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.

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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


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.

const keys = [];

with (Array.prototype) {

// ["copyWithin", "entries", "fill", "find", "findIndex",
//  "includes", "keys", "values"]

Unscopables in objects

You can also set unscopables for your own objects.

const 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


ECMAScript Language Specification
# sec-symbol.unscopables

Browser compatibility

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See also