在此页

• 描述
• 范例
• 规范
• 浏览器兼容性
• 另请参阅
• 相关话题

Symbol.unscopables well-known symbol is used to specify an object value of whose own and inherited property names are excluded from the with environment bindings of the associated object.

The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.

描述

@@unscopables symbol ( Symbol.unscopables ) can be defined on any object to exclude property names from being exposed as lexical variables in with with environment bindings. Note that if using 严格模式 , with statements are not available and will likely also not need this symbol.

Setting a property to true in an unscopables object will make it unscopable and therefore it won't appear in lexical scope variables. Setting a property to false will make it scopable and thus it will appear in lexical scope variables.

特性属性在 Symbol.unscopables
可写	no
可枚举	no
可配置	no

范例

Scoping in with statements

The following code works fine in ES5 and below. However, in ECMAScript 2015 and later, the Array.prototype.keys() method was introduced. That means that inside with environment "keys" would now be the method and not the variable. That's when the unscopable s symbol was introduced. A built-in unscopables setting is implemented as Array.prototype[@@unscopables] to prevent that some of the Array methods are being scoped into the with 语句。

var keys = [];
with (Array.prototype) {
  keys.push('something');
}
Object.keys(Array.prototype[Symbol.unscopables]);
// ["copyWithin", "entries", "fill", "find", "findIndex",
//  "includes", "keys", "values"]
					

Unscopables in objects

You can also set unscopables for your own objects.

var obj = {
  foo: 1,
  bar: 2
};
obj[Symbol.unscopables] = {
  foo: false,
  bar: true
};
with (obj) {
  console.log(foo); // 1
  console.log(bar); // ReferenceError: bar is not defined
}
					

规范

规范
ECMAScript (ECMA-262) The definition of 'Symbol.unscopables' in that specification.

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. 更新 GitHub 上的兼容性数据

	Desktop						Mobile						Server
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet	Node.js
unscopables	Chrome 45	Edge 12	Firefox 48	IE No	Opera 32	Safari 9	WebView Android 45	Chrome Android 45	Firefox Android 48	Opera Android 32	Safari iOS 9	Samsung Internet Android 5.0	nodejs 0.12

图例

完整支持

完整支持

不支持

不支持

另请参阅

• Array.prototype[@@unscopables]
• with statement (not available in 严格模式 )

元数据

• 最后修改： May 7, 2020

相关话题

1. 标准内置对象
2. Symbol
3. 特性
   1. Symbol.asyncIterator
   2. Symbol.hasInstance
   3. Symbol.isConcatSpreadable
   4. Symbol.iterator
   5. Symbol.match
   6. Symbol.matchAll
   7. Symbol.prototype.description
   8. Symbol.replace
   9. Symbol.search
   10. Symbol.species
   11. Symbol.split
   12. Symbol.toPrimitive
   13. Symbol.toStringTag
   14. Symbol.unscopables
4. 方法
   1. Symbol.for()
   2. Symbol.keyFor()
   3. Symbol.prototype.toSource()
   4. Symbol.prototype.toString()
   5. Symbol.prototype.valueOf()
   6. Symbol.prototype[@@toPrimitive]
5. 继承：
6. Function
7. 特性
   1. Function.arguments
   2. Function.caller
   3. Function.displayName
   4. Function.length
   5. Function.name
8. 方法
   1. Function.prototype.apply()
   2. Function.prototype.bind()
   3. Function.prototype.call()
   4. Function.prototype.toSource()
   5. Function.prototype.toString()
9. Object
10. 特性
    1. Object.prototype.__proto__
    2. Object.prototype.constructor
11. 方法
    1. Object.prototype.__defineGetter__()
    2. Object.prototype.__defineSetter__()
    3. Object.prototype.__lookupGetter__()
    4. Object.prototype.__lookupSetter__()
    5. Object.prototype.hasOwnProperty()
    6. Object.prototype.isPrototypeOf()
    7. Object.prototype.propertyIsEnumerable()
    8. Object.prototype.toLocaleString()
    9. Object.prototype.toSource()
    10. Object.prototype.toString()
    11. Object.prototype.valueOf()
    12. Object.setPrototypeOf()