add-to-unscopables.js

var wellKnownSymbol = require('../internals/well-known-symbol');
var create = require('../internals/object-create');
var definePropertyModule = require('../internals/object-define-property');

var UNSCOPABLES = wellKnownSymbol('unscopables');
var ArrayPrototype = Array.prototype;

// Array.prototype[@@unscopables]
// https://tc39.github.io/ecma262/#sec-array.prototype-@@unscopables
if (ArrayPrototype[UNSCOPABLES] == undefined) {
  definePropertyModule.f(ArrayPrototype, UNSCOPABLES, {
    configurable: true,
    value: create(null)
  });
}

// add a key to Array.prototype[@@unscopables]
module.exports = function (key) {
  ArrayPrototype[UNSCOPABLES][key] = true;
};