javascript Promise 一些底層實現

Promise.all 

---

定義：當Promise 結果都爲resolve 返回resolve的數組集，當一個爲reject，將第一個遇到的reject返回。

const p1 = Promise.resolve('ok1')
const p2 = Promise.reject('no2')
const p3 = Promise.resolve('ok3')
const p4 = Promise.reject('no4')

/**
 * 
 * @param {*} promiseArray Promise 數組集
 * 返回結果爲Promise
 */
function PromiseAll(promiseArray) {
    return new Promise(function (resolve, reject) {
        let countNum = 0;
        let promiseArrayResult = new Array()
        let promiseNum = promiseArray.length;
        for (let i = 0; i < promiseNum; i++) {
            promiseArray[i].then((item) => {
                promiseArrayResult.push(item)
                if(i===(promiseNum-1)){
                    resolve(promiseArrayResult)
                    return ;
                }
            },(err)=>{
                reject(err)
            }).catch((errData)=>{
                console.log(errData) //
            })
        }
    })
}

PromiseAll([p1, p2, p3,p4]).then((resultData)=>{
    console.log(resultData) //['ok1','no2','ok3']
},(errDate)=>{
    console.log(errDate)
}).catch((errData)=>{
    console.log(errData) //
})

Promise.race

---

 上面代碼中，只要p1、p2、p3之中有一個實例率先改變狀態，p的狀態就跟着改變。那個率先改變的 Promise 實例的返回值，就傳遞給p的回調函數。

const p1 = new Promise(function (resolve, reject) {
    console.log('這段我也執行了一')
    setTimeout(() => reject(new Error('request timeout')), 2000)
  })
  const p3 = new Promise(function (resolve, reject) {
    console.log('這段我也執行了二')
    setTimeout(() => reject(new Error('request timeout2')), 1000)
  })
  const p2 = new Promise(function (resolve, reject) {
      console.log('這段我也執行了三')
    setTimeout(() => resolve('我成功啦'), 4000)
  })

/**
 * 
 * @param {*} promiseArray Promise 數組集
 * 返回結果爲Promise
 */
function PromiseRace(promiseArray) {
    return new Promise(function (resolve, reject) {
        let countNum = 0;
        let promiseArrayResult = new Array()
        let promiseNum = promiseArray.length;
        for (let i = 0; i < promiseNum; i++) {
            promiseArray[i].then((item) => {
                    resolve(item)
                    return ;
               
            },(item)=>{
                reject(item)
            }).catch((errData)=>{
                reject(errData) //
            })
        }
    })
}

PromiseRace([p1, p2,p3]).then((resultData)=>{
    console.log(resultData) //['ok1','no2','ok3']
},(errDate)=>{
    console.log(errDate)
}).catch((errData)=>{
    console.log(errData) //
})