--- /dev/null
+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.es/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;
+};