Revision 965581 of default

<div>{{jsSidebar("Statements")}}</div>

<p>The <strong>default keyword </strong>can be used in two situations in JavaScript: within a&nbsp;{{jsxref("Statements/switch", "switch")}} statement, or with an&nbsp;{{jsxref("Statements/export", "export")}} statement.</p>

<h2 id="Syntax">Syntax 语法</h2>

<p>在{{jsxref("Statements/switch", "switch")}} 语句中使用:</p>

<pre class="syntaxbox">
switch (expression) {
  case value1:
    //当表达式的值和value1匹配执行这里的语句
    [break;]
  default:
    //当表达式的值没有匹配，执行这里的语句
    [break;]
}</pre>

<p>With&nbsp;{{jsxref("Statements/export", "export")}} statement:</p>

<pre class="syntaxbox">
export default <em>nameN</em> </pre>

<h2 id="Description_描述">Description 描述</h2>

<p>更多细节，请看</p>

<ul>
 <li>{{jsxref("Statements/switch", "switch")}} 语句和</li>
 <li>{{jsxref("Statements/export", "export")}} 语句页面.</li>
</ul>

<h2 id="例子">例子</h2>

<h3 id="Using_default_in_switch_statements在switch语句中使用default">在switch语句中使用default</h3>

<p>In the following example, if <code>expr</code>&nbsp;evaluates to "Bananas" or "Apples", the program matches the values with either the case "Bananas" or "Apples" and executes the corresponding statement. The <code>default</code> keyword will help in any other case and executes the associated statement.</p>

<pre class="brush: js">
switch (expr) {
  case "Oranges":
    console.log("Oranges are $0.59 a pound.");
    break;
  case "Apples":
    console.log("Apples are $0.32 a pound.");
    break;
  default:
    console.log("Sorry, we are out of " + expr + ".");
}</pre>

<h3 id="Using_default_with_export">Using <code>default</code> with <code>export</code></h3>

<p>If you want to export a single value or need a fallback value for a module, a default export can be used:</p>

<pre class="brush: js">
// module "my-module.js"
let cube = function cube(x) {
  return x * x * x;
}
export default cube;</pre>

<p>Then, in another script, it will be straightforward to import the default export:</p>

<pre class="brush: js">
// module "my-module.js"
import myFunction from 'my-module';
console.log(myFunction(3)); // 9
</pre>

<h2 id="Specifications">Specifications 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('ES6', '#sec-switch-statement', 'switch statement')}}</td>
   <td>{{Spec2('ES6')}}</td>
   <td>&nbsp;</td>
  </tr>
  <tr>
   <td>{{SpecName('ES6', '#sec-exports', 'Exports')}}</td>
   <td>{{Spec2('ES6')}}</td>
   <td>&nbsp;</td>
  </tr>
  <tr>
   <td>{{SpecName('ESDraft', '#sec-switch-statement', 'switch statement')}}</td>
   <td>{{Spec2('ESDraft')}}</td>
   <td>&nbsp;</td>
  </tr>
  <tr>
   <td>{{SpecName('ESDraft', '#sec-exports', 'Exports')}}</td>
   <td>{{Spec2('ESDraft')}}</td>
   <td>&nbsp;</td>
  </tr>
 </tbody>
</table>

<h2 id="Browser_compatibility">Browser compatibility 浏览器兼容性</h2>

<p>{{CompatibilityTable}}</p>

<div id="compat-desktop">
<table class="compat-table">
 <tbody>
  <tr>
   <th>Feature</th>
   <th>Chrome</th>
   <th>Firefox (Gecko)</th>
   <th>Internet Explorer</th>
   <th>Opera</th>
   <th>Safari</th>
  </tr>
  <tr>
   <td>Switch default</td>
   <td>{{CompatVersionUnknown}}</td>
   <td>{{CompatVersionUnknown}}</td>
   <td>{{CompatVersionUnknown}}</td>
   <td>{{CompatVersionUnknown}}</td>
   <td>{{CompatVersionUnknown}}</td>
  </tr>
  <tr>
   <td>Export default</td>
   <td>{{CompatNo}}</td>
   <td>{{CompatNo}}</td>
   <td>{{CompatNo}}</td>
   <td>{{CompatNo}}</td>
   <td>{{CompatNo}}</td>
  </tr>
 </tbody>
</table>
</div>

<div id="compat-mobile">
<table class="compat-table">
 <tbody>
  <tr>
   <th>Feature</th>
   <th>Android</th>
   <th>Chrome for Android</th>
   <th>Firefox Mobile (Gecko)</th>
   <th>IE Mobile</th>
   <th>Opera Mobile</th>
   <th>Safari Mobile</th>
  </tr>
  <tr>
   <td>Switch default</td>
   <td>{{CompatVersionUnknown}}</td>
   <td>{{CompatVersionUnknown}}</td>
   <td>{{CompatVersionUnknown}}</td>
   <td>{{CompatVersionUnknown}}</td>
   <td>{{CompatVersionUnknown}}</td>
   <td>{{CompatVersionUnknown}}</td>
  </tr>
  <tr>
   <td>Export default</td>
   <td>{{CompatNo}}</td>
   <td>{{CompatNo}}</td>
   <td>{{CompatNo}}</td>
   <td>{{CompatNo}}</td>
   <td>{{CompatNo}}</td>
   <td>{{CompatNo}}</td>
  </tr>
 </tbody>
</table>
</div>

<h2 id="See_also">See also</h2>

<ul>
 <li>{{jsxref("Statements/export", "export")}}</li>
 <li>{{jsxref("Statements/switch", "switch")}}</li>
</ul>