Promise

异步操作

Promise 使用

1. Promise.then, Promise.catch,Promise.resolve 默认返回一个 resolved 的 Promise

2. Promise.all()只要有一个返回一个拒绝状态就是拒绝状态

3. Promise.race() 只要有一个返回成功状态就是成功状态，哪个返回快就选择哪个作为返回值

   示例：设置接口超时时间

   function query(url, delay = 4000) {
     const promises = [
       ajax(url),
       new Promise((resolve, reject) => {
         setTimeout(() => {
           // eslint-disable-next-line prefer-promise-reject-errors
           reject('请求超时')
         }, delay)
       }),
     ]
     return Promise.race(promises).then(value => console.log(value)).catch(err => console.log(err))
   }

4. Promise.allSettled()，无论成功还是失败，都会返回一个状态

async & await

async 函数错误处理

1. 使用catch

   async function f() {
     return Promise.reject('出错了')
   }
   f().catch(err => console.log(err))

2. 捕获异常

   async function f() {
     try {
       await Promise.reject('出错了')
     }
     catch (e) {
       console.log('捕获异常')
     }
   }

3. 捕获多个异常

   async function f() {
     try {
       await Promise.reject('出错了1')
       await Promise.reject('出错了2')
     }
     catch (e) {
       console.log('捕获异常')
     }
   }

await 并行执行

1. 使用Promise.all

const p1 = () => new Promise(resolve => setTimeout(() => resolve('one'), 1000))
const p2 = () => new Promise(resolve => setTimeout(() => resolve('two'), 2000))
async function f() {
  const p = await Promise.all([p1, p2])
  console.log(p)
}
f()

2. 使用 parallel

const p1 = () => new Promise(resolve => setTimeout(() => resolve('one'), 1000))
const p2 = () => new Promise(resolve => setTimeout(() => resolve('two'), 2000))
async function f() {
  const p = await Promise.allSettled([p1, p2])
  console.log(p)
}
f()

3. 并行执行多个异步函数

const p1 = () => new Promise(resolve => setTimeout(() => resolve('one'), 1000))
const p2 = () => new Promise(resolve => setTimeout(() => resolve('two'), 2000))
async function f() {
  const r1 = p1()
  const r2 = p2()
  const r3 = await r1
  const r4 = await r2
  console.log(r3, r4)
}
f()