`Object.preventExtensions()`方法让一个对象变的不可扩展，也就是永远不能再添加新的属性。

## 语法

    Object.preventExtensions(obj)

### 参数

`obj`
  将要变得不可扩展的对象。

### 返回值

  已经不可扩展的对象。

## 描述

如果一个对象可以添加新的属性，则这个对象是可扩展的。`Object.preventExtensions()`将对象标记为不再可扩展，因此它将永远不会具有超出它被标记为不可扩展的属性。注意，一般来说，不可扩展对象的属性可能仍然可被删除。尝试将新属性添加到不可扩展对象将静默失败或抛出`TypeError`（最常见但不排除其他情况，如在strict mode中）。

`Object.preventExtensions()`仅阻止添加自身的属性。但属性仍然可以添加到对象原型。

一旦使其不可扩展，就无法再对象进行扩展。

## 例子

    // Object.preventExtensions将原对象变的不可扩展,并且返回原对象.
    var obj = {};
    var obj2 = Object.preventExtensions(obj);
    obj === obj2;  // true

    // 字面量方式定义的对象默认是可扩展的.
    var empty = {};
    Object.isExtensible(empty) //=== true

    // ...但可以改变.
    Object.preventExtensions(empty);
    Object.isExtensible(empty) //=== false

    // 使用Object.defineProperty方法为一个不可扩展的对象添加新属性会抛出异常.
    var nonExtensible = { removable: true };
    Object.preventExtensions(nonExtensible);
    Object.defineProperty(nonExtensible, "new", { value: 8675309 }); // 抛出TypeError异常

    // 在严格模式中,为一个不可扩展对象的新属性赋值会抛出TypeError异常.
    function fail()
    {
      "use strict";
      nonExtensible.newProperty = "FAIL"; // throws a TypeError
    }
    fail();

    // 一个不可扩展对象的原型是不可更改的,__proto__是个非标准魔法属性,可以更改一个对象的原型.
    var fixed = Object.preventExtensions({});
    fixed.__proto__ = { oh: "hai" }; // 抛出TypeError异常

## Notes

在 ES5 中，如果参数不是一个对象类型，将抛出一个`TypeError`异常。在 ES2015 中，非对象参数将被视为一个不可扩展的普通对象，因此会被直接返回。

    Object.preventExtensions(1);
    // TypeError: 1 is not an object (ES5 code)

    Object.preventExtensions(1);
    // 1                             (ES2015 code)

## 规范

Specification | Status | Comment
--- | --- | ---
ECMAScript 5.1 (ECMA-262) Object.preventExtensions | Standard | Initial definition. Implemented in JavaScript 1.8.5.
ECMAScript 2015 (6th Edition, ECMA-262) Object.preventExtensions | Standard |
ECMAScript Latest Draft (ECMA-262) Object.preventExtensions | 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
`preventExtensions` | Chrome Full support 6 | Edge Full support 12 | Firefox Full support 4 | IE Full support 9 | Opera Full support 12 | Safari Full support 5.1 | 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
ES2015 behavior for non-object argument | Chrome Full support 44 | Edge ? | Firefox Full support 35 | IE Full support 11 | Opera Full support 31 | Safari Full support 9 | WebView Android Full support Yes | Chrome Android Full support Yes | Edge Mobile ? | Firefox Android Full support 35 | Opera Android Full support Yes | Safari iOS Full support 9 | Samsung Internet Android Full support Yes | nodejs Full support Yes

### Legend

Full support
  Full support
Compatibility unknown
  Compatibility unknown

## 相关链接

- `Object.isExtensible()`
- `Object.seal()`
- `Object.isSealed()`
- `Object.freeze()`
- `Object.isFrozen()`
- `Reflect.preventExtensions()`