// 异步函数a
var a = function () {
 return new Promise(function (resolve, reject) {
  setTimeout(function () {
   resolve('a')
  }, 1000)
 })
}

// 异步函数b
var b = function (data) {
 return new Promise(function (resolve, reject) {
  resolve(data + 'b')
 })
}

// 异步函数c
var c = function (data) {
 return new Promise(function (resolve, reject) {
  setTimeout(function () {
   resolve(data + 'c')
  }, 500)
 })
}

//链式调用
a()
 .then(function (data) {
  return b(data)
 })
 .then(function (data) {
  return c(data)
 })
 .then(function (data) {
  console.log(data)// abc
 })

// 构建队列
function queue(arr) {
 var sequence = Promise.resolve()
 arr.forEach(function (item) {
  sequence = sequence.then(item)
 })
 return sequence
}

// 执行队列
queue([a, b, c])
 .then(data => {
  console.log(data)// abc
 })

async function queue(arr) {
 let res = null
 for (let promise of arr) {
  res = await promise(res)
 }
 return await res
}
queue([a, b, c])
 .then(data => {
  console.log(data)// abc
 })