`length` 属性指明函数的形参个数。

`Function.length` 属性的属性特性：
writable | false
enumerable | false
configurable | true

## 描述

`length` 是函数对象的一个属性值，指该函数有多少个必须要传入的参数，即形参的个数。

形参的数量不包括剩余参数个数，仅包括第一个具有默认值之前的参数个数。

与之对比的是， `arguments.length` 是函数被调用时实际传参的个数。

### `Function` 构造器的属性

`Function` 构造器本身也是个Function。他的 `length` 属性值为 1 。该属性 Writable: `false`, Enumerable: `false`, Configurable: `true`.

### `Function`.prototype 对象的属性

`Function.prototype` 对象的 length 属性值为 0 。

## 示例

    console.log(Function.length); /* 1 */

    console.log((function()        {}).length); /* 0 */
    console.log((function(a)       {}).length); /* 1 */
    console.log((function(a, b)    {}).length); /* 2 etc. */

    console.log((function(...args) {}).length);
    // 0, rest parameter is not counted

    console.log((function(a, b = 1, c) {}).length);
    // 1, only parameters before the first one with
    // a default value is counted

## 规范

Specification | Status | Comment
--- | --- | ---
ECMAScript 1st Edition (ECMA-262) | Standard | Initial definition. Implemented in JavaScript 1.1.
ECMAScript 5.1 (ECMA-262) Function.length | Standard |
ECMAScript 2015 (6th Edition, ECMA-262) Function.length | Standard | The `configurable` attribute of this property is now `true`.
ECMAScript Latest Draft (ECMA-262) Function.length | Draft |

## 浏览器兼容

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
`length` | Chrome Full support Yes | Edge Full support 12 | Firefox Full support 1 | IE Full support Yes | Opera Full support Yes | Safari Full support Yes | WebView Android Full support Yes | Chrome Android Full support Yes | Edge Mobile Full support Yes | Firefox Android Full support 4 | Opera Android Full support Yes | Safari iOS Full support Yes | Samsung Internet Android Full support Yes | nodejs Full support Yes
Configurable: true | Chrome ? | Edge ? | Firefox Full support 37 | IE ? | Opera ? | Safari ? | WebView Android ? | Chrome Android ? | Edge Mobile ? | Firefox Android Full support 37 | Opera Android ? | Safari iOS ? | Samsung Internet Android ? | nodejs ?

### Legend

Full support
  Full support
Compatibility unknown
  Compatibility unknown

## 参见

- `Function`