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uTools-Manuals/docs/javascript/Reference/Statements/const.html
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<article id="wikiArticle">
<div></div>
<p>常量是块级作用域，很像使用 <a href="Reference/Statements/let">let</a> 语句定义的变量。常量的值不能通过重新赋值来改变，并且不能重新声明。</p>
<div><iframe class="interactive interactive-js" frameborder="0" height="250" src="https://interactive-examples.mdn.mozilla.net/pages/js/statement-const.html" width="100%"></iframe></div>
<p class="hidden">The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone <a class="external" href="https://github.com/mdn/interactive-examples" rel="noopener">https://github.com/mdn/interactive-examples</a> and send us a pull request.</p>
<h2 id="语法">语法</h2>
<pre><code class="language-javascript">const <em>name1 = <em>value1 [</em>, <em>name2</em> = <em>value2</em><em> [</em>, ... [</em>, <em>nameN</em> = <em>valueN]]]</em>;</code></pre>
<dl>
<dt><code>nameN</code></dt>
<dd>常量名称，可以是任意合法的<a class="glossaryLink" href="/en-US/docs/Glossary/identifier" title="标识符: A sequence of characters in the code that identifies a variable, function, or property.">标识符</a>。</dd>
<dt><code>valueN</code></dt>
<dd>常量值，可以是任意合法的<a href="Guide/Expressions_and_Operators#Expressions">表达式</a>。</dd>
</dl>
<h2 id="描述">描述</h2>
<p>此声明创建一个常量，其作用域可以是全局或本地声明的块。 与<code><a href="Reference/Statements/var">var</a></code>变量不同，全局常量不会变为窗口对象的属性。需要一个常数的初始化器；也就是说，您必须在声明的同一语句中指定它的值（这是有道理的，因为以后不能更改）。</p>
<p><strong><code>const</code></strong><strong>声明</strong>创建一个值的只读引用。但这并不意味着它所持有的值是不可变的，只是变量标识符不能重新分配。例如，在引用内容是对象的情况下，这意味着可以改变对象的内容（例如，其参数）。</p>
<p>关于“<a href="Reference/Statements/let#Temporal_dead_zone_and_errors_with_let">暂存死区</a>”的所有讨论都适用于<code><a href="Reference/Statements/let">let</a></code>和<code>const</code>。</p>
<p>一个常量不能和它所在作用域内的其他变量或函数拥有相同的名称。</p>
<h2 id="示例">示例</h2>
<p>下面的例子演示了常量的特性。在浏览器的控制台试一下这个例子。</p>
<pre><code class="language-js line-numbers language-js"><code class="language-js"><span class="comment token">// 注意: 常量在声明的时候可以使用大小写，但通常情况下全部用大写字母。 </span>
<span class="comment token">// 定义常量MY_FAV并赋值7</span>
<span class="keyword token">const</span> MY_FAV <span class="operator token">=</span> <span class="number token">7</span><span class="punctuation token">;</span>
<span class="comment token">// 报错</span>
MY_FAV <span class="operator token">=</span> <span class="number token">20</span><span class="punctuation token">;</span>
<span class="comment token">// 输出 7</span>
console<span class="punctuation token">.</span><span class="function token">log</span><span class="punctuation token">(</span><span class="string token">"my favorite number is: "</span> <span class="operator token">+</span> MY_FAV<span class="punctuation token">)</span><span class="punctuation token">;</span>
<span class="comment token">// 尝试重新声明会报错 </span>
<span class="keyword token">const</span> MY_FAV <span class="operator token">=</span> <span class="number token">20</span><span class="punctuation token">;</span>
<span class="comment token">// MY_FAV 保留给上面的常量，这个操作会失败</span>
<span class="keyword token">var</span> MY_FAV <span class="operator token">=</span> <span class="number token">20</span><span class="punctuation token">;</span>
// 也会报错
let MY_FAV = 20;
</code>
<code>// 注意块范围的性质很重要
if (MY_FAV === 7) {
// 没问题，并且创建了一个块作用域变量 MY_FAV
// (works equally well with let to declare a block scoped non const variable)
let MY_FAV = 20;
// MY_FAV 现在为 20
console.log('my favorite number is ' + MY_FAV);
// 这被提升到全局上下文并引发错误
