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---
title: class
slug: Web/JavaScript/Reference/Statements/class
tags:
- Class
- Classes
- Declaration
- ECMAScript 2015
- ES6
- JavaScript
- Reference
- Statement
translation_of: Web/JavaScript/Reference/Statements/class
---
<div>{{jsSidebar("Statements")}}</div>
<p><strong>class 声明</strong>创建一个基于原型继承的具有给定名称的新类。</p>
<p>{{EmbedInteractiveExample("pages/js/statement-class.html")}}</p>
<div class="noinclude">
<p>你也可以使用{{jsxref("Operators/class", "类表达式", "", 1)}}定义类。但是不同于类表达式，类声明不允许再次声明已经存在的类，否则将会抛出一个类型错误。</p>
</div>
<h2 id="语法">语法</h2>
<pre class="brush: js">class <em>name</em> [extends] {
// class body
}
</pre>
<h2 id="描述">描述</h2>
<p>和类表达式一样，类声明体在<a href="https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Strict_mode">严格模式</a>下运行。构造函数是可选的。</p>
<p>类声明不可以提升（这与<a href="https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Statements/function">函数声明</a>不同）。</p>
<h2 id="示例">示例</h2>
<h3 id="声明一个类">声明一个类</h3>
<p>在下面的例子中，我们首先定义一个名为Polygon的类，然后继承它来创建一个名为Square的类。注意，构造函数中使用的 super() 只能在构造函数中使用，并且必须在使用 this 关键字前调用。</p>
<pre class="brush: js">class Polygon {
constructor(height, width) {
this.name = 'Polygon';
this.height = height;
this.width = width;
}
}
class Square extends Polygon {
constructor(length) {
super(length, length);
this.name = 'Square';
}
}</pre>
<div class="warning">
<h3 id="重复定义类">重复定义类</h3>
<p>重复声明一个类会引起类型错误。</p>
<pre class="brush: js">class Foo {};
class Foo {};
// Uncaught TypeError: Identifier 'Foo' has already been declared</pre>
<p>若之前使用类表达式定义了一个类，则再次声明这个类同样会引起类型错误。</p>
<pre class="brush: js">let Foo = class {};
class Foo {};
// Uncaught TypeError: Identifier 'Foo' has already been declared</pre>
</div>
<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('ES2015', '#sec-class-definitions', 'Class definitions')}}</td>
<td>{{Spec2('ES2015')}}</td>
<td>Initial definition.</td>
</tr>
<tr>
<td>{{SpecName('ES2016', '#sec-class-definitions', 'Class definitions')}}</td>
<td>{{Spec2('ES2016')}}</td>
<td> </td>
</tr>
<tr>
<td>{{SpecName('ES2017', '#sec-class-definitions', 'Class definitions')}}</td>
<td>{{Spec2('ES2017')}}</td>
<td> </td>
</tr>
<tr>
<td>{{SpecName('ESDraft', '#sec-class-definitions', 'Class definitions')}}</td>
<td>{{Spec2('ESDraft')}}</td>
<td> </td>
</tr>
</tbody>
</table>
<h2 id="浏览器兼容">浏览器兼容</h2>
<p>{{Compat("javascript.statements.class")}}</p>
<h2 id="See_also">See also</h2>
<ul>
<li><a href="/en-US/docs/Web/JavaScript/Reference/Statements/function"><code>function</code> declaration</a></li>
<li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/class"><code>class</code> expression</a></li>
<li><a href="/en-US/docs/Web/JavaScript/Reference/Classes">Classes</a></li>
</ul>
|
