <a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/eval\"><code>eval</code></a> less bizarrely magical. Both involve a considerable amount of magical behavior in sloppy mode: <code>eval</code> to add or remove bindings and to change binding values, and <code>arguments</code> syncing named arguments with its indexed properties. Strict mode makes great strides toward treating <code>eval</code> and <code>arguments</code> as keywords.</p>\n<h4 id=\"preventing_binding_or_assigning_eval_and_arguments\">Preventing binding or assigning eval and arguments</h4>\n<p>The names <code>eval</code> and <code>arguments</code> can't be bound or assigned in language syntax. All these attempts to do so are syntax errors:</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js example-bad notranslate\"><code>\"use strict\";\neval = 17;\narguments++;\n++eval;\nconst obj = { set p(arguments) {} };\nlet eval;\ntry {\n} catch (arguments) {}\nfunction x(eval) {}\nfunction arguments() {}\nconst y = function eval() {};\nconst f = new Function(\"arguments\", \"'use strict'; return 17;\");\n</code></pre></div>\n<h4 id=\"no_syncing_between_parameters_and_arguments_indices\">No syncing between parameters and arguments indices</h4>\n<p>Strict mode code doesn't sync indices of the <code>arguments</code> object with each parameter binding. In a sloppy mode function whose first argument is <code>arg</code>, setting <code>arg</code> also sets <code>arguments[0]</code>, and vice versa (unless no arguments were provided or <code>arguments[0]</code> is deleted). <code>arguments</code> objects for strict mode functions store the original arguments when the function was invoked. <code>arguments[i]</code> does not track the value of the corresponding named argument, nor does a named argument track the value in the corresponding <code>arguments[i]</code>.</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js notranslate\"><code>function f(a) {\n \"use strict\";\n a = 42;\n return [a, arguments[0]];\n}\nconst pair = f(17);\nconsole.assert(pair[0] === 42);\nconsole.assert(pair[1] === 17);\n</code></pre></div>"}},{"type":"prose","value":{"id":"securing_javascript","title":"\"Securing\" JavaScript","isH3":true,"content":"<p>Strict mode makes it easier to write \"secure\" JavaScript. Some websites now provide ways for users to write JavaScript which will be run by the website <em>on behalf of other users</em>. JavaScript in browsers can access the user's private information, so such JavaScript must be partially transformed before it is run, to censor access to forbidden functionality. JavaScript's flexibility makes it effectively impossible to do this without many runtime checks. Certain language functions are so pervasive that performing runtime checks has a considerable performance cost. A few strict mode tweaks, plus requiring that user-submitted JavaScript be strict mode code and that it be invoked in a certain manner, substantially reduce the need for those runtime checks.</p>\n<h4 id=\"no_this_substitution\">No this substitution</h4>\n<p>The value passed as <code>this</code> to a function in strict mode is not forced into being an object (a.k.a. \"boxed\"). For a sloppy mode function, <code>this</code> is always an object: either the provided object, if called with an object-valued <code>this</code>; or the boxed value of <code>this</code>, if called with a primitive as <code>this</code>; or the global object, if called with <code>undefined</code> or <code>null</code> as <code>this</code>. (Use <a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/call\"><code>call</code></a>, <a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/apply\"><code>apply</code></a>, or <a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/bind\"><code>bind</code></a> to specify a particular <code>this</code>.) Not only is automatic boxing a performance cost, but exposing the global object in browsers is a security hazard because the global object provides access to functionality that \"secure\" JavaScript environments must restrict. Thus for a strict mode function, the specified <code>this</code> is not boxed into an object, and if unspecified, <code>this</code> is <code>undefined</code> instead of <a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/globalThis\"><code>globalThis</code></a>:</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js notranslate\"><code>\"use strict\";\nfunction fun() {\n return this;\n}\nconsole.assert(fun() === undefined);\nconsole.assert(fun.call(2) === 2);\nconsole.assert(fun.apply(null) === null);\nconsole.assert(fun.call(undefined) === undefined);\nconsole.assert(fun.bind(true)() === true);\n</code></pre></div>\n<h4 id=\"removal_of_stack-walking_properties\">Removal of stack-walking properties</h4>\n<p>In strict mode it's no longer possible to \"walk\" the JavaScript stack. Many implementations used to implement some extension features that make it possible to detect the upstream caller of a function. When a function <code>fun</code> is in the middle of being called, <a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/caller\"><code>fun.caller</code></a> is the function that most recently called <code>fun</code>, and <a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/arguments\"><code>fun.arguments</code></a> is the <code>arguments</code> for that invocation of <code>fun</code>. Both extensions are problematic for \"secure\" JavaScript because they allow \"secured\" code to access \"privileged\" functions and their (potentially unsecured) arguments. If <code>fun</code> is in strict mode, both <code>fun.caller</code> and <code>fun.arguments</code> are non-deletable properties which throw when set or retrieved:</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js notranslate\"><code>function restricted() {\n \"use strict\";\n restricted.caller; // throws a TypeError\n restricted.arguments; // throws a TypeError\n}\nfunction privilegedInvoker() {\n return restricted();\n}\nprivilegedInvoker();\n</code></pre></div>\n<p>Similarly, <a href=\"/en-US/docs/Web/JavaScript/Reference/Functions/arguments/callee\"><code>arguments.callee</code></a> is no longer supported. In sloppy mode, <code>arguments.callee</code> refers to the enclosing function. This use case is weak: name the enclosing function! Moreover, <code>arguments.callee</code> substantially hinders optimizations like inlining functions, because it must be made possible to provide a reference to the un-inlined function if <code>arguments.callee</code> is accessed. <code>arguments.callee</code> for strict mode functions is a non-deletable property which throws an error when set or retrieved:</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js notranslate\"><code>\"use strict\";\nconst f = function () {\n return arguments.callee;\n};\nf(); // throws a TypeError\n</code></pre></div>"}},{"type":"prose","value":{"id":"future-proofing_javascript","title":"Future-proofing JavaScript","isH3":true,"content":"<h4 id=\"extra_reserved_words\">Extra reserved words</h4>\n<p><a href=\"/en-US/docs/Web/JavaScript/Reference/Lexical_grammar#reserved_words\">Reserved words</a> are identifiers that can't be used as variable names. Strict mode reserves some more names than sloppy mode, some of which are already used in the language, and some of which are reserved for the future to make future syntax extensions easier to implement.