SOURCE

let compose = function(...args) {
  const init = args.pop()
  return function(...arg) {
    return args.reverse().reduce(function(sequence, func) {
      return sequence.then(function(result) {
        return func.call(null, result)
      })
    }, Promise.resolve(init.apply(null, arg)))
  }
}

let a = async() => {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      console.log('xhr1')
      resolve('xhr1')
    }, 5000)
  })
}

let b = async() => {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      console.log('xhr2')
      resolve('xhr2')
    }, 3000)
  })
}
let steps = [a, b] // 从右向左执行
let composeFn = compose(...steps)

composeFn().then(res => { console.log(666) })


let arr = [1,2,3]
arr.pop()
console.log(arr.reverse())

var func = function(...arg){
    const [item] = arg.flat(Infinity)
    console.log(item)
}

func(1,2,3,4)