用promise实现一个promise.all

let PromiseAll = (promises)=>{
    return new Promise((resolve, reject)=>{
        let results = new Array(promises.length)
        for(let i=0;i<promises.length;i++){
            let promise = promises[i]
            promise.then(res => {
            results[i] = res
            if(i === promises.length-1){
                   resolve(results)
                }
            }).catch(err => {
                    reject(err)
            })
        }
    })
}