异步终极方案-Promise

什么是Promise

青铜段位

 class MyPromise {
     constructor(fn) {
         this.status = 'penging'
         this.fulfilled = null
         this.rejected = null
         this.finallyFn = null
         const onResolvedCallback = (value) => {
             this.status = 'fulfilled'
             this.fulfilled(value)
             this.finallyFn(value)
         }
         const onRejectedCallback = (value) => {
             this.status = 'rejected'
             this.rejected(value)
             this.finallyFn(value)
         }
         fn(onResolvedCallback, onRejectedCallback)
     }

     then(fn) {
         this.fulfilled = fn
         return this
     }
     catch (fn) {
         this.rejected = fn
         return this
     } finally(fn) {
         this.finallyFn = fn
         return this
     }
 }

function post() {
    return new MyPromise((resolve, reject) => {
        setTimeout(() => {
            Math.random() > 0.5 ? resolve('成功') : reject('失败')
        }, 1000);
    })
}

post().then(res => {
    console.log(res)
}).catch(err => {
    console.log(err)
}).finally(res => {
    console.log(res)
})

静态方法

Promise.all

Promise.any

Promise.race

Promise.allSettled