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class="sidebar-inner"><header class="sidebar-actions"><section class="sidebar-filter-container"><div 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">在本文中</h2></header><ul class="document-toc-list"><li class="document-toc-item "><a class="document-toc-link" href="#尝试一下">尝试一下</a></li><li class="document-toc-item "><a class="document-toc-link" href="#语法">语法</a></li><li class="document-toc-item "><a class="document-toc-link" href="#描述">描述</a></li><li class="document-toc-item "><a class="document-toc-link" href="#永远不要使用_eval！">永远不要使用 <code>eval</code>！</a></li><li class="document-toc-item "><a class="document-toc-link" href="#示例">示例</a></li><li class="document-toc-item "><a class="document-toc-link" href="#规范">规范</a></li><li class="document-toc-item "><a class="document-toc-link" href="#浏览器兼容性">浏览器兼容性</a></li><li class="document-toc-item "><a class="document-toc-link" href="#firefox_相关">Firefox 相关</a></li><li class="document-toc-item "><a class="document-toc-link" href="#参见">参见</a></li></ul></section></div></div><div class="sidebar-body"> <ol> <li class="section"><a href="/zh-CN/docs/Web/JavaScript">JavaScript</a></li> <li class="section">教程</li> <li class="toggle"> <details> <summary>快速入门</summary> <ol> <li><a href="/zh-CN/docs/Learn/Getting_started_with_the_web/JavaScript_basics">JavaScript 基础知识</a></li> <li><a href="/zh-CN/docs/Learn/JavaScript/First_steps">JavaScript 第一步</a></li> <li><a href="/zh-CN/docs/Learn/JavaScript/Building_blocks">构建 JavaScript 代码块</a></li> <li><a href="/zh-CN/docs/Learn/JavaScript/Objects">介绍 JavaScript 对象</a></li> </ol> </details> </li> <li class="toggle"> <details> <summary>JavaScript 指南</summary> <ol> <li><a href="/zh-CN/docs/Web/JavaScript/Guide/Introduction">介绍</a></li> <li><a href="/zh-CN/docs/Web/JavaScript/Guide/Grammar_and_types">语法和数据类型</a></li> <li><a href="/zh-CN/docs/Web/JavaScript/Guide/Control_flow_and_error_handling">流程控制与错误处理</a></li> <li><a href="/zh-CN/docs/Web/JavaScript/Guide/Loops_and_iteration">循环与迭代</a></li> <li><a href="/zh-CN/docs/Web/JavaScript/Guide/Functions">函数</a></li> <li><a href="/zh-CN/docs/Web/JavaScript/Guide/Expressions_and_operators">表达式与运算符</a></li> <li><a href="/zh-CN/docs/Web/JavaScript/Guide/Numbers_and_dates">数字和日期</a></li> <li><a href="/zh-CN/docs/Web/JavaScript/Guide/Text_formatting">文本格式化</a></li> <li><a href="/zh-CN/docs/Web/JavaScript/Guide/Regular_expressions">正则表达式</a></li> <li><a href="/zh-CN/docs/Web/JavaScript/Guide/Indexed_collections">索引集合类</a></li> <li><a href="/zh-CN/docs/Web/JavaScript/Guide/Keyed_collections">带键的集合</a></li> <li><a href="/zh-CN/docs/Web/JavaScript/Guide/Working_with_objects">使用对象</a></li> <li><a href="/zh-CN/docs/Web/JavaScript/Guide/Using_classes">使用类</a></li> <li><a href="/zh-CN/docs/Web/JavaScript/Guide/Using_promises">使用 Promise</a></li> <li><a href="/zh-CN/docs/Web/JavaScript/Guide/Typed_arrays">JavaScript 类型化数组</a></li> <li><a href="/zh-CN/docs/Web/JavaScript/Guide/Iterators_and_generators">迭代器和生成器</a></li> <li><a href="/zh-CN/docs/Web/JavaScript/Guide/Meta_programming">元编程</a></li> <li><a href="/zh-CN/docs/Web/JavaScript/Guide/Modules">JavaScript 模块</a></li> </ol> </details> </li> <li class="toggle"> <details> <summary>中级</summary> <ol> <li><a href="/zh-CN/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks">客户端 JavaScript 框架</a></li> <li><a href="/zh-CN/docs/Learn/JavaScript/Client-side_web_APIs">客户端 Web API</a></li> <li><a href="/zh-CN/docs/Web/JavaScript/Language_overview">语言概述</a></li> <li><a href="/zh-CN/docs/Web/JavaScript/Data_structures">JavaScript 数据结构</a></li> <li><a href="/zh-CN/docs/Web/JavaScript/Equality_comparisons_and_sameness">如何正确判断相等性</a></li> <li><a href="/zh-CN/docs/Web/JavaScript/Enumerability_and_ownership_of_properties">属性的可枚举性和所有权</a></li> <li><a href="/zh-CN/docs/Web/JavaScript/Closures">闭包</a></li> </ol> </details> </li> <li class="toggle"> <details> <summary>高级</summary> <ol> <li><a href="/zh-CN/docs/Web/JavaScript/Inheritance_and_the_prototype_chain">继承和原型链</a></li> <li><a href="/zh-CN/docs/Web/JavaScript/Memory_management">内存管理</a></li> <li><a href="/zh-CN/docs/Web/JavaScript/Event_loop">并发模型与事件循环</a></li> </ol> </details> </li> <li class="section"><a href="/zh-CN/docs/Web/JavaScript/Reference">参考</a></li> <li class="toggle"> <details open=""> <summary>内置对象</summary> <ol><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects">JavaScript 标准内置对象</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/AggregateError">AggregateError</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Array">Array</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/ArrayBuffer">ArrayBuffer</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/AsyncFunction">AsyncFunction</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/AsyncGenerator">AsyncGenerator</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/AsyncGeneratorFunction">AsyncGeneratorFunction</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/AsyncIterator">AsyncIterator</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Atomics">Atomics</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/BigInt">BigInt</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/BigInt64Array">BigInt64Array</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/BigUint64Array">BigUint64Array</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Boolean">Boolean</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/DataView">DataView</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Date">Date</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/decodeURI">decodeURI()</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/decodeURIComponent">decodeURIComponent()</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/encodeURI">encodeURI()</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/encodeURIComponent">encodeURIComponent()</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Error">Error</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/escape">escape()</a><abbr class="icon icon-deprecated" title="已弃用。请不要在新的网站中使用。"> <span class="visually-hidden">已弃用</span> </abbr></li><li><em><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/eval" aria-current="page">eval()</a></em></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/EvalError">EvalError</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/FinalizationRegistry">FinalizationRegistry</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Float16Array">Float16Array</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Float32Array">Float32Array</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Float64Array">Float64Array</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Function">Function</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Generator">Generator</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/GeneratorFunction">GeneratorFunction</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/globalThis">globalThis</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Infinity">Infinity</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Int16Array">Int16Array</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Int32Array">Int32Array</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Int8Array">Int8Array</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/InternalError">InternalError</a><abbr class="icon icon-nonstandard" title="非标准。请在使用前检查跨浏览器支持。"> <span class="visually-hidden">非标准</span> </abbr></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Intl">Intl</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/isFinite">isFinite()</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/isNaN">isNaN()</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Iterator">Iterator</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/JSON">JSON</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Map">Map</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Math">Math</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/NaN">NaN</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Number">Number</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Object">Object</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/parseFloat">parseFloat</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/parseInt">parseInt</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Promise">Promise</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Proxy">Proxy</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/RangeError">RangeError</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/ReferenceError">ReferenceError</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Reflect">Reflect</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/RegExp">RegExp</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Set">Set</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/SharedArrayBuffer">SharedArrayBuffer</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/String">String</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Symbol">Symbol</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/SyntaxError">SyntaxError</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/TypedArray">TypedArray</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/TypeError">TypeError</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Uint16Array">Uint16Array</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Uint32Array">Uint32Array</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Uint8Array">Uint8Array</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Uint8ClampedArray">Uint8ClampedArray</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/undefined">undefined</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/unescape">unescape()</a><abbr class="icon icon-deprecated" title="已弃用。请不要在新的网站中使用。"> <span class="visually-hidden">已弃用</span> </abbr></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/URIError">URIError</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/WeakMap">WeakMap</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/WeakRef">WeakRef</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/WeakSet">WeakSet</a></li></ol> </details> </li> <li class="toggle"> <details> <summary>表达式和运算符</summary> <ol><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators">表达式和运算符</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Addition">加法（+）</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Addition_assignment">加法赋值（+=）</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Assignment">赋值（=）</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/async_function">异步函数（async function）表达式</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/async_function*">async function* 表达式</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/await">await</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Bitwise_AND">按位与（&）</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Bitwise_AND_assignment">按位与赋值（&=）</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Bitwise_NOT">按位非（~）</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Bitwise_OR">按位或（|）</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Bitwise_OR_assignment">按位或赋值（|=）</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Bitwise_XOR">按位异或（^）</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Bitwise_XOR_assignment">按位异或赋值 (^=)</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/class">类表达式</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Comma_operator">逗号运算符（,）</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Conditional_operator">条件（三元）运算符</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Decrement">自减（--）</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/delete">delete 运算符</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Destructuring_assignment">解构赋值</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Division">除法（/）</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Division_assignment">除法赋值（/=）</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Equality">相等（==）</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Exponentiation">幂（**）</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Exponentiation_assignment">幂赋值（**=）</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/function">函数表达式</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/function*">function* 表达式</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Greater_than">大于（>）</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Greater_than_or_equal">大于等于（>=）</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Grouping">圆括号运算符</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/import.meta">import.meta</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/import">import()</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/in">in</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Increment">自增（++）</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Inequality">不相等（!=）</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/instanceof">instanceof</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Left_shift">左移 (<<)</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Left_shift_assignment">左移赋值（<<=）</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Less_than">小于（<）</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Less_than_or_equal">小于等于（<=）</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Logical_AND">逻辑与（&&）</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Logical_AND_assignment">逻辑与赋值（&&=）</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Logical_NOT">逻辑非（!）