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.

// 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.

// -----------
// 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-----')
})