</p>\n<ul>\n<li><code>implements</code></li>\n<li><code>interface</code></li>\n<li><a href=\"/en-US/docs/Web/JavaScript/Reference/Statements/let\"><code>let</code></a></li>\n<li><code>package</code></li>\n<li><code>private</code></li>\n<li><code>protected</code></li>\n<li><code>public</code></li>\n<li><a href=\"/en-US/docs/Web/JavaScript/Reference/Classes/static\"><code>static</code></a></li>\n<li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/yield\"><code>yield</code></a></li>\n</ul>"}},{"type":"prose","value":{"id":"transitioning_to_strict_mode","title":"Transitioning to strict mode","isH3":false,"content":"<p>Strict mode has been designed so that the transition to it can be made gradually. It is possible to change each file individually and even to transition code to strict mode down to the function granularity.</p>\n<p>You can migrate a codebase to strict mode by first adding <code>\"use strict\"</code> to a piece of source code, and then fixing all execution errors, while watching out for semantic differences.</p>"}},{"type":"prose","value":{"id":"syntax_errors","title":"Syntax errors","isH3":true,"content":"<p>When adding <code>'use strict';</code>, the following cases will throw a <a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/SyntaxError\"><code>SyntaxError</code></a> before the script is executing:</p>\n<ul>\n<li>Octal syntax <code>const n = 023;</code></li>\n<li><a href=\"/en-US/docs/Web/JavaScript/Reference/Statements/with\"><code>with</code></a> statement</li>\n<li>Using <a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/delete\"><code>delete</code></a> on a variable name <code>delete myVariable</code>;</li>\n<li>Using <a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/eval\"><code>eval</code></a> or <a href=\"/en-US/docs/Web/JavaScript/Reference/Functions/arguments\"><code>arguments</code></a> as variable or function argument name</li>\n<li>Using one of the newly <a href=\"/en-US/docs/Web/JavaScript/Reference/Lexical_grammar#reserved_words\">reserved keywords</a> (in prevision for future language features): <code>implements</code>, <code>interface</code>, <code>let</code>, <code>package</code>, <code>private</code>, <code>protected</code>, <code>public</code>, <code>static</code>, and <code>yield</code></li>\n<li>Declaring two function parameters with the same name <code>function f(a, b, b) {}</code></li>\n<li>Declaring the same property name twice in an object literal <code>{a: 1, b: 3, a: 7}</code>. This constraint was later removed (<a href=\"https://bugzil.la/1041128\" class=\"external\" target=\"_blank\">bug 1041128</a>).</li>\n</ul>\n<p>These errors are good, because they reveal plain errors or bad practices. They occur before the code is running, so they are easily discoverable as long as the code gets parsed by the runtime.</p>"}},{"type":"prose","value":{"id":"new_runtime_errors","title":"New runtime errors","isH3":true,"content":"<p>JavaScript used to silently fail in contexts where what was done should be an error. Strict mode throws in such cases. If your code base contains such cases, testing will be necessary to be sure nothing is broken. You can screen for such errors at the function granularity level.</p>\n<ul>\n<li>Assigning to an undeclared variable throws a <a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/ReferenceError\"><code>ReferenceError</code></a>. This used to set a property on the global object, which is rarely the expected effect. If you really want to set a value to the global object, explicitly assign it as a property on <code>globalThis</code>.</li>\n<li>Failing to assign to an object's property (e.g. it's read-only) throws a <a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypeError\"><code>TypeError</code></a>. In sloppy mode, this would silently fail.</li>\n<li>Deleting a non-deletable property throws a <a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypeError\"><code>TypeError</code></a>. In sloppy mode, this would silently fail.</li>\n<li>Accessing <a href=\"/en-US/docs/Web/JavaScript/Reference/Functions/arguments/callee\"><code>arguments.callee</code></a>, <a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/caller\"><code>strictFunction.caller</code></a>, or <a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/arguments\"><code>strictFunction.arguments</code></a> throws a <a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypeError\"><code>TypeError</code></a> if the function is in strict mode. If you are using <code>arguments.callee</code> to call the function recursively, you can use a named function expression instead.</li>\n</ul>"}},{"type":"prose","value":{"id":"semantic_differences","title":"Semantic differences","isH3":true,"content":"<p>These differences are very subtle differences. It's possible that a test suite doesn't catch this kind of subtle difference. Careful review of your code base will probably be necessary to be sure these differences don't affect the semantics of your code. Fortunately, this careful review can be done gradually down the function granularity.</p>\n<dl>\n<dt id=\"this\"><a href=\"#this\"><code>this</code></a></dt>\n<dd>\n<p>In sloppy mode, function calls like <code>f()</code> would pass the global object as the <code>this</code> value. In strict mode, it is now <code>undefined</code>. When a function was called with <a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/call\"><code>call</code></a> or <a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/apply\"><code>apply</code></a>, if the value was a primitive value, this one was boxed into an object (or the global object for <code>undefined</code> and <code>null</code>). In strict mode, the value is passed directly without conversion or replacement.</p>\n</dd>\n<dt id=\"arguments\"><a href=\"#arguments\"><code>arguments</code></a></dt>\n<dd>\n<p>In sloppy mode, modifying a value in the <code>arguments</code> object modifies the corresponding named argument. This made optimizations complicated for JavaScript engine and made code harder to read/understand. In strict mode, the <code>arguments</code> object is created and initialized with the same values than the named arguments, but changes to either the <code>arguments</code> object or the named arguments aren't reflected in one another.</p>\n</dd>\n<dt id=\"eval\"><a href=\"#eval\"><code>eval</code></a></dt>\n<dd>\n<p>In strict mode code, <code>eval</code> doesn't create a new variable in the scope from which it was called. Also, of course, in strict mode, the string is evaluated with strict mode rules. Thorough testing will need to be performed to make sure nothing breaks. Not using eval if you don't really need it may be another pragmatic solution.</p>\n</dd>\n<dt id=\"block-scoped_function_declarations_2\"><a href=\"#block-scoped_function_declarations_2\">Block-scoped function declarations</a></dt>\n<dd>\n<p>In sloppy mode, a function declaration inside a block may be visible outside the block and even callable. In strict mode, a function declaration inside a block is only visible inside the block.