node promise库bluebird

var fs = require('fs')
var Promise = require("bluebird")

function file1() {
  return new Promise((resolve, reject) => {
    fs.writeFile('cc/file1.txt', 'file1', (err) => {
      if (err) {
        throw new Error()
      }
      console.log('ok1')
      resolve('ok1')
    })
  })
}

function file2() {
  return new Promise((resolve, reject) => {
    fs.writeFile('cc/file2.txt', 'file2', (err) => {
      if (err) {
        reject()
      }
      console.log('ok2')
      resolve('ok2')
    })
  })
}

function file3() {
  return new Promise((resolve, reject) => {
    fs.writeFile('cc/file3.txt', 'file3', (err) => {
      if (err) {
        throw new Error()
      }
      console.log('ok3')
      resolve('ok3')
    })
  })
}

file1().then(file2).then(file3)