var MY_FAV = 20;
}</code><code class="language-js">
<span class="comment token">// MY_FAV 依旧为7</span>
console<span class="punctuation token">.</span><span class="function token">log</span><span class="punctuation token">(</span><span class="string token">"my favorite number is "</span> <span class="operator token">+</span> MY_FAV<span class="punctuation token">)</span><span class="punctuation token">;</span>
<span class="comment token">// 常量要求一个初始值</span>
<span class="keyword token">const</span> FOO<span class="punctuation token">;</span> <span class="comment token">// SyntaxError: missing = in const declaration</span>
<span class="comment token">// 常量可以定义成对象</span>
<span class="keyword token">const</span> MY_OBJECT <span class="operator token">=</span> <span class="punctuation token">{</span><span class="string token">"key"</span><span class="punctuation token">:</span> <span class="string token">"value"</span><span class="punctuation token">}</span><span class="punctuation token">;</span>
<span class="comment token">// 重写对象和上面一样会失败</span>
MY_OBJECT <span class="operator token">=</span> <span class="punctuation token">{</span><span class="string token">"OTHER_KEY"</span><span class="punctuation token">:</span> <span class="string token">"value"</span><span class="punctuation token">}</span><span class="punctuation token">;</span>
<span class="comment token">// 对象属性并不在保护的范围内，下面这个声明会成功执行</span>
MY_OBJECT<span class="punctuation token">.</span>key <span class="operator token">=</span> <span class="string token">"otherValue"</span><span class="punctuation token">;</span></code>
<code>// 也可以用来定义数组
const MY_ARRAY = [];
// It's possible to push items into the array
// 可以向数组填充数据
MY_ARRAY.push('A'); // ["A"]
// 但是，将一个新数组赋给变量会引发错误
MY_ARRAY = ['B']</code></code></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><a class="external" href="https://www.ecma-international.org/ecma-262/6.0/#sec-let-and-const-declarations" hreflang="en" lang="en" rel="noopener">ECMAScript 2015 (6th Edition, ECMA-262)<br/><small lang="zh-CN">Let and Const Declarations</small></a></td>
<td><span class="spec-Standard">Standard</span></td>
<td>Initial definition.</td>
</tr>
<tr>
<td><a class="external" href="https://tc39.github.io/ecma262/#sec-let-and-const-declarations" hreflang="en" lang="en" rel="noopener">ECMAScript Latest Draft (ECMA-262)<br/><small lang="zh-CN">Let and Const Declarations</small></a></td>
<td><span class="spec-Draft">Draft</span></td>
<td>No changes.</td>
</tr>
</tbody>
</table>
<h2 id="浏览器兼容性">浏览器兼容性</h2>
<div class="hidden">The compatibility table on this page is generated from structured data. If you'd like to contribute to the data, please check out <a class="external" href="https://github.com/mdn/browser-compat-data" rel="noopener">https://github.com/mdn/browser-compat-data</a> and send us a pull request.</div>
<p></p><div class="bc-data"><a class="bc-github-link external" href="https://github.com/mdn/browser-compat-data" rel="noopener">Update compatibility data on GitHub</a><table class="bc-table bc-table-js"><thead><tr class="bc-platforms"><td></td><th class="bc-platform-desktop" colspan="6"><span>Desktop</span></th><th class="bc-platform-mobile" colspan="7"><span>Mobile</span></th><th class="bc-platform-server" colspan="1"><span>Server</span></th></tr><tr class="bc-browsers"><td></td><th class="bc-browser-chrome"><span class="bc-head-txt-label bc-head-icon-chrome">Chrome</span></th><th class="bc-browser-edge"><span class="bc-head-txt-label