</p>\n</dd>\n</dl>"}},{"type":"specifications","value":{"id":"specifications","title":"Specifications","isH3":false,"specifications":[{"bcdSpecificationURL":"https://tc39.es/ecma262/multipage/strict-mode-of-ecmascript.html","title":"ECMAScript® 2025 Language Specification"}],"query":"undefined"}},{"type":"prose","value":{"id":"see_also","title":"See also","isH3":false,"content":"<ul>\n<li><a href=\"/en-US/docs/Web/JavaScript/Guide/Modules\">JavaScript modules</a> guide</li>\n<li><a href=\"/en-US/docs/Web/JavaScript/Reference/Lexical_grammar\">Lexical grammar</a></li>\n</ul>"}}],"isActive":true,"isMarkdown":true,"isTranslated":false,"locale":"en-US","mdn_url":"/en-US/docs/Web/JavaScript/Reference/Strict_mode","modified":"2025-01-11T20:52:14.000Z","native":"English (US)","noIndexing":false,"other_translations":[{"locale":"de","title":"Strict mode","native":"Deutsch"},{"locale":"es","title":"Modo Estricto","native":"Español"},{"locale":"fr","title":"Le mode strict","native":"Français"},{"locale":"ja","title":"厳格モード","native":"日本語"},{"locale":"ko","title":"Strict mode","native":"한국어"},{"locale":"pt-BR","title":"Strict mode","native":"Português (do Brasil)"},{"locale":"ru","title":"Strict mode","native":"Русский"},{"locale":"zh-CN","title":"严格模式","native":"中文 (简体)"},{"locale":"zh-TW","title":"Strict mode","native":"正體中文 (繁體)"}],"pageTitle":"Strict mode - JavaScript | MDN","parents":[{"uri":"/en-US/docs/Web","title":"References"},{"uri":"/en-US/docs/Web/JavaScript","title":"JavaScript"},{"uri":"/en-US/docs/Web/JavaScript/Reference","title":"Reference"},{"uri":"/en-US/docs/Web/JavaScript/Reference/Strict_mode","title":"Strict mode"}],"popularity":null,"short_title":"Strict mode","sidebarHTML":"<ol><li class=\"section\"><a href=\"/en-US/docs/Web/JavaScript\">JavaScript</a></li><li class=\"section\">Tutorials and guides</li><li class=\"toggle\"><details><summary>Beginner's tutorials</summary><ol><li><a href=\"/en-US/docs/Learn_web_development/Getting_started/Your_first_website/Adding_interactivity\">Your first website: Adding interactivity</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Scripting\">Dynamic scripting with JavaScript</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Frameworks_libraries\">JavaScript frameworks and libraries</a></li></ol></details></li><li class=\"toggle\"><details><summary>JavaScript Guide</summary><ol><li><a href=\"/en-US/docs/Web/JavaScript/Guide/Introduction\">Introduction</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Guide/Grammar_and_types\">Grammar and types</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Guide/Control_flow_and_error_handling\">Control flow and error handling</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Guide/Loops_and_iteration\">Loops and iteration</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Guide/Functions\">Functions</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Guide/Expressions_and_operators\">Expressions and operators</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Guide/Numbers_and_strings\">Numbers and strings</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Guide/Representing_dates_times\">Representing dates & times</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Guide/Regular_expressions\">Regular expressions</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Guide/Indexed_collections\">Indexed collections</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Guide/Keyed_collections\">Keyed collections</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Guide/Working_with_objects\">Working with objects</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Guide/Using_classes\">Using classes</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Guide/Using_promises\">Using promises</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Guide/Typed_arrays\">JavaScript typed arrays</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Guide/Iterators_and_generators\">Iterators and generators</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Guide/Internationalization\">Internationalization</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Guide/Meta_programming\">Meta programming</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Guide/Modules\">JavaScript modules</a></li></ol></details></li><li class=\"toggle\"><details><summary>Intermediate</summary><ol><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Advanced_JavaScript_objects\">Advanced JavaScript objects</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Async_JS\">Asynchronous JavaScript</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Client-side_APIs\">Client-side web APIs</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Language_overview\">Language overview</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Data_structures\">JavaScript data structures</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Equality_comparisons_and_sameness\">Equality comparisons and sameness</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Enumerability_and_ownership_of_properties\">Enumerability and ownership of properties</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Closures\">Closures</a></li></ol></details></li><li class=\"toggle\"><details><summary>Advanced</summary><ol><li><a href=\"/en-US/docs/Web/JavaScript/Inheritance_and_the_prototype_chain\">Inheritance and the prototype chain</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Memory_management\">Memory Management</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Event_loop\">Concurrency model and Event Loop</a></li></ol></details></li><li class=\"section\"><a href=\"/en-US/docs/Web/JavaScript/Reference\">References</a></li><li class=\"toggle\"><details><summary>Built-in objects</summary><ol><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects\">Overview</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/AggregateError\">AggregateError</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array\">Array</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/ArrayBuffer\">ArrayBuffer</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/AsyncFunction\">AsyncFunction</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/AsyncGenerator\">AsyncGenerator</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/AsyncGeneratorFunction\">AsyncGeneratorFunction</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/AsyncIterator\">AsyncIterator</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/Atomics\">Atomics</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/BigInt\">BigInt</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/BigInt64Array\">BigInt64Array</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/BigUint64Array\">BigUint64Array</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/Boolean\">Boolean</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/DataView\">DataView</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date\">Date</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/decodeURI\">decodeURI()</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/decodeURIComponent\">decodeURIComponent()</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/encodeURI\">encodeURI()</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/encodeURIComponent\">encodeURIComponent()</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error\">Error</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/escape\">escape()</a><abbr class=\"icon icon-deprecated\" title=\"Deprecated. Not for use in new websites.