Symbol.unscopables

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

Document Tags and Contributors

 Contributors to this page: ZeikJT, mata007, JimmyVV, fscholz
 Last updated by: ZeikJT,