Please note, this is a STATIC archive of website developer.mozilla.org from 03 Nov 2016, cach3.com does not collect or store any user information, there is no "phishing" involved.

Symbol.unscopables

我们的志愿者还没有将这篇文章翻译为 中文 (简体)加入我们帮助完成翻译!

The 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.

Property attributes of Symbol.unscopables
Writable no
Enumerable no
Configurable no

Description

The @@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 Strict mode, 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.

Examples

The following code works fine in ES5 and below. However, in ECMAScript 2015 (ES6) 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 unscopables 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 statement.

var keys = [];

with(Array.prototype) {
  keys.push("something");
}

Object.keys(Array.prototype[Symbol.unscopables]); 
// ["copyWithin", "entries", "fill", "find", "findIndex", 
//  "includes", "keys", "values"]

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
}

Specifications

Specification Status Comment
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'Symbol.unscopables' in that specification.
Standard Initial definition.
ECMAScript 2017 Draft (ECMA-262)
The definition of 'Symbol.unscopables' in that specification.
Draft  

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer/Edge Opera Safari
Basic support 38 48 (48) 12 No support 9
Feature Android Chrome for Android Firefox Mobile (Gecko) IE/Edge Mobile Opera Mobile Safari Mobile
Basic support No support No support 48.0 (48) 12 No support 9

See also

文档标签和贡献者

 此页面的贡献者: ZeikJT, mata007, JimmyVV, fscholz
 最后编辑者: ZeikJT,