\">\n<span class=\"visually-hidden\">Deprecated</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/eval\">eval()</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/EvalError\">EvalError</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/FinalizationRegistry\">FinalizationRegistry</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/Float16Array\">Float16Array</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/Float32Array\">Float32Array</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/Float64Array\">Float64Array</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function\">Function</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/Generator\">Generator</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/GeneratorFunction\">GeneratorFunction</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/globalThis\">globalThis</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/Infinity\">Infinity</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/Int16Array\">Int16Array</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/Int32Array\">Int32Array</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/Int8Array\">Int8Array</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/InternalError\">InternalError</a><abbr class=\"icon icon-nonstandard\" title=\"Non-standard. Check cross-browser support before using.\">\n<span class=\"visually-hidden\">Non-standard</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl\">Intl</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/isFinite\">isFinite()</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/isNaN\">isNaN()</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/Iterator\">Iterator</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/JSON\">JSON</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map\">Map</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math\">Math</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/NaN\">NaN</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number\">Number</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object\">Object</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/parseFloat\">parseFloat()</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/parseInt\">parseInt()</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise\">Promise</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy\">Proxy</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/RangeError\">RangeError</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/ReferenceError\">ReferenceError</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/Reflect\">Reflect</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp\">RegExp</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set\">Set</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/SharedArrayBuffer\">SharedArrayBuffer</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/String\">String</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol\">Symbol</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/SyntaxError\">SyntaxError</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/Temporal\">Temporal</a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray\">TypedArray</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypeError\">TypeError</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/Uint16Array\">Uint16Array</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/Uint32Array\">Uint32Array</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/Uint8Array\">Uint8Array</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/Uint8ClampedArray\">Uint8ClampedArray</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/undefined\">undefined</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/unescape\">unescape()</a><abbr class=\"icon icon-deprecated\" title=\"Deprecated. Not for use in new websites.\">\n<span class=\"visually-hidden\">Deprecated</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/URIError\">URIError</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/WeakMap\">WeakMap</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/WeakRef\">WeakRef</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/WeakSet\">WeakSet</a></li></ol></details></li><li class=\"toggle\"><details><summary>Expressions & operators</summary><ol><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators\">Overview</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Addition\">Addition (+)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Addition_assignment\">Addition assignment (+=)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Assignment\">Assignment (=)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/async_function\">async function expression</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/async_function*\">async function* expression</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/await\">await</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Bitwise_AND\">Bitwise AND (&)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Bitwise_AND_assignment\">Bitwise AND assignment (&=)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Bitwise_NOT\">Bitwise NOT (~)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Bitwise_OR\">Bitwise OR (|)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Bitwise_OR_assignment\">Bitwise OR assignment (|=)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Bitwise_XOR\">Bitwise XOR (^)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Bitwise_XOR_assignment\">Bitwise XOR assignment (^=)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/class\">class expression</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Comma_operator\">Comma operator (,)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Conditional_operator\">Conditional (ternary) operator</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Decrement\">Decrement (--)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/delete\">delete</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Destructuring_assignment\">Destructuring assignment</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Division\">Division (/)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Division_assignment\">Division assignment (/=)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Equality\">Equality (==)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Exponentiation\">Exponentiation (**)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Exponentiation_assignment\">Exponentiation assignment (**=)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/function\">function expression</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/function*\">function* expression</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Greater_than\">Greater than (>)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Greater_than_or_equal\">Greater than or equal (>=)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Grouping\">Grouping operator ( )</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/import.meta\">import.meta</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/import\">import()</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/in\">in</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Increment\">Increment (++)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Inequality\">Inequality (!