Symbol.unscopables - JavaScript 编辑

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.

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.

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.

Description

The @@unscopables symbol (Symbol.unscopables) can be defined on any object to exclude property names from being exposed as lexical variables in 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.

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

Examples

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 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"]

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
}

Specifications

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

Browser compatibility

BCD tables only load in the browser

See also

• Array.prototype[@@unscopables]
• with statement (not available in Strict mode)