关于javascript:asyncawait笔记

async/await

1.async关键字只能放在函数前,被async润饰的函数返回的是一个promise，其值主动被包装在promise的resolve中

async function func() {
  return 1
}

2.await只能在async包裹的函数能应用，await会让JavaScript引擎期待直到promise实现并返回后果

async function f() {

  let promise = new Promise((resolve, reject) => {
    setTimeout(() => resolve("done!"), 1000)
  });

  let result = await promise; // 期待，直到 promise resolve (*)

  alert(result); // "done!"
}

f();

async+await相当于promise.then，然而只会承受resolve、不会解决reject

3.try…catch应用try…catch在包裹await来解决promise返回的reject,当遇到reject会间接跳到catch中

    async function fucntionTry() {
      try {
        let promiseResolve =await  new Promise((resolve,reject) => {
          setTimeout(()=>resolve('5秒'),5000)
        })
        console.log(promiseResolve)
        let promiseReject = await  new Promise((resolve,reject) => {
            reject('I am Reject')
        })
        let promiseEnd = await new Promise((resolve,reject) => {
          resolve('Process End')
        })
        console.log(promiseEnd)
      } catch(err) {
        console.log('try catch ', err)
      }
    }

    fucntionTry()

4.prmoise.all([])用于同时执行多个promise，留神promise.all中的promise是异步执行的

async function PromiseAll() {
      try {
        let promiseArray = await Promise.all([
        new Promise((resolve,reject) => {resolve(1)}),
        new Promise((resolve,reject) => {resolve(2)}),
        new Promise((resolve,reject) => {resolve(3)}),
        new Promise((resolve,reject) => {resolve(4)}),
      ])
      console.log('promiseArray', promiseArray)
        let promiseErrArray = await Promise.all([
          new Promise((resolve,reject) => {resolve(5)}),
          new Promise((resolve,reject) => {reject(6)}),
          new Promise((resolve,reject) => {resolve(7)}),
          new Promise((resolve,reject) => {resolve(8)}),
        ])
      }catch(err) {
        console.log('Error', err)
      }

    }

    PromiseAll()

同样，也能够应用try…catch包裹着，来捕捉异样