</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Logical_OR">逻辑或（||）</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Logical_OR_assignment">逻辑或赋值（||=）</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Multiplication">乘法（*）</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Multiplication_assignment">乘法赋值（*=）</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/new">new</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/new.target">new.target</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/null">null</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Nullish_coalescing_assignment">逻辑空赋值（??=）</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Nullish_coalescing">空值合并运算符（??）</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Object_initializer">对象初始化器</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Operator_precedence">运算符优先级</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Optional_chaining">可选链运算符（?.）</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Property_accessors">属性访问器</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Remainder">取余（%）</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Remainder_assignment">取余赋值（%=）</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Right_shift">右移（>>）</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Right_shift_assignment">右移赋值（>>=）</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Spread_syntax">展开语法</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Strict_equality">严格相等（===）</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Strict_inequality">严格不相等（!==）</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Subtraction">减法（-）</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Subtraction_assignment">减法赋值（-=）</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/super">super</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/this">this</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/typeof">typeof</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Unary_negation">一元减（-）</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Unary_plus">一元加（+）</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Unsigned_right_shift">无符号右移（>>>）</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Unsigned_right_shift_assignment">无符号右移赋值（>>>=）</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/void">void 运算符</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/yield">yield</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/yield*">yield*</a></li></ol> </details> </li> <li class="toggle"> <details> <summary>语句和声明</summary> <ol><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Statements">语句和声明</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Statements/async_function">async function</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Statements/async_function*">async function*</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Statements/block">块语句</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Statements/break">break</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Statements/class">class</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Statements/const">const</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Statements/continue">continue</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Statements/debugger">debugger</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Statements/do...while">do...while</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Statements/Empty">空语句</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Statements/export">export</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Statements/Expression_statement">表达式语句</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Statements/for">for</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Statements/for-await...of">for await...of</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Statements/for...in">for...in</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Statements/for...of">for...of</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Statements/function">function</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Statements/function*">function*</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Statements/if...else">if...else</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Statements/import">import</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Statements/label">label</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Statements/let">let</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Statements/return">return</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Statements/switch">switch</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Statements/throw">throw</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Statements/try...catch">try...catch</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Statements/var">var</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Statements/while">while</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Statements/with">with</a><abbr class="icon icon-deprecated" title="已弃用。请不要在新的网站中使用。"> <span class="visually-hidden">已弃用</span> </abbr></li></ol> </details> </li> <li class="toggle"> <details> <summary>函数</summary> <ol><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Functions">函数</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Functions/Arrow_functions">箭头函数表达式</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Functions/Default_parameters">默认参数值</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Functions/get">getter</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Functions/Method_definitions">方法的定义</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Functions/rest_parameters">剩余参数</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Functions/set">setter</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Functions/arguments">Arguments 对象</a></li></ol> </details> </li> <li class="toggle"> <details> <summary>类</summary> <ol><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Classes">类</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Classes/constructor">构造函数</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Classes/extends">extends</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Classes/Private_properties">私有属性</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Classes/Public_class_fields">公有类字段</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Classes/static">static</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Classes/Static_initialization_blocks">静态初始化块</a></li></ol> </details> </li> <li class="toggle"> <details> <summary>正则表达式</summary> <ol><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Regular_expressions">正则表达式</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Regular_expressions/Backreference">Backreference: \1, \2</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Regular_expressions/Capturing_group">Capturing group: (...)</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Regular_expressions/Character_class_escape">Character class escape: \d, \D, \w, \W, \s, \S</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Regular_expressions/Character_class">Character class: [...], [^...]</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Regular_expressions/Character_escape">Character escape: \n, \u{...}</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Regular_expressions/Disjunction">Disjunction: |</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Regular_expressions/Input_boundary_assertion">Input boundary assertion: ^, $</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Regular_expressions/Literal_character">Literal character: a, b</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Regular_expressions/Lookahead_assertion">Lookahead assertion: (?=...), (?!...)</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Regular_expressions/Lookbehind_assertion">Lookbehind assertion: (?<=...), (?<!...)</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Regular_expressions/Modifier">Modifier: (?ims-ims:...)</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Regular_expressions/Named_backreference">Named backreference: \k<name></a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Regular_expressions/Named_capturing_group">Named capturing group: (?<name>...)</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Regular_expressions/Non-capturing_group">Non-capturing group: (?:...)</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Regular_expressions/Quantifier">量词：*、+、?、{n}、{n,}、{n,m}</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Regular_expressions/Unicode_character_class_escape">Unicode 字符类转义：\p{...}、\P{...}</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Regular_expressions/Wildcard">Wildcard: .</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Regular_expressions/Word_boundary_assertion">Word boundary assertion: \b, \B</a></li></ol> </details> </li> <li class="toggle"> <details> <summary>错误</summary> <ol><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors">JavaScript 错误参考</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Promise_any_all_rejected">AggregateError: No Promise in Promise.any was resolved</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Property_access_denied">Error: Permission denied to access property "x"</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Too_much_recursion">InternalError: too much recursion</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Not_a_valid_code_point">RangeError: argument is not a valid code point</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/BigInt_division_by_zero">RangeError: BigInt division by zero</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/BigInt_negative_exponent">RangeError: BigInt negative exponent</a></li><li><a href="/zh-CN/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="/zh-CN/docs/Web/JavaScript/Reference/Errors/Invalid_array_length">RangeError: invalid array length</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Invalid_date">RangeError: invalid date</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Precision_range">RangeError: precision is out of range</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Bad_radix">RangeError: radix must be an integer</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Resulting_string_too_large">RangeError: repeat count must be less than infinity</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Negative_repetition_count">RangeError: repeat count must be non-negative</a></li><li><a href="/zh-CN/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="/zh-CN/docs/Web/JavaScript/Reference/Errors/Not_defined">ReferenceError: "x" is not defined</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Undeclared_var">ReferenceError: assignment to undeclared variable "x"</a></li><li><a href="/zh-CN/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="/zh-CN/docs/Web/JavaScript/Reference/Errors/Deprecated_caller_or_arguments_usage">ReferenceError: deprecated caller or arguments usage</a></li><li><a href="/zh-CN/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="/zh-CN/docs/Web/JavaScript/Reference/Errors/Super_called_twice">ReferenceError: super() called twice in derived class constructor</a></li><li><a href="/zh-CN/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="/zh-CN/docs/Web/JavaScript/Reference/Errors/Deprecated_octal_literal">SyntaxError: "0"-prefixed octal literals are deprecated</a></li><li><a href="/zh-CN/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="/zh-CN/docs/Web/JavaScript/Reference/Errors/Reserved_identifier">SyntaxError: "x" is a reserved identifier</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Regex_backslash_at_end_of_pattern">SyntaxError: \ at end of pattern</a></li><li><a href="/zh-CN/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="/zh-CN/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="/zh-CN/docs/Web/JavaScript/Reference/Errors/Arguments_not_allowed">SyntaxError: arguments is not valid in fields</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Bad_await">SyntaxError: await is only valid in async functions, async generators and modules</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/await_yield_in_parameter">SyntaxError: await/yield expression can't be used in parameter</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Cant_use_nullish_coalescing_unparenthesized">SyntaxError: cannot use `??