=)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/instanceof\">instanceof</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Left_shift\">Left shift (<<)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Left_shift_assignment\">Left shift assignment (<<=)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Less_than\">Less than (<)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Less_than_or_equal\">Less than or equal (<=)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Logical_AND\">Logical AND (&&)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Logical_AND_assignment\">Logical AND assignment (&&=)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Logical_NOT\">Logical NOT (!)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Logical_OR\">Logical OR (||)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Logical_OR_assignment\">Logical OR assignment (||=)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Multiplication\">Multiplication (*)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Multiplication_assignment\">Multiplication assignment (*=)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/new\">new</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/new.target\">new.target</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/null\">null</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Nullish_coalescing_assignment\">Nullish coalescing assignment (??=)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Nullish_coalescing\">Nullish coalescing operator (??)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Object_initializer\">Object initializer</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Operator_precedence\">Operator precedence</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Optional_chaining\">Optional chaining (?.)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Property_accessors\">Property accessors</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Remainder\">Remainder (%)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Remainder_assignment\">Remainder assignment (%=)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Right_shift\">Right shift (>>)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Right_shift_assignment\">Right shift assignment (>>=)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Spread_syntax\">Spread syntax (...)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Strict_equality\">Strict equality (===)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Strict_inequality\">Strict inequality (!==)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Subtraction\">Subtraction (-)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Subtraction_assignment\">Subtraction assignment (-=)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/super\">super</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/this\">this</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/typeof\">typeof</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Unary_negation\">Unary negation (-)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Unary_plus\">Unary plus (+)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Unsigned_right_shift\">Unsigned right shift (>>>)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/Unsigned_right_shift_assignment\">Unsigned right shift assignment (>>>=)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/void\">void operator</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/yield\">yield</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Operators/yield*\">yield*</a></li></ol></details></li><li class=\"toggle\"><details><summary>Statements & declarations</summary><ol><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Statements\">Overview</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Statements/async_function\">async function</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Statements/async_function*\">async function*</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Statements/block\">Block statement</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Statements/break\">break</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Statements/class\">class</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Statements/const\">const</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Statements/continue\">continue</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Statements/debugger\">debugger</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Statements/do...while\">do...while</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Statements/Empty\">Empty statement</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Statements/export\">export</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Statements/Expression_statement\">Expression statement</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Statements/for\">for</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Statements/for-await...of\">for await...of</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Statements/for...in\">for...in</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Statements/for...of\">for...of</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Statements/function\">function</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Statements/function*\">function*</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Statements/if...else\">if...else</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Statements/import\">import</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Statements/label\">Labeled statement</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Statements/let\">let</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Statements/return\">return</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Statements/switch\">switch</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Statements/throw\">throw</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Statements/try...catch\">try...catch</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Statements/var\">var</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Statements/while\">while</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Statements/with\">with</a><abbr class=\"icon icon-deprecated\" title=\"Deprecated. Not for use in new websites.\">\n<span class=\"visually-hidden\">Deprecated</span>\n</abbr></li></ol></details></li><li class=\"toggle\"><details><summary>Functions</summary><ol><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Functions\">Overview</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions\">Arrow function expressions</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Functions/Default_parameters\">Default parameters</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Functions/get\">get</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Functions/Method_definitions\">Method definitions</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Functions/rest_parameters\">Rest parameters</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Functions/set\">set</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Functions/arguments\">The arguments object</a></li></ol></details></li><li class=\"toggle\"><details><summary>Classes</summary><ol><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Classes\">Overview</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Classes/constructor\">constructor</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Classes/extends\">extends</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Classes/Private_properties\">Private properties</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Classes/Public_class_fields\">Public class fields</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Classes/static\">static</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Classes/Static_initialization_blocks\">Static initialization blocks</a></li></ol></details></li><li class=\"toggle\"><details><summary>Regular expressions</summary><ol><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Regular_expressions\">Overview</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Backreference\">Backreference: \\1, \\2</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Capturing_group\">Capturing group: (...)