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

Description

The default array properties that are excluded from with bindings are:

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

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

Examples

Use in with environments

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 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

Browser compatibilityUpdate compatibility data on GitHub

Desktop
Chrome Edge Firefox Internet Explorer Opera Safari
@@unscopables 38 12 48 No 25 10
Mobile
Android webview Chrome for Android Firefox for Android Opera for Android Safari on iOS Samsung Internet
@@unscopables 38 38 48 25 10 3.0
Server
Node.js
@@unscopables 0.12

See also

© 2005–2018 Mozilla Developer Network and individual contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://wiki.developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/@@unscopables