` unparenthesized within `||` and `&&` expressions</a></li><li><a href="/zh-CN/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="/zh-CN/docs/Web/JavaScript/Reference/Errors/Bad_continue">SyntaxError: continue must be inside loop</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Regex_duplicate_capture_group_name">SyntaxError: duplicate capture group name in regular expression</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Duplicate_parameter">SyntaxError: duplicate formal argument x</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Invalid_for-in_initializer">SyntaxError: for-in loop head declarations may not have initializers</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Unnamed_function_statement">SyntaxError: function statement requires a name</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Function_label">SyntaxError: functions cannot be labelled</a></li><li><a href="/zh-CN/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="/zh-CN/docs/Web/JavaScript/Reference/Errors/Getter_no_arguments">SyntaxError: getter functions must have no arguments</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Identifier_after_number">SyntaxError: identifier starts immediately after numeric literal</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Illegal_character">SyntaxError: illegal character</a></li><li><a href="/zh-CN/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="/zh-CN/docs/Web/JavaScript/Reference/Errors/Regex_incomplete_quantifier">SyntaxError: incomplete quantifier in regular expression</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Invalid_assignment_left-hand_side">SyntaxError: invalid assignment left-hand side</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Invalid_BigInt_syntax">SyntaxError: invalid BigInt syntax</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Regex_invalid_capture_group_name">SyntaxError: invalid capture group name in regular expression</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Regex_invalid_char_in_class">SyntaxError: invalid character in class in regular expression</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Regex_invalid_class_set_operation">SyntaxError: invalid class set operation in regular expression</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Regex_invalid_decimal_escape">SyntaxError: invalid decimal escape in regular expression</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Regex_invalid_identity_escape">SyntaxError: invalid identity escape in regular expression</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Regex_invalid_named_capture_reference">SyntaxError: invalid named capture reference in regular expression</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Regex_invalid_property_name">SyntaxError: invalid property name in regular expression</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Regex_invalid_range_in_character_class">SyntaxError: invalid range in character class</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Regex_invalid_group">SyntaxError: invalid regexp group</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Bad_regexp_flag">SyntaxError: invalid regular expression flag "x"</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Regex_invalid_unicode_escape">SyntaxError: invalid unicode escape in regular expression</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/JSON_bad_parse">SyntaxError: JSON.parse: bad parsing</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Label_not_found">SyntaxError: label not found</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Missing_colon_after_property_id">SyntaxError: missing : after property id</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Missing_parenthesis_after_argument_list">SyntaxError: missing ) after argument list</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Missing_parenthesis_after_condition">SyntaxError: missing ) after condition</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Missing_bracket_after_list">SyntaxError: missing ] after element list</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Missing_curly_after_function_body">SyntaxError: missing } after function body</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Missing_curly_after_property_list">SyntaxError: missing } after property list</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Missing_initializer_in_const">SyntaxError: missing = in const declaration</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Missing_formal_parameter">SyntaxError: missing formal parameter</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Missing_name_after_dot_operator">SyntaxError: missing name after . operator</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/No_variable_name">SyntaxError: missing variable name</a></li><li><a href="/zh-CN/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="/zh-CN/docs/Web/JavaScript/Reference/Errors/Bad_new_optional">SyntaxError: new keyword cannot be used with an optional chain</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Regex_nothing_to_repeat">SyntaxError: nothing to repeat</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Regex_numbers_out_of_order_in_quantifier">SyntaxError: numbers out of order in {} quantifier.</a></li><li><a href="/zh-CN/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="/zh-CN/docs/Web/JavaScript/Reference/Errors/Parameter_after_rest_parameter">SyntaxError: parameter after rest parameter</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Cant_delete_private_fields">SyntaxError: private fields can't be deleted</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Duplicate_proto">SyntaxError: property name __proto__ appears more than once in object literal</a></li><li><a href="/zh-CN/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="/zh-CN/docs/Web/JavaScript/Reference/Errors/Redeclared_parameter">SyntaxError: redeclaration of formal parameter "x"</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Undeclared_private_field_or_method">SyntaxError: reference to undeclared private field or method #x</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Rest_with_default">SyntaxError: rest parameter may not have a default</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Bad_return">SyntaxError: return not in function</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Setter_one_argument">SyntaxError: setter functions must have one argument</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/String_literal_EOL">SyntaxError: string literal contains an unescaped line break</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Bad_super_call">SyntaxError: super() is only valid in derived class constructors</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Bad_optional_template">SyntaxError: tagged template cannot be used with optional chain</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Hash_outside_class">SyntaxError: Unexpected '#' used outside of class body</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Unexpected_token">SyntaxError: Unexpected token</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Bad_break">SyntaxError: unlabeled break must be inside loop or switch</a></li><li><a href="/zh-CN/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="/zh-CN/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="/zh-CN/docs/Web/JavaScript/Reference/Errors/Deprecated_source_map_pragma">SyntaxError: Using //@ to indicate sourceURL pragmas is deprecated. Use //# instead</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/is_not_iterable">TypeError: 'x' is not iterable</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Unexpected_type">TypeError: "x" is (not) "y"</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Not_a_constructor">TypeError: "x" is not a constructor</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Not_a_function">TypeError: "x" is not a function</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/No_non-null_object">TypeError: "x" is not a non-null object</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Read-only">TypeError: "x" is read-only</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/BigInt_not_serializable">TypeError: BigInt value can't be serialized in JSON</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Builtin_ctor_no_new">TypeError: calling a builtin X constructor without new is forbidden</a></li><li><a href="/zh-CN/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="/zh-CN/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="/zh-CN/docs/Web/JavaScript/Reference/Errors/Cant_convert_BigInt_to_number">TypeError: can't convert BigInt to number</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Cant_convert_x_to_BigInt">TypeError: can't convert x to BigInt</a></li><li><a href="/zh-CN/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="/zh-CN/docs/Web/JavaScript/Reference/Errors/Non_configurable_array_element">TypeError: can't delete non-configurable array element</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Cant_redefine_property">TypeError: can't redefine non-configurable property "x"</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Cant_set_prototype">TypeError: can't set prototype of this object</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Cyclic_prototype">TypeError: can't set prototype: it would cause a prototype chain cycle</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/in_operator_no_object">TypeError: invalid 'in' operand "x"</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Class_ctor_no_new">TypeError: class constructors must be invoked with 'new'</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Cyclic_object_value">TypeError: cyclic object value</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Invalid_derived_return">TypeError: derived class constructor returned invalid value x</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Private_setter_only">TypeError: getting private setter-only property</a></li><li><a href="/zh-CN/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="/zh-CN/docs/Web/JavaScript/Reference/Errors/invalid_right_hand_side_instanceof_operand">TypeError: invalid 'instanceof' operand 'x'</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Array_sort_argument">TypeError: invalid Array.prototype.sort argument</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Invalid_const_assignment">TypeError: invalid assignment to const "x"</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Constructor_cant_be_used_directly">TypeError: Iterator/AsyncIterator constructor can't be used directly</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Requires_global_RegExp">TypeError: matchAll/replaceAll must be called with a global RegExp</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/More_arguments_needed">TypeError: More arguments needed</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/No_properties">TypeError: "x" has no properties</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Cant_delete">TypeError: property "x" is non-configurable and can't be deleted</a></li><li><a href="/zh-CN/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="/zh-CN/docs/Web/JavaScript/Reference/Errors/Getter_only">TypeError: setting getter-only property "x"</a></li><li><a href="/zh-CN/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="/zh-CN/docs/Web/JavaScript/Reference/Errors/Called_on_incompatible_type">X.prototype.y called on incompatible type</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Malformed_URI">URIError: malformed URI sequence</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Already_has_pragma">Warning: -file- is being assigned a //# sourceMappingURL, but already has one</a></li><li><a href="/zh-CN/docs/Web/JavaScript/Reference/Errors/Stmt_after_return">Warning: unreachable code after return statement</a></li></ol> </details> </li> <li class="toggle"> <details> <summary>更多</summary> <ol> <li><a href="/zh-CN/docs/Web/JavaScript/JavaScript_technologies_overview">JavaScript 技术概要</a></li> <li><a href="/zh-CN/docs/Web/JavaScript/Reference/Lexical_grammar">词法文法</a></li> <li><a href="/zh-CN/docs/Web/JavaScript/Reference/Iteration_protocols">迭代协议</a></li> <li><a href="/zh-CN/docs/Web/JavaScript/Reference/Strict_mode">严格模式</a></li> <li><a href="/zh-CN/docs/Web/JavaScript/Reference/Template_literals">模板字符串</a></li> <li><a href="/zh-CN/docs/Web/JavaScript/Reference/Trailing_commas">尾后逗号</a></li> <li><a href="/zh-CN/docs/Web/JavaScript/Reference/Deprecated_and_obsolete_features">已废弃的特性</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">在本文中</h2></header><ul class="document-toc-list"><li class="document-toc-item "><a class="document-toc-link" href="#尝试一下">尝试一下</a></li><li class="document-toc-item "><a class="document-toc-link" href="#语法">语法</a></li><li class="document-toc-item "><a class="document-toc-link" href="#描述">描述</a></li><li class="document-toc-item "><a class="document-toc-link" href="#永远不要使用_eval！">永远不要使用 <code>eval</code>！</a></li><li class="document-toc-item "><a class="document-toc-link" href="#示例">示例</a></li><li class="document-toc-item "><a class="document-toc-link" href="#规范">规范</a></li><li class="document-toc-item "><a class="document-toc-link" href="#浏览器兼容性">浏览器兼容性</a></li><li class="document-toc-item "><a class="document-toc-link" href="#firefox_相关">Firefox 相关</a></li><li class="document-toc-item "><a class="document-toc-link" href="#参见">参见</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="zh-CN"><header><h1>eval()</h1><details class="baseline-indicator high"><summary><span class="indicator" role="img" aria-label="Baseline Check"></span><h2>Baseline <span class="not-bold">Widely available</span></h2><div class="browsers"><span class="engine" title="Supported in Chrome and Edge"><span class="browser chrome supported" role="img" aria-label="Chrome check"></span><span class="browser edge supported" role="img" aria-label="Edge check"></span></span><span class="engine" title="Supported in Firefox"><span class="browser firefox supported" role="img" aria-label="Firefox check"></span></span><span class="engine" title="Supported in Safari"><span class="browser safari supported" role="img" aria-label="Safari check"></span></span></div><span class="icon icon-chevron "></span></summary><div class="extra"><p>This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.