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](https://github.com/fanfeilong/jsasync/blob/master/code/understanding_async/understanding_async_01.js)
* [understanding_async_02](https://github.com/fanfeilong/jsasync/blob/master/code/understanding_async/understanding_async_02.js)
* [understanding_async_03](https://github.com/fanfeilong/jsasync/blob/master/code/understanding_async/understanding_async_03.js)
* [understanding_async_04](https://github.com/fanfeilong/jsasync/blob/master/code/understanding_async/understanding_async_04.js)
* [understanding_async_05](https://github.com/fanfeilong/jsasync/blob/master/code/understanding_async/understanding_async_05.js)
* [understanding_async_06](https://github.com/fanfeilong/jsasync/blob/master/code/understanding_async/understanding_async_06.js)
* [understanding_async_07](https://github.com/fanfeilong/jsasync/blob/master/code/understanding_async/understanding_async_07.js)
* [understanding_async_08](https://github.com/fanfeilong/jsasync/blob/master/code/understanding_async/understanding_async_08.js)
* [understanding_async_09](https://github.com/fanfeilong/jsasync/blob/master/code/understanding_async/understanding_async_09.js)
* [understanding_async_10](https://github.com/fanfeilong/jsasync/blob/master/code/understanding_async/understanding_async_10.js)
* [understanding_async_11](https://github.com/fanfeilong/jsasync/blob/master/code/understanding_async/understanding_async_11.js)
* [understanding_async_12](https://github.com/fanfeilong/jsasync/blob/master/code/understanding_async/understanding_async_12.js)
* [understanding_async_13](https://github.com/fanfeilong/jsasync/blob/master/code/understanding_async/understanding_async_13.js)
* [understanding_async_14](https://github.com/fanfeilong/jsasync/blob/master/code/understanding_async/understanding_async_14.js)