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Character_class_escape\">Character class escape: \\d, \\D, \\w, \\W, \\s, \\S</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Character_class\">Character class: [...], [^...]</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Character_escape\">Character escape: \\n, \\u{...}</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Disjunction\">Disjunction: |</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Input_boundary_assertion\">Input boundary assertion: ^, $</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Literal_character\">Literal character: a, b</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Lookahead_assertion\">Lookahead assertion: (?=...), (?!...)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Lookbehind_assertion\">Lookbehind assertion: (?<=...), (?<!...)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Modifier\">Modifier: (?ims-ims:...)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Named_backreference\">Named backreference: \\k<name></a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Named_capturing_group\">Named capturing group: (?<name>...)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Non-capturing_group\">Non-capturing group: (?:...)</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Quantifier\">Quantifier: *, +, ?, {n}, {n,}, {n,m}</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Unicode_character_class_escape\">Unicode character class escape: \\p{...}, \\P{...}</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Wildcard\">Wildcard: .</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Word_boundary_assertion\">Word boundary assertion: \\b, \\B</a></li></ol></details></li><li class=\"toggle\"><details><summary>Errors</summary><ol><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors\">Overview</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Promise_any_all_rejected\">AggregateError: No Promise in Promise.any was resolved</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Property_access_denied\">Error: Permission denied to access property \"x\"</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Too_much_recursion\">InternalError: too much recursion</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Not_a_valid_code_point\">RangeError: argument is not a valid code point</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/BigInt_division_by_zero\">RangeError: BigInt division by zero</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/BigInt_negative_exponent\">RangeError: BigInt negative exponent</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Form_must_be_one_of\">RangeError: form must be one of 'NFC', 'NFD', 'NFKC', or 'NFKD'</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Invalid_array_length\">RangeError: invalid array length</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Invalid_date\">RangeError: invalid date</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Precision_range\">RangeError: precision is out of range</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Bad_radix\">RangeError: radix must be an integer</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Resulting_string_too_large\">RangeError: repeat count must be less than infinity</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Negative_repetition_count\">RangeError: repeat count must be non-negative</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Cant_be_converted_to_BigInt_because_it_isnt_an_integer\">RangeError: x can't be converted to BigInt because it isn't an integer</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Not_defined\">ReferenceError: \"x\" is not defined</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Undeclared_var\">ReferenceError: assignment to undeclared variable \"x\"</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Cant_access_lexical_declaration_before_init\">ReferenceError: can't access lexical declaration 'X' before initialization</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Deprecated_caller_or_arguments_usage\">ReferenceError: deprecated caller or arguments usage</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Super_not_called\">ReferenceError: must call super constructor before using 'this' in derived class constructor</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Super_called_twice\">ReferenceError: super() called twice in derived class constructor</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Bad_strict_arguments_eval\">SyntaxError: 'arguments'/'eval' can't be defined or assigned to in strict mode code</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Deprecated_octal_literal\">SyntaxError: \"0\"-prefixed octal literals are deprecated</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Strict_non_simple_params\">SyntaxError: \"use strict\" not allowed in function with non-simple parameters</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Reserved_identifier\">SyntaxError: \"x\" is a reserved identifier</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Regex_backslash_at_end_of_pattern\">SyntaxError: \\ at end of pattern</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Invalid_for-of_initializer\">SyntaxError: a declaration in the head of a for-of loop can't have an initializer</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Delete_in_strict_mode\">SyntaxError: applying the 'delete' operator to an unqualified name is deprecated</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Arguments_not_allowed\">SyntaxError: arguments is not valid in fields</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Bad_await\">SyntaxError: await is only valid in async functions, async generators and modules</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/await_yield_in_parameter\">SyntaxError: await/yield expression can't be used in parameter</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Cant_use_nullish_coalescing_unparenthesized\">SyntaxError: cannot use `??