如果对JavaScript的async的使用不清楚，可以逐个阅读下面的例子理解:
- understanding_async_01
- understanding_async_02
- understanding_async_03
- understanding_async_04
- understanding_async_05
- understanding_async_06
- understanding_async_07
- understanding_async_08
- understanding_async_09
- understanding_async_10
- understanding_async_11
- understanding_async_12
- understanding_async_13
- understanding_async_14