ES6中的 async/await 是什么呢？

async/await 是什么？

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

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

• 自动将常规函数转换成Promise，返回值也是一个Promise对象
• 只有async函数内部的异步操作执行完，才会执行then方法指定的回调函数
• 异步函数内部可以使用 await

2、Await — 暂停异步的功能执行 ( var result = await someAsyncCall() ) 😉

• 放置在Promise调用之前，await强制其他代码等待，直到Promise完成并返回结果
• 只能与Promise一起使用，不适用与回调
• 只能在async函数内部使用

What are the async and await ?

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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