</p><ul><li><a href="/zh-CN/docs/Glossary/Baseline/Compatibility" data-glean="baseline_link_learn_more" target="_blank" class="learn-more">Learn more</a></li><li><a href="#浏览器兼容性" data-glean="baseline_link_bcd_table">See full compatibility</a></li><li><a href="https://survey.alchemer.com/s3/7634825/MDN-baseline-feedback?page=%2Fzh-CN%2Fdocs%2FWeb%2FJavaScript%2FReference%2FGlobal_Objects%2Feval&level=high" data-glean="baseline_link_feedback" class="feedback-link" target="_blank" rel="noreferrer">Report feedback</a></li></ul></div></details></header><div class="section-content"><p><strong><code>eval()</code></strong> 函数会将传入的字符串当做 JavaScript 代码进行执行。</p></div><section aria-labelledby="尝试一下"><h2 id="尝试一下"><a href="#尝试一下">尝试一下</a></h2><div class="section-content"><iframe class="interactive is-js-height" height="200" src="https://interactive-examples.mdn.mozilla.net/pages/js/globalprops-eval.html" title="MDN Web Docs Interactive Example" allow="clipboard-write" loading="lazy"></iframe></div></section><section aria-labelledby="语法"><h2 id="语法"><a href="#语法">语法</a></h2><div class="section-content"><pre class="brush: plain notranslate">eval(string) </pre></div></section><section aria-labelledby="参数"><h3 id="参数"><a href="#参数">参数</a></h3><div class="section-content"><dl> <dt id="string"><a href="#string"><code>string</code></a></dt> <dd> <p>一个表示 JavaScript 表达式、语句或一系列语句的字符串。表达式可以包含变量与已存在对象的属性。</p> </dd> </dl></div></section><section aria-labelledby="返回值"><h3 id="返回值"><a href="#返回值">返回值</a></h3><div class="section-content"><p>返回字符串中代码的返回值。如果返回值为空，则返回 <a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/undefined"><code>undefined</code></a>。</p></div></section><section aria-labelledby="描述"><h2 id="描述"><a href="#描述">描述</a></h2><div class="section-content"><p><code>eval()</code> 是全局对象的一个函数属性。</p> <p><code>eval()</code> 的参数是一个字符串。如果字符串表示的是表达式，<code>eval()</code> 会对表达式进行求值。如果参数表示一个或多个 JavaScript 语句，那么<code>eval()</code> 就会执行这些语句。不需要用 <code>eval()</code> 来执行一个算术表达式：因为 JavaScript 可以自动为算术表达式求值。</p> <p>如果你以字符串的形式构造了算术表达式，那么可以在后面用 <code>eval()</code>对它求值。例如，假设你有一个变量 <code>x</code>，你可以通过将表达式的字符串值（例如 <code>3 * x + 2</code>）赋值给一个变量，然后在你的代码后面的其他地方调用 <code>eval()</code>，来推迟涉及 <code>x</code> 的表达式的求值。</p> <p>如果 <code>eval()</code> 的参数不是字符串， <code>eval()</code> 会将参数原封不动地返回。在下面的例子中，<code>String</code> 构造器被指定，而 <code>eval()</code> 返回了 <code>String</code> 对象而不是执行字符串。</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>eval(new String("2 + 2")); // 返回了包含"2 + 2"的字符串对象 eval("2 + 2"); // returns 4 </code></pre></div> <p>你可以使用一些通用的方法来绕过这个限制，例如使用 <code>toString()</code>。</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>var expression = new String("2 + 2"); eval(expression.toString()); </code></pre></div> <p>如果你间接的使用 <code>eval()</code>，比如通过一个引用来调用它，而不是直接的调用 <code>eval</code>。从 <a href="https://www.ecma-international.org/ecma-262/5.1/#sec-10.4.2" class="external" target="_blank">ECMAScript 5</a> 起，它工作在全局作用域下，而不是局部作用域中。这就意味着，例如，下面的代码的作用声明创建一个全局函数，并且 <code>eval</code> 中的这些代码在执行期间不能在被调用的作用域中访问局部变量。</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>function test() { var x = 2, y = 4; console.log(eval("x + y")); // 直接调用，使用本地作用域，结果是 6 var geval = eval; // 等价于在全局作用域调用 console.log(geval("x + y")); // 间接调用，使用全局作用域，throws ReferenceError 因为 `x` 未定义 (0, eval)("x + y"); // 另一个间接调用的例子 } </code></pre></div></div></section><section aria-labelledby="永远不要使用_eval！"><h2 id="永远不要使用_eval！"><a href="#永远不要使用_eval！">永远不要使用 <code>eval</code>！</a></h2><div class="section-content"><p><code>eval()</code> 是一个危险的函数，它使用与调用者相同的权限执行代码。如果你用 <code>eval()</code> 运行的字符串代码被恶意方（不怀好意的人）修改，你最终可能会在你的网页/扩展程序的权限下，在用户计算机上运行恶意代码。更重要的是，第三方代码可以看到某一个 <code>eval()</code> 被调用时的作用域，这也有可能导致一些不同方式的攻击。相似的 <a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Function"><code>Function</code></a> 就不容易被攻击。</p> <p><code>eval()</code> 通常比其他替代方法更慢，因为它必须调用 JS 解释器，而许多其他结构则可被现代 JS 引擎进行优化。</p> <p>此外，现代 JavaScript 解释器将 JavaScript 转换为机器代码。这意味着任何变量命名的概念都会被删除。因此，任意一个 eval 的使用都会强制浏览器进行冗长的变量名称查找，以确定变量在机器代码中的位置并设置其值。另外，新内容将会通过 <code>eval()</code> 引进给变量，比如更改该变量的类型，因此会强制浏览器重新执行所有已经生成的机器代码以进行补偿。但是（谢天谢地）存在一个非常好的 eval 替代方法：只需使用 <a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Function">window.Function</a>。这有个例子方便你了解如何将<code>eval()</code>的使用转变为<code>Function()</code>。</p> <p>使用 eval 的糟糕代码：</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 looseJsonParse(obj) { return eval("(" + obj + ")"); } console.log(looseJsonParse("{a:(4-1), b:function(){}, c:new Date()}")); </code></pre></div> <p>不用 eval 的更好的代码：</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js example-good notranslate"><code>function looseJsonParse(obj) { return Function('"use strict";return (' + obj + ")")(); } console.log(looseJsonParse("{a:(4-1), b:function(){}, c:new Date()}")); </code></pre></div> <p>比较上面的两个代码片段，两个代码片段似乎是以相同的方式工作，但再想一想：eval 的这个代码的速度要慢得多。注意<code>c: new Date()</code>在执行体中。在没有 eval 的函数中，对象在全局范围内被用来进行计算，因此浏览器可以放心的假设 <code>Date</code> 是来自 <code>window.Date</code> 的而不是一个名为 <code>Date</code> 的局部变量。然而，在使用 <code>eval()</code> 的代码中，浏览器不能假设这一点，因为如果你的代码是下面这个：</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 Date(n) { return [ "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday", ][n % 7 || 0]; } function looseJsonParse(obj) { return eval("(" + obj + ")"); } console.log(looseJsonParse("{a:(4-1), b:function(){}, c:new Date()}")); </code></pre></div> <p>因此，在 <code>eval()</code> 版本的代码中，浏览器被迫进行高代价的查找调用以检查是否存在名为 <code>Date()</code> 的任何局部变量。与 <code>Function()</code> 相比，这是非常低效的。</p> <p>在类似的情况下，如果你确实希望能够从 <code>Function()</code> 内部的代码调用 <code>Date</code> 函数，该怎么办？你应该躲避并退回到 <code>eval()</code> 吗？绝对不是，永远不要这么做。而是尝试下面的方法。</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js example-good notranslate"><code>function Date(n) { return [ "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday", ][n % 7 || 0]; } function runCodeWithDateFunction(obj) { return Function('"use strict";return (' + obj + ")")()(Date); } console.log(runCodeWithDateFunction("function(Date){ return Date(5) }")); </code></pre></div> <p>由于三重嵌套函数，上面的代码似乎效率低下，但让我们分析一下上述有效方法的好处：</p> <ol> <li> <p>它使得传递给<code>runCodeWithDateFunction</code>的字符串中的代码更少；</p> </li> <li> <p>函数调用开销很小，使得代码尺寸小得多，值得获益；</p> </li> <li> <p><code>Function()</code>更容易让你的代码利用特性修饰 <code>"use strict"</code>；</p> </li> <li> <p>代码不使用 <code>eval()</code>，使其比其他方式快几个数量级。</p> </li> </ol> <p>最后，我们来看看简化版。使用如上所示的<code>Function()</code>，你可以更有效地缩小传递给<code>runCodeWithDateFunction</code>的代码字符串，因为函数参数名称也可以缩小，如下面的缩小代码所示。</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>console.log( Function('"use strict";return(function(a){return a(5)})')()(function (a) { return "Monday Tuesday Wednesday Thursday Friday Saturday Sunday".split( " ", )[a % 7 || 0]; }), ); </code></pre></div> <p>对于常见用例，<code>eval()</code>或<code>Function()</code>还有更安全 (而且更快！) 的替代方案。</p></div></section><section aria-labelledby="访问成员属性"><h3 id="访问成员属性"><a href="#访问成员属性">访问成员属性</a></h3><div class="section-content"><p>你不应该去使用 <code>eval()</code> 来将属性名字转化为属性。考虑下面的这个例子，被访问对象的属性在它被执行之前都会未知的。这里可以用 eval 处理：</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>var obj = { a: 20, b: 30 }; var propName = getPropName(); // 返回 "a" 或 "b" eval("var result = obj." + propsName); </code></pre></div> <p>但是，这里并不是必须得使用 <code>eval()</code>。事实上，这里并不建议这样使用。可以使用 <a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Property_accessors">属性访问器</a> 进行代替，它更快、更安全：</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>var obj = { a: 20, b: 30 }; var propName = getPropName(); // 返回 "a" 或 "b" var result = obj[propName]; // obj[ "a" ] 与 obj.a 等价 </code></pre></div> <p>你还可以使用这个方法去访问子代的属性。如下：</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>var obj = { a: { b: { c: 0 } } }; var propPath = getPropPath(); // 例如返回 "a.b.c" eval("var result = obj." + propPath); </code></pre></div> <p>这里，可以通过分割属性路径、循环遍历不同的属性，来避免 <code>eval()</code>：</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>function getDescendantProp(obj, desc) { var arr = desc.split("."); while (arr.length) { obj = obj[arr.shift()]; } return obj; } var obj = { a: { b: { c: 0 } } }; var propPath = getPropPath(); // 例如返回 "a.b.c" var result = getDescendantProp(obj, propPath); </code></pre></div> <p>同样的方法也可实现设置子代的属性值：</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>function setDescendantProp(obj, desc, value) { var arr = desc.split("."); while (arr.length > 1) { obj = obj[arr.shift()]; } return (obj[arr[0]] = value); } var obj = { a: { b: { c: 0 } } }; var propPath = getPropPath(); // 例如，返回 "a.b.c" var result = setDescendantProp(obj, propPath, 1); // a.b.c 值为 1 </code></pre></div></div></section><section aria-labelledby="使用函数而非代码段"><h3 id="使用函数而非代码段"><a href="#使用函数而非代码段">使用函数而非代码段</a></h3><div class="section-content"><p>JavaScript 拥有 <a href="/zh-CN/docs/Glossary/First-class_Function">first-class functions</a>，这意味着你可以将函数直接作为参数传递给其他接口，将他们保存在变量中或者对象的属性中，等等。很多 DOM 的 API 都用这种思路进行设计，你也可以（或者应该）这样子设计你的代码：</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>// 代替 setTimeout(" ... ", 1000) 写法： setTimeout(function() { ... }, 1000); // 代替 elt.setAttribute("onclick", "...") 写法： elt.addEventListener('click', function() { ... } , false); </code></pre></div> <p><a href="/zh-CN/docs/Web/JavaScript/Closures">闭包</a> 也有助于创建参数化函数而不用连接字符串。</p></div></section><section aria-labelledby="解析_json（将字符串转化为_javascript_对象）"><h3 id="解析_json（将字符串转化为_javascript_对象）"><a href="#解析_json（将字符串转化为_javascript_对象）">解析 JSON（将字符串转化为 JavaScript 对象）</a></h3><div class="section-content"><p>如果你在调用 <code>eval()</code> 传入的字符串参数中包含数据（如：一个数组“[1,2,3]”）而不是代码，你应该考虑将其转换为 <a href="/zh-CN/docs/Glossary/JSON">JSON</a> 对象，这允许你用 JavaScript 语法的子集来表示数据。<a class="page-not-created" title="此文档尚未被撰写，期待你的贡献！" data-href="/zh-CN/docs/Downloading_JSON_and_JavaScript_in_extensions">在扩展中下载 JSON 和 JavaScript</a></p> <p>提示：因为 JSON 语法子集相对于 JavaScript 语法子集比较有局限性，很多在 JavaScript 中可用的特性在 JSON 中就不起作用了。比如，后缀逗号在 JSON 中不支持，并且对象中的属性名在 JSON 中必须用引号括起来。请务必使用 JSON 序列化方法来生成稍后将被解析为 JSON 的字符串。</p></div></section><section aria-labelledby="尽量传递数据而非代码"><h3 id="尽量传递数据而非代码"><a href="#尽量传递数据而非代码">尽量传递数据而非代码</a></h3><div class="section-content"><p>例如，设计为抓取网页内容的扩展，可能会在 XPath 中定义抓取规则，而不是在 JavaScript 代码中。</p></div></section><section aria-labelledby="以有限权限运行代码"><h3 id="以有限权限运行代码"><a href="#以有限权限运行代码">以有限权限运行代码</a></h3><div class="section-content"><p>如果你必须执行这段代码，应考虑以更低的权限运行。此建议主要适用于扩展和 XUL 应用程序，可以使用 <a class="page-not-created" title="此文档尚未被撰写，期待你的贡献！" data-href="/zh-CN/docs/Components.utils.evalInSandbox">Components.utils.evalInSandbox</a> 做到降低权限。</p></div></section><section aria-labelledby="示例"><h2 id="示例"><a href="#示例">示例</a></h2><div class="section-content"></div></section><section aria-labelledby="使用_eval"><h3 id="使用_eval"><a href="#使用_eval">使用 <code>eval</code></a></h3><div class="section-content"><p>在下面的代码中，两种包含了 <code>eval()</code> 的声明都返回了 42。第一种是对字符串 "<code>x + y + 1</code>" 求值；第二种是对字符串 "<code>42</code>" 求值。</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>var x = 2; var y = 39; var z = "42"; eval("x + y + 1"); // returns 42 eval(z); // returns 42 </code></pre></div></div></section><section aria-labelledby="使用_eval_执行一串_javascript_语句"><h3 id="使用_eval_执行一串_javascript_语句"><a href="#使用_eval_执行一串_javascript_语句">使用 <code>eval</code> 执行一串 JavaScript 语句</a></h3><div class="section-content"><p>下面的例子使用 <code>eval()</code> 来执行 <code>str</code> 字符串。这个字符串包含了如果 <code>x</code> 等于 5，就打开一个 Alert 对话框并对 <code>z</code> 赋值 42，否则就对 <code>z</code> 赋值 0 的 JavaScript 语句。当第二个声明被执行，<code>eval()</code> 将会令字符串被执行，并最终返回赋值给 <code>z</code> 的 42。</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>var x = 5; var str = "if (x == 5) {console.log('z is 42'); z = 42;} else z = 0;"; console.log("z is ", eval(str)); </code></pre></div> <p>如果你定义了多个值，则会返回最后一个值。</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>var x = 5; var str = "if (x == 5) {console.log('z is 42'); z = 42; x = 420; } else z = 0;"; console.log("x is ", eval(str)); // z is 42 x is 420 </code></pre></div></div></section><section aria-labelledby="返回值_2"><h3 id="返回值_2"><a href="#返回值_2">返回值</a></h3><div class="section-content"><p><code>eval</code> 返回最后一个表达式的值。</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>var str = "if ( a ) { 1 + 1; } else { 1 + 2; }"; var a = true; var b = eval(str); // returns 2 console.log("b is : " + b); a = false; b = eval(str); // returns 3 console.log("b is : " + b); </code></pre></div></div></section><section aria-labelledby="eval_中函数作为字符串被定义需要“（”和“）”作为前缀和后缀"><h3 id="eval_中函数作为字符串被定义需要“（”和“）”作为前缀和后缀"><a href="#eval_中函数作为字符串被定义需要“（”和“）”作为前缀和后缀"><code>eval</code> 中函数作为字符串被定义需要“（”和“）”作为前缀和后缀</a></h3><div class="section-content"><div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>var fctStr1 = "function a() {}"; var fctStr2 = "(function a() {})"; var fct1 = eval(fctStr1); // 返回 undefined var fct2 = eval(fctStr2); // 返回一个函数 </code></pre></div></div></section><h2 id="规范"><a href="#规范">规范</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/global-object.html#sec-eval-x">ECMAScript Language Specification <br/><small># sec-eval-x</small></a></td></tr></tbody></table><h2 id="浏览器兼容性"><a href="#浏览器兼容性">浏览器兼容性</a></h2><p>BCD tables only load in the browser<noscript> with JavaScript enabled. Enable JavaScript to view data.</noscript></p><section aria-labelledby="firefox_相关"><h2 id="firefox_相关"><a href="#firefox_相关">Firefox 相关</a></h2><div class="section-content"><ul> <li>从历史上看，<code>eval()</code> 有一个可选的第二个参数，指定上下文执行对象。