sync和await
1. Role?
Promise to simplify the use of the object: no need to use then () to specify the success / failure of a callback function
to synchronize coding (not a callback) way to implement asynchronous processes
2. Where write await?
Write await the promise of expression on the left returns: do not want to promise, want to promise asynchronous execution of a successful value data
3. Where write async?
Where the left side await function (recent) defined write async