Promise-then的链式调用

const fs = require('fs')
function pReadFile(filePath){
  return  new Promise((resolve, reject) => {
    fs.readFile(filePath, (err, data) => {
      if (err) {
        reject(err)
      } else {
        resolve(data)
      }
    })
  })
}

pReadFile('../data/a.txt')
  .then(data => {
    console.log(data.toString())
    return pReadFile('../data/b.txt')
},err => {
    console.log('读取文件失败',err)
})
  .then(data => { //该方法作为第二个promise实例对象的resolve
    console.log(data.toString())
    return pReadFile('../data/c.txt')
  })
  .then(data => { //该方法作为第三个promise实例对象的resolve
    console.log(data.toString())
  })