bc-head-icon-edge">Edge</span></th><th class="bc-browser-firefox"><span class="bc-head-txt-label bc-head-icon-firefox">Firefox</span></th><th class="bc-browser-ie"><span class="bc-head-txt-label bc-head-icon-ie">Internet Explorer</span></th><th class="bc-browser-opera"><span class="bc-head-txt-label bc-head-icon-opera">Opera</span></th><th class="bc-browser-safari"><span class="bc-head-txt-label bc-head-icon-safari">Safari</span></th><th class="bc-browser-webview_android"><span class="bc-head-txt-label bc-head-icon-webview_android">Android webview</span></th><th class="bc-browser-chrome_android"><span class="bc-head-txt-label bc-head-icon-chrome_android">Chrome for Android</span></th><th class="bc-browser-edge_mobile"><span class="bc-head-txt-label bc-head-icon-edge_mobile">Edge Mobile</span></th><th class="bc-browser-firefox_android"><span class="bc-head-txt-label bc-head-icon-firefox_android">Firefox for Android</span></th><th class="bc-browser-opera_android"><span class="bc-head-txt-label bc-head-icon-opera_android">Opera for Android</span></th><th class="bc-browser-safari_ios"><span class="bc-head-txt-label bc-head-icon-safari_ios">Safari on iOS</span></th><th class="bc-browser-samsunginternet_android"><span class="bc-head-txt-label bc-head-icon-samsunginternet_android">Samsung Internet</span></th><th class="bc-browser-nodejs"><span class="bc-head-txt-label bc-head-icon-nodejs">Node.js</span></th></tr></thead><tbody><tr><th scope="row"><code>const</code></th><td class="bc-supports-yes bc-browser-chrome"><span class="bc-browser-name">Chrome</span><abbr class="bc-level-yes only-icon" title="Full support">
<span>Full support</span>
</abbr>
21</td><td class="bc-supports-yes bc-browser-edge"><span class="bc-browser-name">Edge</span><abbr class="bc-level-yes only-icon" title="Full support">
<span>Full support</span>
</abbr>
Yes</td><td class="bc-supports-yes bc-browser-firefox bc-has-history"><span class="bc-browser-name">Firefox</span><abbr class="bc-level-yes only-icon" title="Full support">
<span>Full support</span>
</abbr>
36<div class="bc-icons"><abbr class="only-icon" title="See implementation notes"><span>Notes</span><i class="ic-footnote"></i></abbr> </div><section class="bc-history" id="sect1"><dl><dt class="bc-supports-yes bc-supports"><abbr class="bc-level-yes only-icon" title="Full support">
<span>Full support</span>
</abbr>
36<div class="bc-icons"><abbr class="only-icon" title="See implementation notes"><span>Notes</span><i class="ic-footnote"></i></abbr> </div></dt><dd><abbr class="only-icon" title="See implementation notes"><span>Notes</span><i class="ic-footnote"></i></abbr> Prior to Firefox 13, <code>const</code> is implemented, but re-assignment is not failing.</dd><dd><abbr class="only-icon" title="See implementation notes"><span>Notes</span><i class="ic-footnote"></i></abbr> Prior to Firefox 46, a <code>TypeError</code> was thrown on redeclaration instead of a <code>SyntaxError</code>.</dd></dl></section></td><td class="bc-supports-yes bc-browser-ie"><span class="bc-browser-name">IE</span><abbr class="bc-level-yes only-icon" title="Full support">
<span>Full support</span>
</abbr>
11</td><td class="bc-supports-yes bc-browser-opera"><span class="bc-browser-name">Opera</span><abbr class="bc-level-yes only-icon" title="Full support">
<span>Full support</span>
</abbr>
Yes</td><td class="bc-supports-yes bc-browser-safari"><span class="bc-browser-name">Safari</span><abbr class="bc-level-yes only-icon" title="Full support">
