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300 lines
21 KiB
HTML
<article id="wikiArticle">
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<p></p><p></p>
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<p><strong>函数默认参数</strong>允许在没有值或<code>undefined</code>被传入时使用默认形参。</p>
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<div><iframe class="interactive interactive-js" frameborder="0" height="250" src="https://interactive-examples.mdn.mozilla.net/pages/js/functions-default.html" width="100%"></iframe></div>
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<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>
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<h2 id="语法" name="语法">语法</h2>
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<pre><code class="language-javascript">function [<em>name</em>]([<em>param1</em>[ = defaultValue1 ][, ..., <em>paramN</em>[ = defaultValueN ]]]) {
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<em> statements</em>
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}
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</code></pre>
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<h2 id="描述" name="描述">描述</h2>
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<p>JavaScript 中函数的参数默认是<code><a href="Reference/Global_Objects/undefined" title="undefined是全局对象的一个属性。也就是说,它是全局作用域的一个变量。undefined的最初值就是原始数据类型undefined。"><code>undefined</code></a></code>。然而,在某些情况下可能需要设置一个不同的默认值。这是默认参数可以帮助的地方。</p>
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<p>以前,一般设置默认参数的方法是在函数体测试参数是否为<code>undefined</code>,如果是的话就设置为默认的值。</p>
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<p>下面的例子中,如果在调用<code>multiply</code>时,参数<code>b</code>的值没有提供,那么它的值就为<code>undefined</code>。如果直接执行<code>a * b</code>,函数会返回 <code>NaN</code>。</p>
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<pre><code class="language-javascript">function multiply(a, b) {
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return a * b;
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}
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multiply(5, 2); // 10
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multiply(5); // NaN !
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</code></pre>
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<p>为了防止这种情况,第二行代码解决了这个问题,其中如果只使用一个参数调用<code>multiply</code>,则<code>b</code>设置为<code>1</code>:</p>
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<pre><code class="language-javascript">function multiply(a, b) {
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b = (typeof b !== 'undefined') ? b : 1;
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return a * b;
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}
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multiply(5, 2); // 10
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multiply(5); // 5</code></pre>
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<p>有了默认参数,我们不需要再在函数体内做不必要的检查。现在你可以在函数头将<code>b</code>的默认值置为<code>1</code>:</p>
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<pre><code class="language-javascript">function multiply(a, b = 1) {
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return a * b;
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}
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multiply(5, 2); // 10
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multiply(5); // 5</code></pre>
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<h2 id="示例" name="示例">示例</h2>
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<h3 id="传入_undefined_vs_其他假值">传入 <code>undefined</code> vs 其他假值</h3>
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<p>在第二次调用中,即使第一个参数在调用时显式设置为<code>undefined</code>(虽然不是<code>null</code>或其他<a href="/zh-CN/docs/Glossary/Falsy">falsy</a>值),但是<code>num</code>参数的值是默认值。</p>
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<pre><code class="language-javascript">function test(num = 1) {
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console.log(typeof num);
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}
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test(); // 'number' (num is set to 1)
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test(undefined); // 'number' (num is set to 1 too)
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// test with other falsy values:
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test(''); // 'string' (num is set to '')
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test(null); // 'object' (num is set to null)</code></pre>
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<h3 id="调用时解析">调用时解析</h3>
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<p>在函数被调用时,参数默认值会被解析,所以不像Python中的例子,每次函数调用时都会创建一个新的参数对象。</p>
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<pre><code class="language-javascript">function append(value, array = []) {
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array.push(value);
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return array;
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}
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append(1); //[1]
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append(2); //[2], not [1, 2]</code></pre>
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<p>这个规则对于函数和变量也是适用的。</p>
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<pre><code class="language-javascript">function callSomething(thing = something()) {
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return thing;
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}
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let numberOfTimesCalled = 0;
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function something() {
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numberOfTimesCalled += 1;
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return numberOfTimesCalled;
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}
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callSomething(); // 1
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callSomething(); // 2</code></pre>
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<h3 id="默认参数可用于后面的默认参数">默认参数可用于后面的默认参数</h3>
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<p>已经遇到的参数可用于以后的默认参数:</p>
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<pre><code class="language-javascript">function greet(name, greeting, message = greeting + ' ' + name) {
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return [name, greeting, message];
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}
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greet('David', 'Hi'); // ["David", "Hi", "Hi David"]
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greet('David', 'Hi', 'Happy Birthday!'); // ["David", "Hi", "Happy Birthday!"]
