7.4 Operations on Iterator Objects https://tc39.github.io/ecma262/#sec-operations-on-iterator-objects
function GetIterator(obj) {
            var method = GetMethod(obj, iteratorSymbol);
            if (!IsCallable(method))
                throw new TypeError(); // from Call
            var iterator = method.call(obj);
            if (!IsObject(iterator))
                throw new TypeError();
            return iterator;
        }