<span>Full support</span>
</abbr>
5.1</td><td class="bc-supports-yes bc-browser-webview_android"><span class="bc-browser-name">WebView Android</span><abbr class="bc-level-yes only-icon" title="Full support">
<span>Full support</span>
</abbr>
Yes</td><td class="bc-supports-yes bc-browser-chrome_android"><span class="bc-browser-name">Chrome Android</span><abbr class="bc-level-yes only-icon" title="Full support">
<span>Full support</span>
</abbr>
Yes</td><td class="bc-supports-yes bc-browser-edge_mobile"><span class="bc-browser-name">Edge Mobile</span><abbr class="bc-level-yes only-icon" title="Full support">
<span>Full support</span>
</abbr>
Yes</td><td class="bc-supports-yes bc-browser-firefox_android bc-has-history"><span class="bc-browser-name">Firefox Android</span><abbr class="bc-level-yes only-icon" title="Full support">
<span>Full support</span>
</abbr>
36<div class="bc-icons"><abbr class="only-icon" title="See implementation notes"><span>Notes</span><i class="ic-footnote"></i></abbr> </div><section class="bc-history" id="sect2"><dl><dt class="bc-supports-yes bc-supports"><abbr class="bc-level-yes only-icon" title="Full support">
<span>Full support</span>
</abbr>
36<div class="bc-icons"><abbr class="only-icon" title="See implementation notes"><span>Notes</span><i class="ic-footnote"></i></abbr> </div></dt><dd><abbr class="only-icon" title="See implementation notes"><span>Notes</span><i class="ic-footnote"></i></abbr> Prior to Firefox 13, <code>const</code> is implemented, but re-assignment is not failing.</dd><dd><abbr class="only-icon" title="See implementation notes"><span>Notes</span><i class="ic-footnote"></i></abbr> Prior to Firefox 46, a <code>TypeError</code> was thrown on redeclaration instead of a <code>SyntaxError</code>.</dd></dl></section></td><td class="bc-supports-yes bc-browser-opera_android"><span class="bc-browser-name">Opera Android</span><abbr class="bc-level-yes only-icon" title="Full support">
<span>Full support</span>
</abbr>
Yes</td><td class="bc-supports-yes bc-browser-safari_ios"><span class="bc-browser-name">Safari iOS</span><abbr class="bc-level-yes only-icon" title="Full support">
<span>Full support</span>
</abbr>
Yes</td><td class="bc-supports-yes bc-browser-samsunginternet_android"><span class="bc-browser-name">Samsung Internet Android</span><abbr class="bc-level-yes only-icon" title="Full support">
<span>Full support</span>
</abbr>
Yes</td><td class="bc-supports-yes bc-browser-nodejs"><span class="bc-browser-name">nodejs</span><abbr class="bc-level-yes only-icon" title="Full support">
<span>Full support</span>
</abbr>
Yes</td></tr></tbody></table><section class="bc-legend" id="sect3"><h3 class="offscreen" id="Legend">Legend</h3><dl><dt><span class="bc-supports-yes bc-supports">
<abbr class="bc-level bc-level-yes only-icon" title="Full support">
<span>Full support</span>
 
</abbr></span></dt><dd>Full support</dd><dt><abbr class="only-icon" title="See implementation notes."><span>See implementation notes.</span><i class="ic-footnote"></i></abbr></dt><dd>See implementation notes.</dd></dl></section></div><p></p>
<h2 id="相关链接">相关链接</h2>
<ul>
<li><a href="Reference/Statements/var"><code>var</code></a></li>
<li><a href="/zh-CN/docs/JavaScript/Reference/Statements/let"><code>let</code></a></li>
<li><a href="/zh-CN/docs/JavaScript/Guide/Values,_variables,_and_literals#Constants">Constants in JavaScript Guide</a></li>
</ul>
</article>
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