promise.all简易实现

function all (arr) {
  return new Promise((resolve, reject) => {
    let rtnArr = []
    let count = 0
    for (let i = 0; i < arr.length; i++) {
      Promise.resolve(arr[i]).then(res => {
        rtnArr[i] = res
        count += 1
        count == arr.length && resolve(rtnArr)
      }, reject)
    }
  })
}