这个参数是非标准的，并且明确地从 Firefox 4 中删除。请参阅 <a href="https://bugzil.la/531675" class="external" target="_blank">Firefox bug 531675</a>。</li> </ul></div></section><section aria-labelledby="参见"><h2 id="参见"><a href="#参见">参见</a></h2><div class="section-content"><ul> <li><a href="/zh-CN/docs/Web/JavaScript/Reference/Operators/Property_accessors">属性访问器</a></li> <li><a href="/zh-CN/docs/Mozilla/Add-ons/WebExtensions/Content_scripts#%e5%9c%a8_content_script_%e4%b8%ad%e4%bd%bf%e7%94%a8_eval">WebExtension：在 content script 中使用 eval</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 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href=\"/zh-CN/docs/Learn/Getting_started_with_the_web/JavaScript_basics\">JavaScript 基础知识</a></li>\n <li><a href=\"/zh-CN/docs/Learn/JavaScript/First_steps\">JavaScript 第一步</a></li>\n <li><a href=\"/zh-CN/docs/Learn/JavaScript/Building_blocks\">构建 JavaScript 代码块</a></li>\n <li><a href=\"/zh-CN/docs/Learn/JavaScript/Objects\">介绍 JavaScript 对象</a></li>\n </ol>\n </details>\n </li>\n <li class=\"toggle\">\n <details>\n <summary>JavaScript 指南</summary>\n <ol>\n <li><a href=\"/zh-CN/docs/Web/JavaScript/Guide/Introduction\">介绍</a></li>\n <li><a href=\"/zh-CN/docs/Web/JavaScript/Guide/Grammar_and_types\">语法和数据类型</a></li>\n <li><a href=\"/zh-CN/docs/Web/JavaScript/Guide/Control_flow_and_error_handling\">流程控制与错误处理</a></li>\n <li><a href=\"/zh-CN/docs/Web/JavaScript/Guide/Loops_and_iteration\">循环与迭代</a></li>\n <li><a href=\"/zh-CN/docs/Web/JavaScript/Guide/Functions\">函数</a></li>\n <li><a href=\"/zh-CN/docs/Web/JavaScript/Guide/Expressions_and_operators\">表达式与运算符</a></li>\n <li><a href=\"/zh-CN/docs/Web/JavaScript/Guide/Numbers_and_dates\">数字和日期</a></li>\n <li><a href=\"/zh-CN/docs/Web/JavaScript/Guide/Text_formatting\">文本格式化</a></li>\n <li><a href=\"/zh-CN/docs/Web/JavaScript/Guide/Regular_expressions\">正则表达式</a></li>\n <li><a href=\"/zh-CN/docs/Web/JavaScript/Guide/Indexed_collections\">索引集合类</a></li>\n <li><a href=\"/zh-CN/docs/Web/JavaScript/Guide/Keyed_collections\">带键的集合</a></li>\n <li><a href=\"/zh-CN/docs/Web/JavaScript/Guide/Working_with_objects\">使用对象</a></li>\n <li><a href=\"/zh-CN/docs/Web/JavaScript/Guide/Using_classes\">使用类</a></li>\n <li><a href=\"/zh-CN/docs/Web/JavaScript/Guide/Using_promises\">使用 Promise</a></li>\n <li><a href=\"/zh-CN/docs/Web/JavaScript/Guide/Typed_arrays\">JavaScript 类型化数组</a></li>\n <li><a href=\"/zh-CN/docs/Web/JavaScript/Guide/Iterators_and_generators\">迭代器和生成器</a></li>\n <li><a href=\"/zh-CN/docs/Web/JavaScript/Guide/Meta_programming\">元编程</a></li>\n <li><a href=\"/zh-CN/docs/Web/JavaScript/Guide/Modules\">JavaScript 模块</a></li>\n </ol>\n </details>\n </li>\n <li class=\"toggle\">\n <details>\n <summary>中级</summary>\n <ol>\n <li><a href=\"/zh-CN/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks\">客户端 JavaScript 框架</a></li>\n <li><a href=\"/zh-CN/docs/Learn/JavaScript/Client-side_web_APIs\">客户端 Web API</a></li>\n <li><a href=\"/zh-CN/docs/Web/JavaScript/Language_overview\">语言概述</a></li>\n <li><a href=\"/zh-CN/docs/Web/JavaScript/Data_structures\">JavaScript 数据结构</a></li>\n <li><a href=\"/zh-CN/docs/Web/JavaScript/Equality_comparisons_and_sameness\">如何正确判断相等性</a></li>\n <li><a href=\"/zh-CN/docs/Web/JavaScript/Enumerability_and_ownership_of_properties\">属性的可枚举性和所有权</a></li>\n <li><a href=\"/zh-CN/docs/Web/JavaScript/Closures\">闭包</a></li>\n </ol>\n </details>\n </li>\n <li class=\"toggle\">\n <details>\n <summary>高级</summary>\n <ol>\n <li><a href=\"/zh-CN/docs/Web/JavaScript/Inheritance_and_the_prototype_chain\">继承和原型链</a></li>\n <li><a href=\"/zh-CN/docs/Web/JavaScript/Memory_management\">内存管理</a></li>\n <li><a href=\"/zh-CN/docs/Web/JavaScript/Event_loop\">并发模型与事件循环</a></li>\n </ol>\n </details>\n </li>\n <li class=\"section\"><a href=\"/zh-CN/docs/Web/JavaScript/Reference\">参考</a></li>\n <li class=\"toggle\">\n <details open=\"\">\n <summary>内置对象</summary>\n <ol><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects\">JavaScript 标准内置对象</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/AggregateError\">AggregateError</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Array\">Array</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/ArrayBuffer\">ArrayBuffer</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/AsyncFunction\">AsyncFunction</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/AsyncGenerator\">AsyncGenerator</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/AsyncGeneratorFunction\">AsyncGeneratorFunction</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/AsyncIterator\">AsyncIterator</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Atomics\">Atomics</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/BigInt\">BigInt</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/BigInt64Array\">BigInt64Array</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/BigUint64Array\">BigUint64Array</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Boolean\">Boolean</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/DataView\">DataView</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Date\">Date</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/decodeURI\">decodeURI()</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/decodeURIComponent\">decodeURIComponent()</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/encodeURI\">encodeURI()</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/encodeURIComponent\">encodeURIComponent()</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Error\">Error</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/escape\">escape()</a><abbr class=\"icon icon-deprecated\" title=\"已弃用。请不要在新的网站中使用。\">\n <span class=\"visually-hidden\">已弃用</span>\n</abbr></li><li><em><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/eval\" aria-current=\"page\">eval()</a></em></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/EvalError\">EvalError</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/FinalizationRegistry\">FinalizationRegistry</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Float16Array\">Float16Array</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Float32Array\">Float32Array</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Float64Array\">Float64Array</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Function\">Function</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Generator\">Generator</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/GeneratorFunction\">GeneratorFunction</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/globalThis\">globalThis</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Infinity\">Infinity</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Int16Array\">Int16Array</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Int32Array\">Int32Array</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Int8Array\">Int8Array</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/InternalError\">InternalError</a><abbr class=\"icon icon-nonstandard\" title=\"非标准。请在使用前检查跨浏览器支持。\">\n <span class=\"visually-hidden\">非标准</span>\n</abbr></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Intl\">Intl</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/isFinite\">isFinite()</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/isNaN\">isNaN()</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Iterator\">Iterator</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/JSON\">JSON</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Map\">Map</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Math\">Math</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/NaN\">NaN</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Number\">Number</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Object\">Object</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/parseFloat\">parseFloat</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/parseInt\">parseInt</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Promise\">Promise</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Proxy\">Proxy</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/RangeError\">RangeError</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/ReferenceError\">ReferenceError</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Reflect\">Reflect</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/RegExp\">RegExp</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Set\">Set</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/SharedArrayBuffer\">SharedArrayBuffer</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/String\">String</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Symbol\">Symbol</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/SyntaxError\">SyntaxError</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/TypedArray\">TypedArray</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/TypeError\">TypeError</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Uint16Array\">Uint16Array</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Uint32Array\">Uint32Array</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Uint8Array\">Uint8Array</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Uint8ClampedArray\">Uint8ClampedArray</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/undefined\">undefined</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/unescape\">unescape()</a><abbr class=\"icon icon-deprecated\" title=\"已弃用。请不要在新的网站中使用。\">\n <span class=\"visually-hidden\">已弃用</span>\n</abbr></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/URIError\">URIError</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/WeakMap\">WeakMap</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/WeakRef\">WeakRef</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/WeakSet\">WeakSet</a></li></ol>\n </details>\n </li>\n <li class=\"toggle\">\n <details>\n <summary>表达式和运算符</summary>\n <ol><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators\">表达式和运算符</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Addition\">加法（+）</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Addition_assignment\">加法赋值（+=）</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Assignment\">赋值（=）</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/async_function\">异步函数（async function）表达式</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/async_function*\">async function* 表达式</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/await\">await</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Bitwise_AND\">按位与（&）</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Bitwise_AND_assignment\">按位与赋值（&=）</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Bitwise_NOT\">按位非（~）</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Bitwise_OR\">按位或（|）</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Bitwise_OR_assignment\">按位或赋值（|=）</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Bitwise_XOR\">按位异或（^）</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Bitwise_XOR_assignment\">按位异或赋值 (^=)</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/class\">类表达式</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Comma_operator\">逗号运算符（,）</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Conditional_operator\">条件（三元）运算符</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Decrement\">自减（--）</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/delete\">delete 运算符</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Destructuring_assignment\">解构赋值</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Division\">除法（/）</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Division_assignment\">除法赋值（/=）</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Equality\">相等（==）</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Exponentiation\">幂（**）</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Exponentiation_assignment\">幂赋值（**=）</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/function\">函数表达式</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/function*\">function* 表达式</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Greater_than\">大于（>）</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Greater_than_or_equal\">大于等于（>=）</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Grouping\">圆括号运算符</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/import.meta\">import.meta</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/import\">import()</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/in\">in</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Increment\">自增（++）</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Inequality\">不相等（!=）</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/instanceof\">instanceof</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Left_shift\">左移 (<<)</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Left_shift_assignment\">左移赋值（<<=）</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Less_than\">小于（<）</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Less_than_or_equal\">小于等于（<=）</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Logical_AND\">逻辑与（&&）</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Logical_AND_assignment\">逻辑与赋值（&&=）</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Logical_NOT\">逻辑非（!）