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 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 unscopable
s 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 Latest Draft (ECMA-262) The definition of 'Symbol.unscopables' in that specification. |
Draft |
Browser compatibility
Feature | Chrome | Edge | Firefox (Gecko) | Internet Explorer/Edge | Opera | Safari |
---|---|---|---|---|---|---|
Basic support | 38 | (Yes) | 48 (48) | 12 | No support | 9 |
Feature | Android | Chrome for Android | Edge | Firefox Mobile (Gecko) | IE/Edge Mobile | Opera Mobile | Safari Mobile |
---|---|---|---|---|---|---|---|
Basic support | No support | No support | (Yes) | 48.0 (48) | 12 | No support | 9 |
See also
Array.prototype[@@unscopables]
with
statement (not available in Strict mode)