Array.prototype[@@unscopables]

The @@unscopables data property 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.

The default Array properties that are ignored for with statement-binding purposes are:

• at()
• copyWithin()
• entries()
• fill()
• find()
• findIndex()
• includes()
• keys()
• values()

See Symbol.unscopables for how to set unscopables for your own objects.

Imagine the keys.push('something') call below is in code that was written prior to ECMAScript 2015.

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.

var keys = [];

with (Array.prototype) {
  keys.push('something');
}

Object.keys(Array.prototype[Symbol.unscopables]);
// ["at", "copyWithin", "entries", "fill", "find", "findIndex",
//  "includes", "keys", "values"]

• Symbol.unscopables