【Javascript Iterator】
1、@@iterator
Whenever an object needs to be iterated (such as at the beginning of a for..of loop), its @@iteratormethod is called with no arguments, and the returned iterator is used to obtain the values to be iterated.
示例：
指定的方法必须是funtion*，否则出错
2、for...of
Whenever an object needs to be iterated (such as at the beginning of a for..of loop), its @@iteratormethod is called with no arguments, and the returned iterator is used to obtain the values to be iterated.
3、The iterator protocol
参考：
1、https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol/iterator
2、https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Iteration_protocols
