read 3 file with callback , Promise and generator

readfile.js

'use strict'
var co = require('co')
var fs = require('fs')
var cofs = require('co-fs')


var r1 = exports.r1 = function(fileName){
  fs.readFile(fileName, (err,data)=>{
    fs.readFile(data.toString().trim(), (err,data)=>{
      fs.readFile(data.toString().trim(),(err,data)=>{
        fs.readFile(data.toString().trim(), (err,data)=>{
          console.log(data.toString())
        })
      })
    })
  })
}

function readFileQ(fileName){
  return new Promise(
      function(resolve, reject) {
        fs.readFile(fileName, (err,data)=>{
          if(!!err){
            reject(err)
          }else{
            resolve(data.toString().trim())
          }
        })
      }
    )
}

var r2 = exports.r2 = function(fileName){
  readFileQ(fileName).
  then((d)=>{
    return readFileQ(d)
  }).
  then((d)=>{
    return readFileQ(d)
  }).
  then((d)=>{
    return readFileQ(d)
  }).
  then((d)=>{
    console.log(d)
  })
}

var r3 = exports.r3 = function(fileName){
  co(function*(){
    var a = yield cofs.readFile(fileName)
    var b = yield cofs.readFile(a.toString().trim())
    var c = yield cofs.readFile(b.toString().trim())
    var d = yield cofs.readFile(c.toString().trim())
    console.log(d.toString().trim())
  })
}


r1('./a')
r2('./a')
r3('./a')