Es6实现Promise.all和Promise.race

Promise.all([p1,p2,p3])中，all方法的参数条件

• 必须为数组
• 可以传入非promise，all自动将其转换为promise对象
• 传入的值必须按顺序输出
• 一旦又一个reject则状态立马变为reject，并将错误原因抛出

function myPromiseAll(promiseArr){
    // 为了让传入的值不是promise也返回promise
    return new Promise((resolve,reject) => {
        if(!Array.isArray(promiseArr)){
            throw('promiseArr必须为数组')
        }
        let resArr = []
        let len = promiseArr.length;
        let count = 0;
        for(let i = 0; i < len; i++){
            // Promise.resolve将数组中非promise转为promise
            Promise.resolve(promiseArr[i])
            .then(value => {
                count++
                resArr[i] = value
                
                if(count == len) return resolve(resArr)
            })
            .catch(err => {
                return reject(err)
            })
           
        }
    })
    
}

let p1 = Promise.resolve(1)
let p2 = Promise.resolve(2)
let p3 = Promise.resolve(3)

myPromiseAll([p1,p3,p2]).then(res => {
    console.log(res); // [1,3,2]
    
})

Promise.race()实现

function myPromiseRace(promiseArr){
    return new Promise((resolve,reject) => {
        if(!Array.isArray(promiseArr)) throw('参数必须为数组')
        let len = promiseArr.length
        for(let i = 0; i < len; i++){
            Promise.resolve(promiseArr[i]).then(val => {
                resolve(val)
            }).catch(err => {
                reject(err)
            })
        }
        
    })
   

}

测试

let p1 = new Promise((resolve,reject) => {
    setTimeout(() => {
        reject(1)
    },50)
})
let p2 = new Promise((resolve,reject) => {
    setTimeout(() => {
        reject(22)
    },100)
})
let p3 = new Promise((resolve,reject) => {
    setTimeout(() => {
        resolve(3)
    },700)
})
myPromiseRace([p2,p3,p1])
.then(res => console.log(res)) 
.catch(err=>console.log(err)) // 1

Promise实战，QPS为3，但是同时发出5个请求

• 实现一个直接的PromiseAll

function myPromiseAll(promiseArr){
    // 为了让传入的值不是promise也返回promise
    return new Promise((resolve,reject) => {
        if(!Array.isArray(promiseArr)){
            throw('promiseArr必须为数组')
        }
        let resArr = []
        let len = promiseArr.length;
        let count = 0;
        for(let i = 0; i < len; i++){
            // Promise.resolve将数组中非promise转为promise
            Promise.resolve(promiseArr[i])
            .then(value => {
                count++
                resArr[i] = value
                
                if(count == 3) return resolve(resArr)  //这里设置了并发最大为3，就返回结果
            })
            .catch(err => {
                return reject(err)
            })
           
        }
    })
    
}

let wake = (time) => {
    return new Promise((resolve, reject) => {
      setTimeout(() => {
        resolve(`${time / 1000}秒后醒来`)
      }, time)
    })
  }
  
  let p1 = wake(2000)
  let p2 = wake(3000)
  let p3 = wake(4000)
  let p4 = wake(5000)
  let p5 = wake(6000)
  
 let start = new Date().getSeconds()
  myPromiseAll([p1,p2,p3,p4,p5]).then(res => {
    console.log(res); // [ '2秒后醒来', '3秒后醒来', '4秒后醒来' ]
    console.log(`耗时:${new Date().getSeconds() -start }`); //4
    
})