</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Logical_OR\">逻辑或（||）</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Logical_OR_assignment\">逻辑或赋值（||=）</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Multiplication\">乘法（*）</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Multiplication_assignment\">乘法赋值（*=）</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/new\">new</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/new.target\">new.target</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/null\">null</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Nullish_coalescing_assignment\">逻辑空赋值（??=）</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Nullish_coalescing\">空值合并运算符（??）</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Object_initializer\">对象初始化器</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Operator_precedence\">运算符优先级</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Optional_chaining\">可选链运算符（?.）</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Property_accessors\">属性访问器</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Remainder\">取余（%）</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Remainder_assignment\">取余赋值（%=）</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Right_shift\">右移（>>）</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Right_shift_assignment\">右移赋值（>>=）</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Spread_syntax\">展开语法</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Strict_equality\">严格相等（===）</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Strict_inequality\">严格不相等（!==）</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Subtraction\">减法（-）</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Subtraction_assignment\">减法赋值（-=）</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/super\">super</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/this\">this</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/typeof\">typeof</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Unary_negation\">一元减（-）</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Unary_plus\">一元加（+）</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Unsigned_right_shift\">无符号右移（>>>）</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Unsigned_right_shift_assignment\">无符号右移赋值（>>>=）</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/void\">void 运算符</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/yield\">yield</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/yield*\">yield*</a></li></ol>\n </details>\n </li>\n <li class=\"toggle\">\n <details>\n <summary>语句和声明</summary>\n <ol><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Statements\">语句和声明</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Statements/async_function\">async function</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Statements/async_function*\">async function*</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Statements/block\">块语句</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Statements/break\">break</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Statements/class\">class</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Statements/const\">const</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Statements/continue\">continue</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Statements/debugger\">debugger</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Statements/do...while\">do...while</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Statements/Empty\">空语句</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Statements/export\">export</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Statements/Expression_statement\">表达式语句</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Statements/for\">for</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Statements/for-await...of\">for await...of</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Statements/for...in\">for...in</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Statements/for...of\">for...of</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Statements/function\">function</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Statements/function*\">function*</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Statements/if...else\">if...else</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Statements/import\">import</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Statements/label\">label</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Statements/let\">let</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Statements/return\">return</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Statements/switch\">switch</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Statements/throw\">throw</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Statements/try...catch\">try...catch</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Statements/var\">var</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Statements/while\">while</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Statements/with\">with</a><abbr class=\"icon icon-deprecated\" title=\"已弃用。请不要在新的网站中使用。\">\n <span class=\"visually-hidden\">已弃用</span>\n</abbr></li></ol>\n </details>\n </li>\n <li class=\"toggle\">\n <details>\n <summary>函数</summary>\n <ol><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Functions\">函数</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Functions/Arrow_functions\">箭头函数表达式</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Functions/Default_parameters\">默认参数值</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Functions/get\">getter</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Functions/Method_definitions\">方法的定义</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Functions/rest_parameters\">剩余参数</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Functions/set\">setter</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Functions/arguments\">Arguments 对象</a></li></ol>\n </details>\n </li>\n <li class=\"toggle\">\n <details>\n <summary>类</summary>\n <ol><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Classes\">类</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Classes/constructor\">构造函数</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Classes/extends\">extends</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Classes/Private_properties\">私有属性</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Classes/Public_class_fields\">公有类字段</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Classes/static\">static</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Classes/Static_initialization_blocks\">静态初始化块</a></li></ol>\n </details>\n </li>\n <li class=\"toggle\">\n <details>\n <summary>正则表达式</summary>\n <ol><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Regular_expressions\">正则表达式</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Regular_expressions/Backreference\">Backreference: \\1, \\2</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Regular_expressions/Capturing_group\">Capturing group: (...)</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Regular_expressions/Character_class_escape\">Character class escape: \\d, \\D, \\w, \\W, \\s, \\S</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Regular_expressions/Character_class\">Character class: [...], [^...]</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Regular_expressions/Character_escape\">Character escape: \\n, \\u{...}</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Regular_expressions/Disjunction\">Disjunction: |</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Regular_expressions/Input_boundary_assertion\">Input boundary assertion: ^, $</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Regular_expressions/Literal_character\">Literal character: a, b</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Regular_expressions/Lookahead_assertion\">Lookahead assertion: (?=...), (?!...)</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Regular_expressions/Lookbehind_assertion\">Lookbehind assertion: (?<=...), (?<!...)</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Regular_expressions/Modifier\">Modifier: (?ims-ims:...)</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Regular_expressions/Named_backreference\">Named backreference: \\k<name></a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Regular_expressions/Named_capturing_group\">Named capturing group: (?<name>...)</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Regular_expressions/Non-capturing_group\">Non-capturing group: (?:...)</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Regular_expressions/Quantifier\">量词：*、+、?、{n}、{n,}、{n,m}</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Regular_expressions/Unicode_character_class_escape\">Unicode 字符类转义：\\p{...}、\\P{...}</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Regular_expressions/Wildcard\">Wildcard: .</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Regular_expressions/Word_boundary_assertion\">Word boundary assertion: \\b, \\B</a></li></ol>\n </details>\n </li>\n <li class=\"toggle\">\n <details>\n <summary>错误</summary>\n <ol><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors\">JavaScript 错误参考</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Promise_any_all_rejected\">AggregateError: No Promise in Promise.any was resolved</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Property_access_denied\">Error: Permission denied to access property \"x\"</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Too_much_recursion\">InternalError: too much recursion</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Not_a_valid_code_point\">RangeError: argument is not a valid code point</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/BigInt_division_by_zero\">RangeError: BigInt division by zero</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/BigInt_negative_exponent\">RangeError: BigInt negative exponent</a></li><li><a href=\"/zh-CN/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=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Invalid_array_length\">RangeError: invalid array length</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Invalid_date\">RangeError: invalid date</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Precision_range\">RangeError: precision is out of range</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Bad_radix\">RangeError: radix must be an integer</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Resulting_string_too_large\">RangeError: repeat count must be less than infinity</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Negative_repetition_count\">RangeError: repeat count must be non-negative</a></li><li><a href=\"/zh-CN/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=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Not_defined\">ReferenceError: \"x\" is not defined</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Undeclared_var\">ReferenceError: assignment to undeclared variable \"x\"</a></li><li><a href=\"/zh-CN/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=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Deprecated_caller_or_arguments_usage\">ReferenceError: deprecated caller or arguments usage</a></li><li><a href=\"/zh-CN/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=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Super_called_twice\">ReferenceError: super() called twice in derived class constructor</a></li><li><a href=\"/zh-CN/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=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Deprecated_octal_literal\">SyntaxError: \"0\"-prefixed octal literals are deprecated</a></li><li><a href=\"/zh-CN/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=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Reserved_identifier\">SyntaxError: \"x\" is a reserved identifier</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Regex_backslash_at_end_of_pattern\">SyntaxError: \\ at end of pattern</a></li><li><a href=\"/zh-CN/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=\"/zh-CN/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=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Arguments_not_allowed\">SyntaxError: arguments is not valid in fields</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Bad_await\">SyntaxError: await is only valid in async functions, async generators and modules</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/await_yield_in_parameter\">SyntaxError: await/yield expression can't be used in parameter</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Cant_use_nullish_coalescing_unparenthesized\">SyntaxError: cannot use `??