` unparenthesized within `||` and `&&` expressions</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Regex_character_class_escape_in_class_range\">SyntaxError: character class escape cannot be used in class range in regular expression</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Bad_continue\">SyntaxError: continue must be inside loop</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Regex_duplicate_capture_group_name\">SyntaxError: duplicate capture group name in regular expression</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Duplicate_parameter\">SyntaxError: duplicate formal argument x</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Invalid_for-in_initializer\">SyntaxError: for-in loop head declarations may not have initializers</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Unnamed_function_statement\">SyntaxError: function statement requires a name</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Function_label\">SyntaxError: functions cannot be labelled</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Either_be_both_static_or_non-static\">SyntaxError: getter and setter for private name #x should either be both static or non-static</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Getter_no_arguments\">SyntaxError: getter functions must have no arguments</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Identifier_after_number\">SyntaxError: identifier starts immediately after numeric literal</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Illegal_character\">SyntaxError: illegal character</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/import_decl_module_top_level\">SyntaxError: import declarations may only appear at top level of a module</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Regex_incomplete_quantifier\">SyntaxError: incomplete quantifier in regular expression</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Invalid_assignment_left-hand_side\">SyntaxError: invalid assignment left-hand side</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Invalid_BigInt_syntax\">SyntaxError: invalid BigInt syntax</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Regex_invalid_capture_group_name\">SyntaxError: invalid capture group name in regular expression</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Regex_invalid_char_in_class\">SyntaxError: invalid character in class in regular expression</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Regex_invalid_class_set_operation\">SyntaxError: invalid class set operation in regular expression</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Regex_invalid_decimal_escape\">SyntaxError: invalid decimal escape in regular expression</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Regex_invalid_identity_escape\">SyntaxError: invalid identity escape in regular expression</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Regex_invalid_named_capture_reference\">SyntaxError: invalid named capture reference in regular expression</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Regex_invalid_property_name\">SyntaxError: invalid property name in regular expression</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Regex_invalid_range_in_character_class\">SyntaxError: invalid range in character class</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Regex_invalid_group\">SyntaxError: invalid regexp group</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Bad_regexp_flag\">SyntaxError: invalid regular expression flag \"x\"</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Regex_invalid_unicode_escape\">SyntaxError: invalid unicode escape in regular expression</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/JSON_bad_parse\">SyntaxError: JSON.parse: bad parsing</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Label_not_found\">SyntaxError: label not found</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Missing_colon_after_property_id\">SyntaxError: missing : after property id</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Missing_parenthesis_after_argument_list\">SyntaxError: missing ) after argument list</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Missing_parenthesis_after_condition\">SyntaxError: missing ) after condition</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Missing_bracket_after_list\">SyntaxError: missing ] after element list</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Missing_curly_after_function_body\">SyntaxError: missing } after function body</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Missing_curly_after_property_list\">SyntaxError: missing } after property list</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Missing_initializer_in_const\">SyntaxError: missing = in const declaration</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Missing_formal_parameter\">SyntaxError: missing formal parameter</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Missing_name_after_dot_operator\">SyntaxError: missing name after . operator</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/No_variable_name\">SyntaxError: missing variable name</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Regex_negated_char_class_with_strings\">SyntaxError: negated character class with strings in regular expression</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Bad_new_optional\">SyntaxError: new keyword cannot be used with an optional chain</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Regex_nothing_to_repeat\">SyntaxError: nothing to repeat</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Regex_numbers_out_of_order_in_quantifier\">SyntaxError: numbers out of order in {} quantifier.</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Deprecated_octal_escape_sequence\">SyntaxError: octal escape sequences can't be used in untagged template literals or in strict mode code</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Parameter_after_rest_parameter\">SyntaxError: parameter after rest parameter</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Cant_delete_private_fields\">SyntaxError: private fields can't be deleted</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Duplicate_proto\">SyntaxError: property name __proto__ appears more than once in object literal</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Regex_raw_bracket\">SyntaxError: raw bracket is not allowed in regular expression with unicode flag</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Redeclared_parameter\">SyntaxError: redeclaration of formal parameter \"x\"</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Undeclared_private_field_or_method\">SyntaxError: reference to undeclared private field or method #x</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Rest_with_default\">SyntaxError: rest parameter may not have a default</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Bad_return\">SyntaxError: return not in function</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Setter_one_argument\">SyntaxError: setter functions must have one argument</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/String_literal_EOL\">SyntaxError: string literal contains an unescaped line break</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Bad_super_call\">SyntaxError: super() is only valid in derived class constructors</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Bad_optional_template\">SyntaxError: tagged template cannot be used with optional chain</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Hash_outside_class\">SyntaxError: Unexpected '#' used outside of class body</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Unexpected_token\">SyntaxError: Unexpected token</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Bad_break\">SyntaxError: unlabeled break must be inside loop or