async/await笔记

async/await

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

async function func() {
  return 1
}

2.await只能在async包裹的函数能使用，await会让JavaScript引擎等待直到promise完成并返回结果

async function f() {

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

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

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

f();

async+await相当于promise.then，但是只会接受resolve、不会处理reject

3.try...catch使用try...catch在包裹await来处理promise返回的reject,当遇到reject会直接跳到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，注意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，也可以使用try...catch包裹着，来捕获异常