Array.prototype[@@unscopables]

The @@unscopable symbol property contains property names that were not included in the ECMAScript standard prior to the ES2015 (ES6) version. These properties are excluded from with statement bindings.

Property attributes of Array.prototype[@@unscopables]
Writable no
Enumerable no
Configurable yes

Syntax

arr[Symbol.unscopables]

Description

The default array properties that are excluded from with bindings are: copyWithin, entries, fill, find, findIndex, includes, keys, and values.

See Symbol.unscopables for how to set unscopables for your own objects.

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 environments, "keys" would now be the method and not the variable. This is where now the built-in @@unscopables Array.prototype[@@unscopables] symbol property comes into play and prevents 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"]

Specifications

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

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
Basic support No support 48 (48) No support No support No support
Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support No support No support 48.0 (48) No support No support No support

See also

Document Tags and Contributors

 Contributors to this page: matt-tingen, evilpie, fscholz
 Last updated by: matt-tingen,