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</code></pre>
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<p>以下这个例子近似模拟了一些比较简单的情况,并说明了特殊情况是怎么被处理的。</p>
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<pre><code class="language-javascript">function go() {
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return ':P';
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}
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function withDefaults(a, b = 5, c = b, d = go(), e = this,
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f = arguments, g = this.value) {
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return [a, b, c, d, e, f, g];
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}
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function withoutDefaults(a, b, c, d, e, f, g) {
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switch (arguments.length) {
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case 0:
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a;
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case 1:
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b = 5;
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case 2:
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c = b;
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case 3:
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d = go();
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case 4:
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e = this;
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case 5:
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f = arguments;
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case 6:
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g = this.value;
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default:
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}
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return [a, b, c, d, e, f, g];
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}
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withDefaults.call({value: '=^_^='});
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// [undefined, 5, 5, ":P", {value:"=^_^="}, arguments, "=^_^="]
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withoutDefaults.call({value: '=^_^='});
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// [undefined, 5, 5, ":P", {value:"=^_^="}, arguments, "=^_^="]</code></pre>
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<h3 id="函数嵌套定义">函数嵌套定义</h3>
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<p>在 Gecko 33 (Firefox 33 / Thunderbird 33 / SeaMonkey 2.30) 中引入。在函数体内的函数声明不能引用内部的默认参数,并且会在 SpiderMonkey 抛出一个<a href="Reference/Global_Objects/ReferenceError" title="ReferenceError(引用错误) 对象代表当一个不存在的变量被引用时发生的错误。"><code>ReferenceError</code></a>(现在是 <a href="Reference/Global_Objects/TypeError" title="TypeError(类型错误) 对象用来表示值的类型非预期类型时发生的错误。"><code>TypeError</code></a>),参见 <a class="external" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1022967" rel="noopener" title="FIXED: Separate environment for default parameter and function body if default parameter contains expression">bug 1022967</a>。默认参数总是会被首先执行,而在函数体内部的函数声明会在之后生效。</p>
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<pre><code class="language-javascript">// Doesn't work! Throws ReferenceError.
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function f(a = go()) {
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function go() { return ':P'; }
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}</code></pre>
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<h3 id="位于默认参数之后非默认参数">位于默认参数之后非默认参数</h3>
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<p>在Gecko 26 (Firefox 26 / Thunderbird 26 / SeaMonkey 2.23 / Firefox OS 1.2)之前,以下代码会造成<a href="Reference/Global_Objects/SyntaxError" title="SyntaxError 对象代表尝试解析语法上不合法的代码的错误。"><code>SyntaxError</code></a>错误。这已经在<a class="external" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1022967" rel="noopener" title="FIXED: Separate environment for default parameter and function body if default parameter contains expression">bug 1022967</a>中修复,并在以后的版本中按预期方式工作。参数仍然设置为从左到右,覆盖默认参数,即使后面的参数没有默认值。</p>
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<pre><code class="language-javascript">function f(x = 1, y) {
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return [x, y];
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}
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f(); // [1, undefined]
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f(2); // [2, undefined]</code></pre>
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<h3 id="有默认值的解构参数">有默认值的解构参数</h3>
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<p>你可以通过<a href="Reference/Operators/Destructuring_assignment">解构赋值</a>为参数赋值:</p>
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<pre><code class="language-javascript">function f([x, y] = [1, 2], {z: z} = {z: 3}) {
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return x + y + z;
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}
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f(); // 6</code></pre>
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<h2 id="规范">规范</h2>
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<table class="standard-table">
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<tbody>
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<tr>
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<th scope="col">Specification</th>
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<th scope="col">Status</th>
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<th scope="col">Comment</th>
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</tr>
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<tr>
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<td><a class="external" href="https://www.ecma-international.org/ecma-262/6.0/#sec-function-definitions" hreflang="en" lang="en" rel="noopener">ECMAScript 2015 (6th Edition, ECMA-262)<br/><small lang="zh-CN">Function Definitions</small></a></td>
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<td><span class="spec-Standard">Standard</span></td>
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<td>Initial definition.</td>
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</tr>
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<tr>
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<td><a class="external" href="https://tc39.github.io/ecma262/#sec-function-definitions" hreflang="en" lang="en" rel="noopener">ECMAScript Latest Draft (ECMA-262)<br/><small lang="zh-CN">Function Definitions</small></a></td>
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<td><span class="spec-Draft">Draft</span></td>
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<td> </td>
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</tr>
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</tbody>
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</table>
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<h2 id="浏览器兼容" name="浏览器兼容">浏览器兼容</h2>
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<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>
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<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">Default parameters</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">
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<span>Full support</span>
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</abbr>
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49</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">
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<span>Full support</span>
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</abbr>
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14</td><td class="bc-supports-yes bc-browser-firefox"><span class="bc-browser-name">Firefox</span><abbr class="bc-level-yes only-icon" title="Full support">
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<span>Full support</span>
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</abbr>
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15</td><td class="bc-supports-no bc-browser-ie"><span class="bc-browser-name">IE</span><abbr class="bc-level-no only-icon" title="No support">
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<span>No support</span>
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</abbr>
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No</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">
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<span>Full support</span>
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</abbr>
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36</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">
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<span>Full support</span>
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</abbr>
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10</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">
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<span>Full support</span>
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</abbr>
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49</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">
