函数默认参数允许在没有值或`undefined`被传入时使用默认形参。

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## 语法

    function [name]([param1[ = defaultValue1 ][, ..., paramN[ = defaultValueN ]]]) {
        statements
    }

## 描述

JavaScript 中函数的参数默认是``undefined``。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。

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`undefined`，如果是的话就设置为默认的值。

下面的例子中，如果在调用`multiply`时，参数`b`的值没有提供，那么它的值就为`undefined`。如果直接执行`a * b`，函数会返回 `NaN`。

    function multiply(a, b) {
      return a * b;
    }

    multiply(5, 2); // 10
    multiply(5);    // NaN !

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`multiply`，则`b`设置为`1`：

    function multiply(a, b) {
      b = (typeof b !== 'undefined') ?  b : 1;
      return a * b;
    }

    multiply(5, 2); // 10
    multiply(5);    // 5

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`b`的默认值置为`1`：

    function multiply(a, b = 1) {
      return a * b;
    }

    multiply(5, 2); // 10
    multiply(5);    // 5

## 示例

### 传入 `undefined` vs 其他假值

在第二次调用中，即使第一个参数在调用时显式设置为`undefined`（虽然不是`null`或其他falsy值），但是`num`参数的值是默认值。

    function test(num = 1) {
      console.log(typeof num);
    }

    test();          // 'number' (num is set to 1)
    test(undefined); // 'number' (num is set to 1 too)

    // test with other falsy values:
    test('');        // 'string' (num is set to '')
    test(null);      // 'object' (num is set to null)

### 调用时解析

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。

    function append(value, array = []) {
      array.push(value);
      return array;
    }

    append(1); //[1]
    append(2); //[2], not [1, 2]

这个规则对于函数和变量也是适用的。

    function callSomething(thing = something()) {
     return thing;
    }

    let numberOfTimesCalled = 0;
    function something() {
      numberOfTimesCalled += 1;
      return numberOfTimesCalled;
    }

    callSomething(); // 1
    callSomething(); // 2

### 默认参数可用于后面的默认参数

已经遇到的参数可用于以后的默认参数：

    function greet(name, greeting, message = greeting + ' ' + name) {
        return [name, greeting, message];
    }

    greet('David', 'Hi');  // ["David", "Hi", "Hi David"]
    greet('David', 'Hi', 'Happy Birthday!');  // ["David", "Hi", "Happy Birthday!"]

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。

    function go() {
      return ':P';
    }

    function withDefaults(a, b = 5, c = b, d = go(), e = this,
                          f = arguments, g = this.value) {
      return [a, b, c, d, e, f, g];
    }

    function withoutDefaults(a, b, c, d, e, f, g) {
      switch (arguments.length) {
        case 0:
          a;
        case 1:
          b = 5;
        case 2:
          c = b;
        case 3:
          d = go();
        case 4:
          e = this;
        case 5:
          f = arguments;
        case 6:
          g = this.value;
        default:
      }
      return [a, b, c, d, e, f, g];
    }

    withDefaults.call({value: '=^_^='});
    // [undefined, 5, 5, ":P", {value:"=^_^="}, arguments, "=^_^="]

    withoutDefaults.call({value: '=^_^='});
    // [undefined, 5, 5, ":P", {value:"=^_^="}, arguments, "=^_^="]

### 函数嵌套定义

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`ReferenceError`（现在是 `TypeError`），参见 bug 1022967。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。

    // Doesn't work! Throws ReferenceError.
    function f(a = go()) {
      function go() { return ':P'; }
    }

### 位于默认参数之后非默认参数

在Gecko 26 (Firefox 26 / Thunderbird 26 / SeaMonkey 2.23 / Firefox OS 1.2)之前，以下代码会造成`SyntaxError`错误。这已经在bug 1022967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。

    function f(x = 1, y) {
      return [x, y];
    }

    f(); // [1, undefined]
    f(2); // [2, undefined]

### 有默认值的解构参数

你可以通过解构赋值为参数赋值：

    function f([x, y] = [1, 2], {z: z} = {z: 3}) {
      return x + y + z;
    }

    f(); // 6

## 规范

Specification | Status | Comment
--- | --- | ---
ECMAScript 2015 (6th Edition, ECMA-262) Function Definitions | Standard | Initial definition.
ECMAScript Latest Draft (ECMA-262) Function Definitions | Draft |

## 浏览器兼容

The compatibility table on this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request.

Update compatibility data on GitHub

| Desktop | Mobile | Server
| Chrome | Edge | Firefox | Internet Explorer | Opera | Safari | Android webview | Chrome for Android | Edge Mobile | Firefox for Android | Opera for Android | Safari on iOS | Samsung Internet | Node.js
Default parameters | Chrome Full support 49 | Edge Full support 14 | Firefox Full support 15 | IE No support No | Opera Full support 36 | Safari Full support 10 | WebView Android Full support 49 | Chrome Android Full support 49 | Edge Mobile Full support 14 | Firefox Android Full support 15 | Opera Android Full support 36 | Safari iOS Full support 10 | Samsung Internet Android Full support 5.0 | nodejs Full support 6.0.0
Parameters without defaults after default parameters | Chrome Full support 49 | Edge Full support 14 | Firefox Full support 26 | IE No support No | Opera Full support 36 | Safari Full support 10 | WebView Android Full support 49 | Chrome Android Full support 49 | Edge Mobile Full support 14 | Firefox Android Full support 26 | Opera Android Full support 36 | Safari iOS Full support 10 | Samsung Internet Android Full support 5.0 | nodejs Full support Yes
Destructured parameter with default value assignment | Chrome Full support 49 | Edge ? | Firefox Full support 41 | IE No support No | Opera ? | Safari ? | WebView Android Full support 49 | Chrome Android Full support 49 | Edge Mobile ? | Firefox Android Full support 41 | Opera Android ? | Safari iOS ? | Samsung Internet Android Full support 5.0 | nodejs Full support Yes

### Legend

Full support
  Full support
No support
  No support
Compatibility unknown
  Compatibility unknown

## 相关链接

- Original proposal at ecmascript.org