promise async

Posted 2020-10-29 gyz418

最简用  promise

let res = function () {
  return new Promise((resolve, reject) => {   // 返回一个promise
    setTimeout(() => {
        resolve(10)
    }, 3000)
  })
}
res().then(x => {    // promise.then()
  let a = 20;
  a += x
  console.log(a);
})

多个异步用 async 

let res = function () {
  return new Promise((resolve,reject)=>{
    setTimeout(()=>{
      resolve(10)
    },3000)
  })
}
async function test(){
  console.log(‘before‘);  // 按顺序输出 before  b  after
  let b=await res()       // 可以按顺序执行多个 promise
  b+=30;
  console.log(‘b‘,b);
  console.log(‘after‘);
}

test();

async 返回 promise (直接在async函数里面操作就行了，不要再返回了)

let res = function () {
  return new Promise((resolve,reject)=>{
    setTimeout(()=>{
      resolve(10)
    },3000)
  })
}
async function test(){
  let b=await res()
  console.log(‘b‘,b);
  b+=30;
  return b;   // async  返回一个 promise
}

let cc=0
test().then(x=>{   // promise then
  cc=x;
  console.log(‘cc‘,cc);
})

async  try.. catch..

let res = function (val) {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      if (val > 3) {

        resolve(10)
      } else {
        reject(false)
      }
    }, 3000)
  })
}

async function test (val) {
  try {
    let b = await res(val)
    b+=20;
    console.log(‘b‘,b);
  } catch (e) {    //  promise有 reject时 要 try()catch{}
    if (e) {
      console.log(‘b is true‘);
    }else{
      console.log(‘b is false‘);
    }
  }
}

test(10);

promise try-catch

let res = function (val) {
  return new Promise((resolve, reject) => {   // 返回一个promise
    setTimeout(() => {
      if (val > 10) {
        resolve(10)
      }else{
        reject(‘no4‘)
      }
    }, 3000)
  })
}
res(1).then(x => {    // promise.then()
  let a = 20;
  a += x
  console.log(a);
}).catch(e=>{
  if(e===‘no‘){
    console.log(‘hahaha‘);
  }else{
    console.log(‘no input‘);
  }
})

 传统回调函数

let f = function (test) { // 传入一个函数
  setTimeout(()=>{
    let a = 10
    test(a)  // 把结果给一个函数
  },3000)
}
f(function (val) {  
  console.log(val);  // 10
})