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---
title: in
slug: Web/JavaScript/Reference/Operators/in
tags:
- JavaScript
- Operator
- Relational Operators
translation_of: Web/JavaScript/Reference/Operators/in
---
<p>{{jsSidebar("Operators")}}</p>
<p>如果指定的属性在指定的对象或其原型链中，则<strong><code>in</code> 运算符</strong>返回<code>true</code>。</p>
<div>{{EmbedInteractiveExample("pages/js/expressions-inoperator.html")}}</div>
<h2 id="Syntax" name="Syntax">语法</h2>
<pre class="syntaxbox"><em>prop</em> in <em>object</em></pre>
<h3 id="Parameters" name="Parameters">参数</h3>
<dl>
<dt><code>prop</code></dt>
<dd>一个字符串类型或者 symbol 类型的属性名或者数组索引（非symbol类型将会强制转为字符串）。</dd>
</dl>
<dl>
<dt><code>objectName</code></dt>
<dd>检查它（或其原型链）是否包含具有指定名称的属性的对象。</dd>
</dl>
<h2 id="Description" name="Description">描述</h2>
<p>下面的例子演示了一些 <code>in</code> 运算符的用法。</p>
<pre class="brush:js">// 数组
var trees = new Array("redwood", "bay", "cedar", "oak", "maple");
0 in trees // 返回true
3 in trees // 返回true
6 in trees // 返回false
"bay" in trees // 返回false (必须使用索引号,而不是数组元素的值)
"length" in trees // 返回true (length是一个数组属性)
Symbol.iterator in trees // 返回true (数组可迭代，只在ES2015+上有效)
// 内置对象
"PI" in Math // 返回true
// 自定义对象
var mycar = {make: "Honda", model: "Accord", year: 1998};
"make" in mycar // 返回true
"model" in mycar // 返回true
</pre>
<p><code>in</code>右操作数必须是一个对象值。例如，你可以指定使用<code>String</code>构造函数创建的字符串，但不能指定字符串文字。</p>
<pre class="brush:js">var color1 = new String("green");
"length" in color1 // 返回true
var color2 = "coral";
"length" in color2 // 报错(color2不是对象)
</pre>
<h3 id="Using_in_with_deleted_or_undefined_properties" name="Using_in_with_deleted_or_undefined_properties">对被删除或值为 undefined 的属性使用<code>in</code></h3>
<p>如果你使用 <code><a href="/zh-CN/docs/JavaScript/Reference/Operators/delete" title="zh-CN/docs/JavaScript/Reference/Operators/Special/delete">delete</a></code> 运算符删除了一个属性，则 <code>in</code> 运算符对所删除属性返回 <code>false</code>。</p>
<pre class="brush:js">var mycar = {make: "Honda", model: "Accord", year: 1998};
delete mycar.make;
"make" in mycar; // 返回false
var trees = new Array("redwood", "bay", "cedar", "oak", "maple");
delete trees[3];
3 in trees; // 返回false
</pre>
<p>如果你只是将一个属性的值赋值为{{jsxref("Global_Objects/undefined", "undefined")}}，而没有删除它，则 <code>in</code> 运算仍然会返回<code>true</code>。</p>
<pre class="brush:js">var mycar = {make: "Honda", model: "Accord", year: 1998};
mycar.make = undefined;
"make" in mycar; // 返回true
</pre>
<pre class="brush:js">var trees = new Array("redwood", "bay", "cedar", "oak", "maple");
trees[3] = undefined;
3 in trees; // 返回true
</pre>
<h3 id="Inherited_properties" name="Inherited_properties">继承属性</h3>
<p>如果一个属性是从原型链上继承来的，<code>in</code> 运算符也会返回 <code>true</code>。</p>
<pre class="brush:js">"toString" in {}; // 返回true
</pre>
<h2 id="规范">规范</h2>
<table class="standard-table">
<tbody>
<tr>
<th scope="col">Specification</th>
<th scope="col">Status</th>
<th scope="col">Comment</th>
</tr>
<tr>
<td>{{SpecName('ESDraft', '#sec-relational-operators', 'Relational Operators')}}</td>
<td>{{Spec2('ESDraft')}}</td>
<td></td>
</tr>
<tr>
<td>{{SpecName('ES2015', '#sec-relational-operators', 'Relational Operators')}}</td>
<td>{{Spec2('ES2015')}}</td>
<td></td>
</tr>
<tr>
<td>{{SpecName('ES5.1', '#sec-11.8.7', 'The in Operator')}}</td>
<td>{{Spec2('ES5.1')}}</td>
<td></td>
</tr>
<tr>
<td>{{SpecName('ES3', '#sec-11.8.7', 'The in Operator')}}</td>
<td>{{Spec2('ES3')}}</td>
<td>Initial definition. Implemented in JavaScript 1.4.</td>
</tr>
</tbody>
</table>
<h2 id="浏览器兼容">浏览器兼容</h2>
<p>{{Compat("javascript.operators.in")}}</p>
<h2 id="See_also" name="See_also">相关链接</h2>
<ul>
<li><code><a href="/zh-CN/docs/Web/JavaScript/Reference/Statements/for...in">for...in</a></code></li>
<li><code><a href="/zh-CN/docs/JavaScript/Reference/Operators/delete" title="zh-CN/docs/JavaScript/Reference/Operators/Special/delete">delete</a></code></li>
<li>{{jsxref("Object.prototype.hasOwnProperty()")}}</li>
<li>{{jsxref("Reflect.has()")}}</li>
<li><a href="/zh-CN/docs/Enumerability_and_ownership_of_properties" title="/zh-CN/docs/Enumerability_and_ownership_of_properties">属性的可枚举性和所有权</a></li>
</ul>
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