switch</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Unparenthesized_unary_expr_lhs_exponentiation\">SyntaxError: unparenthesized unary expression can't appear on the left-hand side of '**'</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Bad_super_prop\">SyntaxError: use of super property/member accesses only valid within methods or eval code within methods</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Deprecated_source_map_pragma\">SyntaxError: Using //@ to indicate sourceURL pragmas is deprecated. Use //# instead</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/is_not_iterable\">TypeError: 'x' is not iterable</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Unexpected_type\">TypeError: \"x\" is (not) \"y\"</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Not_a_constructor\">TypeError: \"x\" is not a constructor</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Not_a_function\">TypeError: \"x\" is not a function</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/No_non-null_object\">TypeError: \"x\" is not a non-null object</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Read-only\">TypeError: \"x\" is read-only</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/BigInt_not_serializable\">TypeError: BigInt value can't be serialized in JSON</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Builtin_ctor_no_new\">TypeError: calling a builtin X constructor without new is forbidden</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Get_set_missing_private\">TypeError: can't access/set private field or method: object is not the right class</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Cant_assign_to_property\">TypeError: can't assign to property \"x\" on \"y\": not an object</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Cant_convert_BigInt_to_number\">TypeError: can't convert BigInt to number</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Cant_convert_x_to_BigInt\">TypeError: can't convert x to BigInt</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Cant_define_property_object_not_extensible\">TypeError: can't define property \"x\": \"obj\" is not extensible</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Non_configurable_array_element\">TypeError: can't delete non-configurable array element</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Cant_redefine_property\">TypeError: can't redefine non-configurable property \"x\"</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Cant_set_prototype\">TypeError: can't set prototype of this object</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Cyclic_prototype\">TypeError: can't set prototype: it would cause a prototype chain cycle</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/in_operator_no_object\">TypeError: cannot use 'in' operator to search for 'x' in 'y'</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Class_ctor_no_new\">TypeError: class constructors must be invoked with 'new'</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Cyclic_object_value\">TypeError: cyclic object value</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Invalid_derived_return\">TypeError: derived class constructor returned invalid value x</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Private_setter_only\">TypeError: getting private setter-only property</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Private_double_initialization\">TypeError: Initializing an object twice is an error with private fields/methods</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/invalid_right_hand_side_instanceof_operand\">TypeError: invalid 'instanceof' operand 'x'</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Array_sort_argument\">TypeError: invalid Array.prototype.sort argument</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Invalid_const_assignment\">TypeError: invalid assignment to const \"x\"</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Constructor_cant_be_used_directly\">TypeError: Iterator/AsyncIterator constructor can't be used directly</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Requires_global_RegExp\">TypeError: matchAll/replaceAll must be called with a global RegExp</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/More_arguments_needed\">TypeError: More arguments needed</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/No_properties\">TypeError: null/undefined has no properties</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Cant_delete\">TypeError: property \"x\" is non-configurable and can't be deleted</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Reduce_of_empty_array_with_no_initial_value\">TypeError: Reduce of empty array with no initial value</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Getter_only\">TypeError: setting getter-only property \"x\"</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Key_not_weakly_held\">TypeError: WeakSet key/WeakMap value 'x' must be an object or an unregistered symbol</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Called_on_incompatible_type\">TypeError: X.prototype.y called on incompatible type</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Malformed_URI\">URIError: malformed URI sequence</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Already_has_pragma\">Warning: -file- is being assigned a //# sourceMappingURL, but already has one</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Errors/Stmt_after_return\">Warning: unreachable code after return statement</a></li></ol></details></li><li class=\"toggle\"><details open=\"\"><summary>Misc</summary><ol><li><a href=\"/en-US/docs/Web/JavaScript/JavaScript_technologies_overview\">JavaScript technologies overview</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Lexical_grammar\">Lexical grammar</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Iteration_protocols\">Iteration protocols</a></li><li><em><a href=\"/en-US/docs/Web/JavaScript/Reference/Strict_mode\" aria-current=\"page\">Strict mode</a></em></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Template_literals\">Template literals</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Trailing_commas\">Trailing commas</a></li><li><a href=\"/en-US/docs/Web/JavaScript/Reference/Deprecated_and_obsolete_features\">Deprecated features</a></li></ol></details></li></ol>","source":{"folder":"en-us/web/javascript/reference/strict_mode","github_url":"https://github.com/mdn/content/blob/main/files/en-us/web/javascript/reference/strict_mode/index.md","last_commit_url":"https://github.com/mdn/content/commit/99a44005f9d90b831526e9d0ffd7803cdafa4a72","filename":"index.md"},"summary":"JavaScript's strict mode is a way to opt in to a restricted variant of JavaScript, thereby implicitly opting-out of \"sloppy mode\". Strict mode isn't just a subset: it intentionally has different semantics from normal code. Strict mode code and non-strict mode code can coexist, so scripts can opt into strict mode incrementally.","title":"Strict mode","toc":[{"text":"Invoking strict mode","id":"invoking_strict_mode"},{"text":"Changes in strict mode","id":"changes_in_strict_mode"},{"text":"Transitioning to strict mode","id":"transitioning_to_strict_mode"},{"text":"Specifications","id":"specifications"},{"text":"See also","id":"see_also"}],"pageType":"guide"}}</script></body></html>