`handler.ownKeys()` 方法用于拦截 `Reflect.ownKeys()`.

## 语法

    var p = new Proxy(target, {
      ownKeys: function(target) {
      }
    });

### 参数

下面的参数被传递给`ownKeys。this`被绑定在`handler上。`

`target`
  目标对象.

### 返回值

`ownKeys` 方法必须返回一个可枚举对象.

## 描述

`handler.ownKeys()` 方法用于拦截 `Reflect.ownKeys()`.

### 拦截

该拦截器可以拦截以下操作::

- `Object.getOwnPropertyNames()`
- `Object.getOwnPropertySymbols()`
- `Object.keys()`
- `Reflect.ownKeys()`

### 约定

如果违反了下面的约定，proxy将抛出错误 `TypeError`:

- `ownKeys` 的结果必须是一个数组.
- 数组的元素类型要么是一个 `String` ，要么是一个 `Symbol`.
- 结果列表必须包含目标对象的所有不可配置（non-configurable ）、自有（own）属性的key.
- 如果目标对象不可扩展，那么结果列表必须包含目标对象的所有自有（own）属性的key，不能有其它值.

## 示例

下面的代码拦截 `Object.getOwnPropertyNames()`.

    var p = new Proxy({}, {
      ownKeys: function(target) {
        console.log('called');
        return ['a', 'b', 'c'];
      }
    });

    console.log(Object.getOwnPropertyNames(p)); // "called"
                                                // [ 'a', 'b', 'c' ]

下面的代码违反了约定

    var obj = {};
    Object.defineProperty(obj, 'a', {
      configurable: false,
      enumerable: true,
      value: 10 }
    );

    var p = new Proxy(obj, {
      ownKeys: function(target) {
        return [123, 12.5, true, false, undefined, null, {}, []];
      }
    });

    console.log(Object.getOwnPropertyNames(p));

    // TypeError: proxy [[OwnPropertyKeys]] 必须返回一个数组
    // 数组元素类型只能是String或Symbol

## 标准

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

## 浏览器兼容

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 | ? | 18 (18) | ? | ? | ?

Feature | Android | Chrome for Android | Firefox Mobile (Gecko) | IE Mobile | Opera Mobile | Safari Mobile
--- | --- | --- | --- | --- | --- | ---
Basic support | ? | ? | 18.0 (18) | ? | ? | ?

## 兼容性注意事项

### Firefox火狐

- 在Gecko 42 (Firefox 42 / Thunderbird 42 / SeaMonkey 2.39)版本中, `ownKey` 的实施已经更新了，为了反映最终的ES5标准 (see bug 1049662):
  - 现在需要检查结果是不是数组以及数组元素类型是不是string或symbol.
  - 枚举重复的自有的属性名称不再失败.

## 另见

- `Proxy`
- `handler`
- `Object.getOwnPropertyNames()`
- `Reflect.ownKeys()`