`toString()` 方法返回一个表示当前函数源代码的字符串。

## 语法

    function.toString()

### 返回值

表示函数源代码的一个字符串

## 描述

`Function`对象覆盖了从`Object`继承来的`toString` 方法。对于用户定义的 `Function` 对象，`toString`方法返回一个字符串，其中包含用于定义函数的源文本段。

在`Function`需要转换为字符串时，通常会自动调用函数的 `toString` 方法。

若 `this` 不是 `Function` 对象，则 `toString()` 方法将抛出 `TypeError` ("Function.prototype.toString called on incompatible object") 异常，比如 `Proxy` 对象就会抛出异常。

    Function.prototype.toString.call('foo'); // TypeError

如果是在内置函数或由 `Function.prototype.bind` 返回的函数上调用 `toString()`，则`toString()` 返回原生代码字符串，如下

    "function () {\n    [native code]\n}"

若是在由 `Function` 构造器生成的函数上调用 `toString()` ，则 `toString()` 返回创建后的函数源码，包括形参和函数体，函数名为 "anonymous"。

## 示例

Function | Function.prototype.toString result
--- | ---
    function f(){}
    "function f(){}"
    class A { a(){} }
    "class A { a(){} }"
    function* g(){}
    "function* g(){}"
    a => a
    "a => a"
    ({ a(){} }.a)
    "a(){}"
    ({ *a(){} }.a)
    "*a(){}"
    ({ [0](){} }[0])
    "[0](){}"
    Object.getOwnPropertyDescriptor({
        get a(){}
    }, "a").get
    "get a(){}"
    Object.getOwnPropertyDescriptor({
        set a(x){}
    }, "a").set
    "set a(x){}"
    Function.prototype.toString
    "function toString() { [native code] }"
    (function f(){}.bind(0))
    "function () { [native code] }"
    Function("a", "b")
    "function anonymous(a\n) {\nb\n}"

## 规范

规范版本 | 规范状态 | 注解
--- | --- | ---
ECMAScript 1st Edition (ECMA-262) | Standard | 初始定义。在 JavaScript 1.1 中实现。
ECMAScript 2015 (6th Edition, ECMA-262) Function.prototype.toString | Standard | 对字符串表示增加了更多的特定需求。
Function.prototype.toString revision | Draft | 对内置函数与行尾表示进行标准化。
ECMAScript Latest Draft (ECMA-262) Function.prototype.toString | Draft |

## 浏览器兼容性

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
`toString` | Chrome Full support Yes | Edge Full support 12 | Firefox Full support 1 | IE Full support 5 | 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
Support of toString revision | Chrome No support No | Edge No support No | Firefox Full support 54 | IE No support No | Opera No support No | Safari No support No | WebView Android No support No | Chrome Android No support No | Edge Mobile No support No | Firefox Android Full support 54 | Opera Android No support No | Safari iOS No support No | Samsung Internet Android No support No | nodejs No support No

### Legend

Full support
  Full support
No support
  No support

## 附注（针对Firefox）

- 从Firefox 17开始，`Function.prototype.toString()` 通过保存函数源码的方式来实现，而之前是通过反编译器反编译函数字节码的方式来实现。反编译器已经被移除，因此我们不再需要 `indentation` 参数。查看 bug 761723 获得更多信息。
- 从Firefox 38开始，`Function.prototype.toString()` 会对 `Proxy` 对象抛出异常 (bug 1100936)。

## 相关链接

- `Object.prototype.toString()`