` unparenthesized within `||` and `&&` expressions</a></li><li><a href=\"/zh-CN/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=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Bad_continue\">SyntaxError: continue must be inside loop</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Regex_duplicate_capture_group_name\">SyntaxError: duplicate capture group name in regular expression</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Duplicate_parameter\">SyntaxError: duplicate formal argument x</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Invalid_for-in_initializer\">SyntaxError: for-in loop head declarations may not have initializers</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Unnamed_function_statement\">SyntaxError: function statement requires a name</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Function_label\">SyntaxError: functions cannot be labelled</a></li><li><a href=\"/zh-CN/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=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Getter_no_arguments\">SyntaxError: getter functions must have no arguments</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Identifier_after_number\">SyntaxError: identifier starts immediately after numeric literal</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Illegal_character\">SyntaxError: illegal character</a></li><li><a href=\"/zh-CN/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=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Regex_incomplete_quantifier\">SyntaxError: incomplete quantifier in regular expression</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Invalid_assignment_left-hand_side\">SyntaxError: invalid assignment left-hand side</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Invalid_BigInt_syntax\">SyntaxError: invalid BigInt syntax</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Regex_invalid_capture_group_name\">SyntaxError: invalid capture group name in regular expression</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Regex_invalid_char_in_class\">SyntaxError: invalid character in class in regular expression</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Regex_invalid_class_set_operation\">SyntaxError: invalid class set operation in regular expression</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Regex_invalid_decimal_escape\">SyntaxError: invalid decimal escape in regular expression</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Regex_invalid_identity_escape\">SyntaxError: invalid identity escape in regular expression</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Regex_invalid_named_capture_reference\">SyntaxError: invalid named capture reference in regular expression</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Regex_invalid_property_name\">SyntaxError: invalid property name in regular expression</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Regex_invalid_range_in_character_class\">SyntaxError: invalid range in character class</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Regex_invalid_group\">SyntaxError: invalid regexp group</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Bad_regexp_flag\">SyntaxError: invalid regular expression flag \"x\"</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Regex_invalid_unicode_escape\">SyntaxError: invalid unicode escape in regular expression</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/JSON_bad_parse\">SyntaxError: JSON.parse: bad parsing</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Label_not_found\">SyntaxError: label not found</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Missing_colon_after_property_id\">SyntaxError: missing : after property id</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Missing_parenthesis_after_argument_list\">SyntaxError: missing ) after argument list</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Missing_parenthesis_after_condition\">SyntaxError: missing ) after condition</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Missing_bracket_after_list\">SyntaxError: missing ] after element list</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Missing_curly_after_function_body\">SyntaxError: missing } after function body</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Missing_curly_after_property_list\">SyntaxError: missing } after property list</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Missing_initializer_in_const\">SyntaxError: missing = in const declaration</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Missing_formal_parameter\">SyntaxError: missing formal parameter</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Missing_name_after_dot_operator\">SyntaxError: missing name after . operator</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/No_variable_name\">SyntaxError: missing variable name</a></li><li><a href=\"/zh-CN/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=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Bad_new_optional\">SyntaxError: new keyword cannot be used with an optional chain</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Regex_nothing_to_repeat\">SyntaxError: nothing to repeat</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Regex_numbers_out_of_order_in_quantifier\">SyntaxError: numbers out of order in {} quantifier.</a></li><li><a href=\"/zh-CN/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=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Parameter_after_rest_parameter\">SyntaxError: parameter after rest parameter</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Cant_delete_private_fields\">SyntaxError: private fields can't be deleted</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Duplicate_proto\">SyntaxError: property name __proto__ appears more than once in object literal</a></li><li><a href=\"/zh-CN/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=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Redeclared_parameter\">SyntaxError: redeclaration of formal parameter \"x\"</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Undeclared_private_field_or_method\">SyntaxError: reference to undeclared private field or method #x</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Rest_with_default\">SyntaxError: rest parameter may not have a default</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Bad_return\">SyntaxError: return not in function</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Setter_one_argument\">SyntaxError: setter functions must have one argument</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/String_literal_EOL\">SyntaxError: string literal contains an unescaped line break</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Bad_super_call\">SyntaxError: super() is only valid in derived class constructors</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Bad_optional_template\">SyntaxError: tagged template cannot be used with optional chain</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Hash_outside_class\">SyntaxError: Unexpected '#' used outside of class body</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Unexpected_token\">SyntaxError: Unexpected token</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Bad_break\">SyntaxError: unlabeled break must be inside loop or switch</a></li><li><a href=\"/zh-CN/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=\"/zh-CN/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=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Deprecated_source_map_pragma\">SyntaxError: Using //@ to indicate sourceURL pragmas is deprecated. Use //# instead</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/is_not_iterable\">TypeError: 'x' is not iterable</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Unexpected_type\">TypeError: \"x\" is (not) \"y\"</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Not_a_constructor\">TypeError: \"x\" is not a constructor</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Not_a_function\">TypeError: \"x\" is not a function</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/No_non-null_object\">TypeError: \"x\" is not a non-null object</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Read-only\">TypeError: \"x\" is read-only</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/BigInt_not_serializable\">TypeError: BigInt value can't be serialized in JSON</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Builtin_ctor_no_new\">TypeError: calling a builtin X constructor without new is forbidden</a></li><li><a href=\"/zh-CN/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=\"/zh-CN/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=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Cant_convert_BigInt_to_number\">TypeError: can't convert BigInt to number</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Cant_convert_x_to_BigInt\">TypeError: can't convert x to BigInt</a></li><li><a href=\"/zh-CN/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=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Non_configurable_array_element\">TypeError: can't delete non-configurable array element</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Cant_redefine_property\">TypeError: can't redefine non-configurable property \"x\"</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Cant_set_prototype\">TypeError: can't set prototype of this object</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Cyclic_prototype\">TypeError: can't set prototype: it would cause a prototype chain cycle</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/in_operator_no_object\">TypeError: invalid 'in' operand \"x\"</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Class_ctor_no_new\">TypeError: class constructors must be invoked with 'new'</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Cyclic_object_value\">TypeError: cyclic object value</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Invalid_derived_return\">TypeError: derived class constructor returned invalid value x</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Private_setter_only\">TypeError: getting private setter-only property</a></li><li><a href=\"/zh-CN/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=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/invalid_right_hand_side_instanceof_operand\">TypeError: invalid 'instanceof' operand 'x'</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Array_sort_argument\">TypeError: invalid Array.prototype.sort argument</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Invalid_const_assignment\">TypeError: invalid assignment to const \"x\"</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Constructor_cant_be_used_directly\">TypeError: Iterator/AsyncIterator constructor can't be used directly</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Requires_global_RegExp\">TypeError: matchAll/replaceAll must be called with a global RegExp</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/More_arguments_needed\">TypeError: More arguments needed</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/No_properties\">TypeError: \"x\" has no properties</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Cant_delete\">TypeError: property \"x\" is non-configurable and can't be deleted</a></li><li><a href=\"/zh-CN/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=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Getter_only\">TypeError: setting getter-only property \"x\"</a></li><li><a href=\"/zh-CN/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=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Called_on_incompatible_type\">X.prototype.y called on incompatible type</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Malformed_URI\">URIError: malformed URI sequence</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Already_has_pragma\">Warning: -file- is being assigned a //# sourceMappingURL, but already has one</a></li><li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Errors/Stmt_after_return\">Warning: unreachable code after return statement</a></li></ol>\n </details>\n </li>\n <li class=\"toggle\">\n <details>\n <summary>更多</summary>\n <ol>\n <li><a href=\"/zh-CN/docs/Web/JavaScript/JavaScript_technologies_overview\">JavaScript 技术概要</a></li>\n <li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Lexical_grammar\">词法文法</a></li>\n <li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Iteration_protocols\">迭代协议</a></li>\n <li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Strict_mode\">严格模式</a></li>\n <li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Template_literals\">模板字符串</a></li>\n <li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Trailing_commas\">尾后逗号</a></li>\n <li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Deprecated_and_obsolete_features\">已废弃的特性</a></li>\n </ol>\n </details>\n </li>\n </ol>\n","sidebarMacro":"JsSidebar","body":[{"type":"prose","value":{"id":null,"title":null,"isH3":false,"content":"<p><strong><code>eval()</code></strong> 函数会将传入的字符串当做 JavaScript 代码进行执行。</p>"}},{"type":"prose","value":{"id":"尝试一下","title":"尝试一下","isH3":false,"content":"<iframe class=\"interactive is-js-height\" height=\"200\" src=\"https://interactive-examples.mdn.mozilla.net/pages/js/globalprops-eval.html\" title=\"MDN Web Docs Interactive Example\" allow=\"clipboard-write\" loading=\"lazy\"></iframe>"}},{"type":"prose","value":{"id":"语法","title":"语法","isH3":false,"content":"<pre class=\"brush: plain notranslate\">eval(string)\n</pre>"}},{"type":"prose","value":{"id":"参数","title":"参数","isH3":true,"content":"<dl>\n <dt id=\"string\"><a href=\"#string\"><code>string</code></a></dt>\n <dd>\n <p>一个表示 JavaScript 表达式、语句或一系列语句的字符串。表达式可以包含变量与已存在对象的属性。</p>\n </dd>\n</dl>"}},{"type":"prose","value":{"id":"返回值","title":"返回值","isH3":true,"content":"<p>返回字符串中代码的返回值。如果返回值为空，则返回 <a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/undefined\"><code>undefined</code></a>。</p>"}},{"type":"prose","value":{"id":"描述","title":"描述","isH3":false,"content":"<p><code>eval()</code> 是全局对象的一个函数属性。</p>\n<p><code>eval()</code> 的参数是一个字符串。如果字符串表示的是表达式，<code>eval()</code> 会对表达式进行求值。如果参数表示一个或多个 JavaScript 语句，那么<code>eval()</code> 就会执行这些语句。