async/await 是什么？

async 函数，就是 Generator 函数的语法糖，它建设在Promises上，并且与所有现有的基于Promise的API兼容。

1、Async — 申明一个异步函数 ( async function someName(){...} )

  • 主动将惯例函数转换成Promise，返回值也是一个Promise对象
  • 只有async函数外部的异步操作执行完，才会执行then办法指定的回调函数
  • 异步函数外部能够应用 await

2、Await — 暂停异步的性能执行 ( var result = await someAsyncCall() ) ????

  • 搁置在Promise调用之前，await强制其余代码期待，直到Promise实现并返回后果
  • 只能与Promise一起应用，不实用与回调
  • 只能在async函数外部应用

What are the async and await ?

The async function, it's a syntactic sugar of Geneartor function, it is built on Promise, and is compatible with all existing Promise-based API.

1、The async -- declare a async function ( async function someName(){...} )

  • Automatically convert regular functions to Promise, and the return value is also a Promise object.
  • Only after asynchronous operation inside async function is executed, then execute the callback function specified by then method.
  • we can use await inseide asynchronous function

2、The await -- Paused the asnychronous function execution ( var result = await someAsyncCall() )

  • Placed before the Promise Call, the await forced other codes to wait until the Promise completes and return the result.
  • Can only be used with Promise toghter, it's not suitable for callback
  • Can only be used inside async function