1. 入参必须实现iterator接口，一般为数组
2. 全部成功才算成功，有一个失败就失败, 如果成功，结果顺序和入参顺序保持一致
3. 返回一个新的promise,

    Promise._all = function (promises) {
        if (!promises instanceof Array) throw 'promises must be an array'
        const length = promises.length
        let count = 0
        const ret = []
        if (!length) return []
        return new Promise((resolve,reject) => {
            for (let i = 0; i < promises.length; i++) {
                promises[i].then(v => {
                    count++
                    ret[i] = v
                    count === length && resolve(ret)
                }, r => {
                    reject(r)
                })
            }
        })
    }

    const doAsyncTask = (fn, timeout) => setTimeout(() => fn(), timeout)
    const p1 = new Promise((resolve, reject) => doAsyncTask(()=>{reject(1)},2000))
    const p2 = new Promise((resolve, reject) => doAsyncTask(()=>{resolve(2)},1000))
    const p3 = new Promise((resolve, reject) => doAsyncTask(()=>{resolve(3)},3000))
    const res = Promise._all([p1,p2,p3])
    res.then(v=>console.log(v),r=>console.log(r))
    console.log(res);