`valueOf()` 方法返回一个`String`对象的原始值（primitive value）。

## 语法

    str.valueOf()

## 描述

`String` 对象的 `valueOf` 方法返回一个`String`对象的原始值。该值等同于`String.prototype.toString()`。

该方法通常在 JavaScript 内部被调用，而不是在代码里显示调用。

## 示例

### 例子：使用 `valueOf`

    x = new String("Hello world");
    alert(x.valueOf())          // Displays "Hello world"

## 规范

Specification | Status | Comment
--- | --- | ---
ECMAScript 1st Edition (ECMA-262) | Standard | Initial definition. Implemented in JavaScript 1.1.
ECMAScript 5.1 (ECMA-262) String.prototype.valueOf | Standard |
ECMAScript 2015 (6th Edition, ECMA-262) String.prototype.valueOf | Standard |

## 浏览器兼容性

We're converting our compatibility data into a machine-readable JSON format. This compatibility table still uses the old format, because we haven't yet converted the data it contains. Find out how you can help!

- Desktop
- Mobile

Feature | Chrome | Firefox (Gecko) | Internet Explorer | Opera | Safari
--- | --- | --- | --- | --- | ---
Basic support | (Yes) | (Yes) | (Yes) | (Yes) | (Yes)

Feature | Android | Chrome for Android | Firefox Mobile (Gecko) | IE Mobile | Opera Mobile | Safari Mobile
--- | --- | --- | --- | --- | --- | ---
Basic support | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) | (Yes)

## 相关链接

- `String.prototype.toString()`
- `Object.prototype.valueOf()`