实现promiseAll

这个难度比较底，面试遇到的频率也挺高的

第一种

• 要求：必须等全部都resolved或者reject才返回
• 有一个reject就走catch
• 返回数组:（包括每一个resolved/reject的内容）
• 返回的数组结果对应的顺序与传入的promise顺序一致

朴素写法

/**
 * Promise.All
 * @param {Array<promise>} promises 
 */
function PromiseAll(promises) {
    const data = []
    let count = 0
    return new Promise((res, rej) => {
        for (let i = 0; i < promises.length; i++) {
            const p = promises[i]
            p.then(r => {
                data[i] = r
                count++
                if (count === promises.length) {
                    res(data)
                }
            }).catch(err => {
                data[i] = err
                count++
                if (count === promises.length) {
                    rej(data)
                }
            })
        }
    })
}

/**
 * Promise.All
 * @param {Array<promise>} promises 
 */
function PromiseAll(promises) {
    const data = []
    let count = 0
    return new Promise((res, rej) => {
        for (let i = 0; i < promises.length; i++) {
            const p = promises[i]
            p.then(r => {
                data[i] = r
                count++
                if (count === promises.length) {
                    res(data)
                }
            }).catch(err => {
                data[i] = err
                count++
                if (count === promises.length) {
                    rej(data)
                }
            })
        }
    })
}

测试

function createPromise(timeout, success = true) {
    return new Promise((res, rej) => {
        setTimeout(() => {
            if (success) {
                res(timeout)
                return
            }
            rej(timeout)
        }, timeout)
    })
}

const p1 = createPromise(1000)
const p2 = createPromise(3000)
const p3 = createPromise(2000,false)

PromiseAll([p1, p2, p3]).then(console.log).catch(console.error)
// [1000,3000,2000]

function createPromise(timeout, success = true) {
    return new Promise((res, rej) => {
        setTimeout(() => {
            if (success) {
                res(timeout)
                return
            }
            rej(timeout)
        }, timeout)
    })
}

const p1 = createPromise(1000)
const p2 = createPromise(3000)
const p3 = createPromise(2000,false)

PromiseAll([p1, p2, p3]).then(console.log).catch(console.error)
// [1000,3000,2000]