Array.prototype[@@unscopables]
The @@unscopables
data property of Array.prototype
is shared by all Array
instances. It contains property names that were not included in the ECMAScript standard prior to the ES2015 version and that are ignored for with
statement-binding purposes.
Value
A null
-prototype object with property names given below and their values set to true
.
Property attributes of Array.prototype[@@unscopables] |
|
---|---|
Writable | no |
Enumerable | no |
Configurable | yes |
Description
The default Array
properties that are ignored for with
statement-binding purposes are:
at()
copyWithin()
entries()
fill()
find()
findIndex()
findLast()
findLastIndex()
flat()
flatMap()
includes()
keys()
toReversed()
toSorted()
toSpliced()
values()
Array.prototype[@@unscopables]
is an empty object only containing all the above property names with the value true
. Its prototype is null
, so Object.prototype
properties like toString
won't accidentally be made unscopable, and a toString()
within the with
statement will continue to be called on the array.
See Symbol.unscopables
for how to set unscopable properties for your own objects.
Examples
Imagine the keys.push('something')
call below is in code that was written prior to ECMAScript 2015.
js
var keys = [];
with (Array.prototype) {
keys.push("something");
}
When ECMAScript 2015 introduced the Array.prototype.keys()
method, if the @@unscopables
data property had not also been introduced, that keys.push('something')
call would break — because the JavaScript runtime would have interpreted keys
as being the Array.prototype.keys()
method, rather than the keys
array defined in the example code.
So the @@unscopables
data property for Array.prototype
causes the Array
properties introduced in ECMAScript 2015 to be ignored for with
statement-binding purposes — allowing code that was written prior to ECMAScript 2015 to continue working as expected, rather than breaking.
Specifications
Specification |
---|
ECMAScript Language Specification # sec-array.prototype-@@unscopables |
Browser compatibility
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