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()
• 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.

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