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<span>Full support</span>
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</abbr>
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49</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">
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<span>Full support</span>
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</abbr>
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14</td><td class="bc-supports-yes bc-browser-firefox_android"><span class="bc-browser-name">Firefox Android</span><abbr class="bc-level-yes only-icon" title="Full support">
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<span>Full support</span>
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</abbr>
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15</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">
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<span>Full support</span>
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</abbr>
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36</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">
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<span>Full support</span>
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</abbr>
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10</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">
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<span>Full support</span>
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</abbr>
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5.0</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">
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<span>Full support</span>
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</abbr>
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6.0.0</td></tr><tr><th scope="row">Parameters without defaults after default parameters</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">
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<span>Full support</span>
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</abbr>
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49</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">
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<span>Full support</span>
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</abbr>
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14</td><td class="bc-supports-yes bc-browser-firefox"><span class="bc-browser-name">Firefox</span><abbr class="bc-level-yes only-icon" title="Full support">
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<span>Full support</span>
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</abbr>
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26</td><td class="bc-supports-no bc-browser-ie"><span class="bc-browser-name">IE</span><abbr class="bc-level-no only-icon" title="No support">
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<span>No support</span>
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</abbr>
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No</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">
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<span>Full support</span>
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</abbr>
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36</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">
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<span>Full support</span>
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</abbr>
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10</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">
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<span>Full support</span>
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</abbr>
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49</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">
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<span>Full support</span>
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</abbr>
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49</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">
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<span>Full support</span>
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</abbr>
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14</td><td class="bc-supports-yes bc-browser-firefox_android"><span class="bc-browser-name">Firefox Android</span><abbr class="bc-level-yes only-icon" title="Full support">
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<span>Full support</span>
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||
</abbr>
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||
26</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">
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<span>Full support</span>
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||
</abbr>
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36</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">
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<span>Full support</span>
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||
</abbr>
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10</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">
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<span>Full support</span>
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</abbr>
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5.0</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">
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<span>Full support</span>
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</abbr>
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Yes</td></tr><tr><th scope="row">Destructured parameter with default value assignment</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">
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<span>Full support</span>
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</abbr>
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49</td><td class="bc-supports-unknown bc-browser-edge"><span class="bc-browser-name">Edge</span><abbr title="Compatibility unknown; please update this.">
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?
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</abbr></td><td class="bc-supports-yes bc-browser-firefox"><span class="bc-browser-name">Firefox</span><abbr class="bc-level-yes only-icon" title="Full support">
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<span>Full support</span>
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</abbr>
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41</td><td class="bc-supports-no bc-browser-ie"><span class="bc-browser-name">IE</span><abbr class="bc-level-no only-icon" title="No support">
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<span>No support</span>
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||
</abbr>
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||
No</td><td class="bc-supports-unknown bc-browser-opera"><span class="bc-browser-name">Opera</span><abbr title="Compatibility unknown; please update this.">
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?
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</abbr></td><td class="bc-supports-unknown bc-browser-safari"><span class="bc-browser-name">Safari</span><abbr title="Compatibility unknown; please update this.">
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?
|
||
</abbr></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>
|
||
49</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>
|
||
49</td><td class="bc-supports-unknown bc-browser-edge_mobile"><span class="bc-browser-name">Edge Mobile</span><abbr title="Compatibility unknown; please update this.">
|
||
?
|
||
</abbr></td><td class="bc-supports-yes bc-browser-firefox_android"><span class="bc-browser-name">Firefox Android</span><abbr class="bc-level-yes only-icon" title="Full support">
|
||
<span>Full support</span>
|
||
</abbr>
|
||
41</td><td class="bc-supports-unknown bc-browser-opera_android"><span class="bc-browser-name">Opera Android</span><abbr title="Compatibility unknown; please update this.">
|
||
?
|
||
</abbr></td><td class="bc-supports-unknown bc-browser-safari_ios"><span class="bc-browser-name">Safari iOS</span><abbr title="Compatibility unknown; please update this.">
|
||
?
|
||
</abbr></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>
|
||
5.0</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="sect1"><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><span class="bc-supports-no bc-supports">
|
||
<abbr class="bc-level bc-level-no only-icon" title="No support">
|
||
<span>No support</span>
|
||
|
||
</abbr></span></dt><dd>No support</dd><dt><span class="bc-supports-unknown bc-supports">
|
||
<abbr class="bc-level bc-level-unknown only-icon" title="Compatibility unknown">
|
||
<span>Compatibility unknown</span>
|
||
|
||
</abbr></span></dt><dd>Compatibility unknown</dd></dl></section></div><p></p>
|
||
<h2 id="相关链接" name="相关链接">相关链接</h2>
|
||
<ul>
|
||
<li><a class="external external-icon" href="http://wiki.ecmascript.org/doku.php?id=harmony:parameter_default_values" rel="external noopener" title="http://wiki.ecmascript.org/doku.php?id=harmony:parameter_default_values">Original proposal at ecmascript.org</a></li>
|
||
</ul>
|
||
<p> </p>
|
||
</article> |