不需要用 <code>eval()</code> 来执行一个算术表达式：因为 JavaScript 可以自动为算术表达式求值。</p>\n<p>如果你以字符串的形式构造了算术表达式，那么可以在后面用 <code>eval()</code>对它求值。例如，假设你有一个变量 <code>x</code>，你可以通过将表达式的字符串值（例如 <code>3 * x + 2</code>）赋值给一个变量，然后在你的代码后面的其他地方调用 <code>eval()</code>，来推迟涉及 <code>x</code> 的表达式的求值。</p>\n<p>如果 <code>eval()</code> 的参数不是字符串， <code>eval()</code> 会将参数原封不动地返回。在下面的例子中，<code>String</code> 构造器被指定，而 <code>eval()</code> 返回了 <code>String</code> 对象而不是执行字符串。</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js notranslate\"><code>eval(new String(\"2 + 2\")); // 返回了包含\"2 + 2\"的字符串对象\neval(\"2 + 2\"); // returns 4\n</code></pre></div>\n<p>你可以使用一些通用的方法来绕过这个限制，例如使用 <code>toString()</code>。</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js notranslate\"><code>var expression = new String(\"2 + 2\");\neval(expression.toString());\n</code></pre></div>\n<p>如果你间接的使用 <code>eval()</code>，比如通过一个引用来调用它，而不是直接的调用 <code>eval</code>。从 <a href=\"https://www.ecma-international.org/ecma-262/5.1/#sec-10.4.2\" class=\"external\" target=\"_blank\">ECMAScript 5</a> 起，它工作在全局作用域下，而不是局部作用域中。这就意味着，例如，下面的代码的作用声明创建一个全局函数，并且 <code>eval</code> 中的这些代码在执行期间不能在被调用的作用域中访问局部变量。</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js notranslate\"><code>function test() {\n var x = 2,\n y = 4;\n console.log(eval(\"x + y\")); // 直接调用，使用本地作用域，结果是 6\n var geval = eval; // 等价于在全局作用域调用\n console.log(geval(\"x + y\")); // 间接调用，使用全局作用域，throws ReferenceError 因为 `x` 未定义\n (0, eval)(\"x + y\"); // 另一个间接调用的例子\n}\n</code></pre></div>"}},{"type":"prose","value":{"id":"永远不要使用_eval！","title":"永远不要使用 <code>eval</code>！","isH3":false,"content":"<p><code>eval()</code> 是一个危险的函数，它使用与调用者相同的权限执行代码。如果你用 <code>eval()</code> 运行的字符串代码被恶意方（不怀好意的人）修改，你最终可能会在你的网页/扩展程序的权限下，在用户计算机上运行恶意代码。更重要的是，第三方代码可以看到某一个 <code>eval()</code> 被调用时的作用域，这也有可能导致一些不同方式的攻击。相似的 <a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Function\"><code>Function</code></a> 就不容易被攻击。</p>\n<p><code>eval()</code> 通常比其他替代方法更慢，因为它必须调用 JS 解释器，而许多其他结构则可被现代 JS 引擎进行优化。</p>\n<p>此外，现代 JavaScript 解释器将 JavaScript 转换为机器代码。这意味着任何变量命名的概念都会被删除。因此，任意一个 eval 的使用都会强制浏览器进行冗长的变量名称查找，以确定变量在机器代码中的位置并设置其值。另外，新内容将会通过 <code>eval()</code> 引进给变量，比如更改该变量的类型，因此会强制浏览器重新执行所有已经生成的机器代码以进行补偿。但是（谢天谢地）存在一个非常好的 eval 替代方法：只需使用 <a href=\"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Function\">window.Function</a>。这有个例子方便你了解如何将<code>eval()</code>的使用转变为<code>Function()</code>。</p>\n<p>使用 eval 的糟糕代码：</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 looseJsonParse(obj) {\n return eval(\"(\" + obj + \")\");\n}\nconsole.log(looseJsonParse(\"{a:(4-1), b:function(){}, c:new Date()}\"));\n</code></pre></div>\n<p>不用 eval 的更好的代码：</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>function looseJsonParse(obj) {\n return Function('\"use strict\";return (' + obj + \")\")();\n}\nconsole.log(looseJsonParse(\"{a:(4-1), b:function(){}, c:new Date()}\"));\n</code></pre></div>\n<p>比较上面的两个代码片段，两个代码片段似乎是以相同的方式工作，但再想一想：eval 的这个代码的速度要慢得多。注意<code>c: new Date()</code>在执行体中。在没有 eval 的函数中，对象在全局范围内被用来进行计算，因此浏览器可以放心的假设 <code>Date</code> 是来自 <code>window.Date</code> 的而不是一个名为 <code>Date</code> 的局部变量。然而，在使用 <code>eval()</code> 的代码中，浏览器不能假设这一点，因为如果你的代码是下面这个：</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 Date(n) {\n return [\n \"Monday\",\n \"Tuesday\",\n \"Wednesday\",\n \"Thursday\",\n \"Friday\",\n \"Saturday\",\n \"Sunday\",\n ][n % 7 || 0];\n}\nfunction looseJsonParse(obj) {\n return eval(\"(\" + obj + \")\");\n}\nconsole.log(looseJsonParse(\"{a:(4-1), b:function(){}, c:new Date()}\"));\n</code></pre></div>\n<p>因此，在 <code>eval()</code> 版本的代码中，浏览器被迫进行高代价的查找调用以检查是否存在名为 <code>Date()</code> 的任何局部变量。与 <code>Function()</code> 相比，这是非常低效的。</p>\n<p>在类似的情况下，如果你确实希望能够从 <code>Function()</code> 内部的代码调用 <code>Date</code> 函数，该怎么办？你应该躲避并退回到 <code>eval()</code> 吗？绝对不是，永远不要这么做。而是尝试下面的方法。</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>function Date(n) {\n return [\n \"Monday\",\n \"Tuesday\",\n \"Wednesday\",\n \"Thursday\",\n \"Friday\",\n \"Saturday\",\n \"Sunday\",\n ][n % 7 || 0];\n}\nfunction runCodeWithDateFunction(obj) {\n return Function('\"use strict\";return (' + obj + \")\")()(Date);\n}\nconsole.log(runCodeWithDateFunction(\"function(Date){ return Date(5) }\"));\n</code></pre></div>\n<p>由于三重嵌套函数，上面的代码似乎效率低下，但让我们分析一下上述有效方法的好处：</p>\n<ol>\n <li>\n <p>它使得传递给<code>runCodeWithDateFunction</code>的字符串中的代码更少；</p>\n </li>\n <li>\n <p>函数调用开销很小，使得代码尺寸小得多，值得获益；</p>\n </li>\n <li>\n <p><code>Function()</code>更容易让你的代码利用特性修饰 <code>\"use strict\"</code>；</p>\n </li>\n <li>\n <p>代码不使用 <code>eval()</code>，使其比其他方式快几个数量级。</p>\n </li>\n</ol>\n<p>最后，我们来看看简化版。使用如上所示的<code>Function()</code>，你可以更有效地缩小传递给<code>runCodeWithDateFunction</code>的代码字符串，因为函数参数名称也可以缩小，如下面的缩小代码所示。</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js notranslate\"><code>console.log(\n Function('\"use strict\";return(function(a){return a(5)})')()(function (a) {\n return \"Monday Tuesday Wednesday Thursday Friday Saturday Sunday\".split(\n \" \",\n )[a % 7 || 0];\n }),\n);\n</code></pre></div>\n<p>对于常见用例，<code>eval()</code>或<code>Function()</code>还有更安全 (而且更快！) 的替代方案。</p>"}},{"type":"prose","value":{"id":"访问成员属性","title":"访问成员属性","isH3":true,"content":"<p>你不应该去使用 <code>eval()</code> 来将属性名字转化为属性。考虑下面的这个例子，被访问对象的属性在它被执行之前都会未知的。这里可以用 eval 处理：</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js notranslate\"><code>var obj = { a: 20, b: 30 };\nvar propName = getPropName(); // 返回 \"a\" 或 \"b\"\n\neval(\"var result = obj.\" + propsName);\n</code></pre></div>\n<p>但是，这里并不是必须得使用 <code>eval()</code>。事实上，这里并不建议这样使用。可以使用 <a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Property_accessors\">属性访问器</a> 进行代替，它更快、更安全：</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js notranslate\"><code>var obj = { a: 20, b: 30 };\nvar propName = getPropName(); // 返回 \"a\" 或 \"b\"\nvar result = obj[propName]; // obj[ \"a\" ] 与 obj.a 等价\n</code></pre></div>\n<p>你还可以使用这个方法去访问子代的属性。如下：</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js notranslate\"><code>var obj = { a: { b: { c: 0 } } };\nvar propPath = getPropPath(); // 例如返回 \"a.b.c\"\n\neval(\"var result = obj.\" + propPath);\n</code></pre></div>\n<p>这里，可以通过分割属性路径、循环遍历不同的属性，来避免 <code>eval()</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 getDescendantProp(obj, desc) {\n var arr = desc.split(\".\");\n while (arr.length) {\n obj = obj[arr.shift()];\n }\n return obj;\n}\n\nvar obj = { a: { b: { c: 0 } } };\nvar propPath = getPropPath(); // 例如返回 \"a.b.c\"\nvar result = getDescendantProp(obj, propPath);\n</code></pre></div>\n<p>同样的方法也可实现设置子代的属性值：</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js notranslate\"><code>function setDescendantProp(obj, desc, value) {\n var arr = desc.split(\".\");\n while (arr.length > 1) {\n obj = obj[arr.shift()];\n }\n return (obj[arr[0]] = value);\n}\n\nvar obj = { a: { b: { c: 0 } } };\nvar propPath = getPropPath(); // 例如，返回 \"a.b.c\"\nvar result = setDescendantProp(obj, propPath, 1); // a.b.c 值为 1\n</code></pre></div>"}},{"type":"prose","value":{"id":"使用函数而非代码段","title":"使用函数而非代码段","isH3":true,"content":"<p>JavaScript 拥有 <a href=\"/zh-CN/docs/Glossary/First-class_Function\">first-class functions</a>，这意味着你可以将函数直接作为参数传递给其他接口，将他们保存在变量中或者对象的属性中，等等。很多 DOM 的 API 都用这种思路进行设计，你也可以（或者应该）这样子设计你的代码：</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js notranslate\"><code>// 代替 setTimeout(\" ... \", 1000) 写法：\nsetTimeout(function() { ... }, 1000);\n\n// 代替 elt.setAttribute(\"onclick\", \"...\") 写法：\nelt.addEventListener('click', function() { ... } , false);\n</code></pre></div>\n<p><a href=\"/zh-CN/docs/Web/JavaScript/Closures\">闭包</a> 也有助于创建参数化函数而不用连接字符串。</p>"}},{"type":"prose","value":{"id":"解析_json（将字符串转化为_javascript_对象）","title":"解析 JSON（将字符串转化为 JavaScript 对象）","isH3":true,"content":"<p>如果你在调用 <code>eval()</code> 传入的字符串参数中包含数据（如：一个数组“[1,2,3]”）而不是代码，你应该考虑将其转换为 <a href=\"/zh-CN/docs/Glossary/JSON\">JSON</a> 对象，这允许你用 JavaScript 语法的子集来表示数据。<a class=\"page-not-created\" title=\"此文档尚未被撰写，期待你的贡献！\" data-href=\"/zh-CN/docs/Downloading_JSON_and_JavaScript_in_extensions\">在扩展中下载 JSON 和 JavaScript</a></p>\n<p>提示：因为 JSON 语法子集相对于 JavaScript 语法子集比较有局限性，很多在 JavaScript 中可用的特性在 JSON 中就不起作用了。比如，后缀逗号在 JSON 中不支持，并且对象中的属性名在 JSON 中必须用引号括起来。请务必使用 JSON 序列化方法来生成稍后将被解析为 JSON 的字符串。</p>"}},{"type":"prose","value":{"id":"尽量传递数据而非代码","title":"尽量传递数据而非代码","isH3":true,"content":"<p>例如，设计为抓取网页内容的扩展，可能会在 XPath 中定义抓取规则，而不是在 JavaScript 代码中。</p>"}},{"type":"prose","value":{"id":"以有限权限运行代码","title":"以有限权限运行代码","isH3":true,"content":"<p>如果你必须执行这段代码，应考虑以更低的权限运行。此建议主要适用于扩展和 XUL 应用程序，可以使用 <a class=\"page-not-created\" title=\"此文档尚未被撰写，期待你的贡献！\" data-href=\"/zh-CN/docs/Components.utils.evalInSandbox\">Components.utils.evalInSandbox</a> 做到降低权限。</p>"}},{"type":"prose","value":{"id":"示例","title":"示例","isH3":false,"content":""}},{"type":"prose","value":{"id":"使用_eval","title":"使用 <code>eval</code>","isH3":true,"content":"<p>在下面的代码中，两种包含了 <code>eval()</code> 的声明都返回了 42。第一种是对字符串 \"<code>x + y + 1</code>\" 求值；第二种是对字符串 \"<code>42</code>\" 求值。</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 = 2;\nvar y = 39;\nvar z = \"42\";\neval(\"x + y + 1\"); // returns 42\neval(z); // returns 42\n</code></pre></div>"}},{"type":"prose","value":{"id":"使用_eval_执行一串_javascript_语句","title":"使用 <code>eval</code> 执行一串 JavaScript 语句","isH3":true,"content":"<p>下面的例子使用 <code>eval()</code> 来执行 <code>str</code> 字符串。这个字符串包含了如果 <code>x</code> 等于 5，就打开一个 Alert 对话框并对 <code>z</code> 赋值 42，否则就对 <code>z</code> 赋值 0 的 JavaScript 语句。当第二个声明被执行，<code>eval()</code> 将会令字符串被执行，并最终返回赋值给 <code>z</code> 的 42。</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 = 5;\nvar str = \"if (x == 5) {console.log('z is 42'); z = 42;} else z = 0;\";\n\nconsole.log(\"z is \", eval(str));\n</code></pre></div>\n<p>如果你定义了多个值，则会返回最后一个值。</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 = 5;\nvar str = \"if (x == 5) {console.log('z is 42'); z = 42; x = 420; } else z = 0;\";\n\nconsole.log(\"x is \", eval(str)); // z is 42 x is 420\n</code></pre></div>"}},{"type":"prose","value":{"id":"返回值_2","title":"返回值","isH3":true,"content":"<p><code>eval</code> 返回最后一个表达式的值。</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js notranslate\"><code>var str = \"if ( a ) { 1 + 1; } else { 1 + 2; }\";\nvar a = true;\nvar b = eval(str); // returns 2\n\nconsole.log(\"b is : \" + b);\n\na = false;\nb = eval(str); // returns 3\n\nconsole.log(\"b is : \" + b);\n</code></pre></div>"}},{"type":"prose","value":{"id":"eval_中函数作为字符串被定义需要“（”和“）”作为前缀和后缀","title":"<code>eval</code> 中函数作为字符串被定义需要“（”和“）”作为前缀和后缀","isH3":true,"content":"<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js notranslate\"><code>var fctStr1 = \"function a() {}\";\nvar fctStr2 = \"(function a() {})\";\nvar fct1 = eval(fctStr1); // 返回 undefined\nvar fct2 = eval(fctStr2); // 返回一个函数\n</code></pre></div>"}},{"type":"specifications","value":{"title":"规范","id":"规范","isH3":false,"specifications":[{"bcdSpecificationURL":"https://tc39.es/ecma262/multipage/global-object.html#sec-eval-x","title":"ECMAScript Language Specification"}],"query":"javascript.builtins.eval"}},{"type":"browser_compatibility","value":{"title":"浏览器兼容性","id":"浏览器兼容性","isH3":false,"query":"javascript.builtins.eval"}},{"type":"prose","value":{"id":"firefox_相关","title":"Firefox 相关","isH3":false,"content":"<ul>\n <li>从历史上看，<code>eval()</code> 有一个可选的第二个参数，指定上下文执行对象。这个参数是非标准的，并且明确地从 Firefox 4 中删除。请参阅 <a href=\"https://bugzil.la/531675\" class=\"external\" target=\"_blank\">Firefox bug 531675</a>。</li>\n</ul>"}},{"type":"prose","value":{"id":"参见","title":"参见","isH3":false,"content":"<ul>\n <li><a href=\"/zh-CN/docs/Web/JavaScript/Reference/Operators/Property_accessors\">属性访问器</a></li>\n <li><a href=\"/zh-CN/docs/Mozilla/Add-ons/WebExtensions/Content_scripts#%e5%9c%a8_content_script_%e4%b8%ad%e4%bd%bf%e7%94%a8_eval\">WebExtension：在 content script 中使用 eval</a></li>\n</ul>"}}],"toc":[{"text":"尝试一下","id":"尝试一下"},{"text":"语法","id":"语法"},{"text":"描述","id":"描述"},{"text":"永远不要使用 <code>eval</code>！","id":"永远不要使用_eval！"},{"text":"示例","id":"示例"},{"text":"规范","id":"规范"},{"text":"浏览器兼容性","id":"浏览器兼容性"},{"text":"Firefox 相关","id":"firefox_相关"},{"text":"参见","id":"参见"}],"summary":"eval() 函数会将传入的字符串当做 JavaScript 代码进行执行。","popularity":0.0059,"modified":"2023-12-05T11:53:18.000Z","other_translations":[{"locale":"de","title":"eval()","native":"Deutsch"},{"locale":"en-US","title":"eval()","native":"English (US)"},{"locale":"es","title":"eval","native":"Español"},{"locale":"fr","title":"eval()","native":"Français"},{"locale":"ja","title":"eval()","native":"日本語"},{"locale":"ko","title":"eval()","native":"한국어"},{"locale":"pt-BR","title":"eval()","native":"Português (do Brasil)"},{"locale":"ru","title":"eval()","native":"Русский"}],"pageType":"unknown","source":{"folder":"zh-cn/web/javascript/reference/global_objects/eval","github_url":"https://github.com/mdn/translated-content/blob/main/files/zh-cn/web/javascript/reference/global_objects/eval/index.md","last_commit_url":"https://github.com/mdn/translated-content/commit/0b7a24463a14f1edc1fc1f9a070f7d54f4797f35","filename":"index.md"},"short_title":"eval()","parents":[{"uri":"/zh-CN/docs/Web","title":"面向开发者的 Web 技术"},{"uri":"/zh-CN/docs/Web/JavaScript","title":"JavaScript"},{"uri":"/zh-CN/docs/Web/JavaScript/Reference","title":"JavaScript 参考"},{"uri":"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects","title":"JavaScript 标准内置对象"},{"uri":"/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/eval","title":"eval()"}],"pageTitle":"eval() - JavaScript | MDN","noIndexing":false}}</script></body></html>