async await与promise

1.async 的返回值一定是一个promise。,即使你的函数里没有return.

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// 测试async的返回值 async function testAsync() { } let result = testAsync(); console.log(result);   // 结果 Promise { undefined }

　　

2.因为有了await的限制，所以这里的async加不加都一样。

 3.

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// ----------- // 一个函数前如果加async和没有加sync的区别 function testSometing() {  console.log("进入testSometing");  return "执行testSometingreturn"; } async function testAsync() {  console.log("进入testAsync");  return Promise.resolve("hello async"); } async function test() {   console.log("进入test")   let aa = await testSometing();   console.log(aa)   let a1 = await testAsync();   console.log(a1)   console.log("离开test") } test() var promise = new Promise((resolve)=> {   console.log("进入promise");   resolve("执行promise回调"); });//关键点2 promise.then((val)=> console.log(val)); console.log("进程结束")

　　

 

 

例子3 

var fs = require('fs');
async function asyncPrint() {
  await new Promise(function (resolve,reject) {
    fs.readFile('await1_attach1.js','utf8',function(err,data){console.log('1----',data)
    if(err){
        reject(err);
    }else{
        resolve(data);
    }})
  })
  await new Promise(function (resolve,reject) {
    fs.readFile('await1_attach1.js','utf8',function(err,data){console.log('2----',data)
    if(err){
        reject(err);
    }else{
        resolve(data);
    }})
  })

  console.log('3----')
}
asyncPrint().then(() => {
  console.log('4-----')
}).then(() => {
  console.log('5-----')
})