core-js/internals/add-to-unscopables.js

'use strict';
var wellKnownSymbol = require('../internals/well-known-symbol');
var create = require('../internals/object-create');
var defineProperty = require('../internals/object-define-property').f;

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

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

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