Symbol.unscopables
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.
Description
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 |
Examples
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 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"]
Unscopables in objects
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 |
---|
ECMAScript Language Specification # sec-symbol.unscopables |
Browser compatibility
BCD tables only load in the browser
See also
Array.prototype[@@unscopables]
with
statement (not available in Strict mode)