array.@@unscopables

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.

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.

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

Examples

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

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

See also

• Symbol.unscopables