flattened-callbacks.js

// another kind of flattening; see: http://blog.vullum.io/javascript-flow-callback-hell-vs-async-vs-highland/

var express = require('express')  
var fs = require('fs')  
var app = express()

app.post('/process-file', onProcessFile)

function onProcessFile(req, res) {
  var inputFile = 'input.txt'
  var outputFile = 'output.txt'

  fs.readFile(inputFile, onReadFile)
  
  function onReadFile(err, data) {
    if (err) return res.status(500).send(err)

    process1(data, onProcess1)
  }  
  
  function onProcess1(err, data) {
    if (err) return res.status(500).send(err)

    process2(data, onProcess2)
  }  

  function onProcess2(err, data) {
    if (err) return res.status(500).send(err)

    process3(data, onProcess3)
  }  

  function onProcess3(err, data) {
    if (err) return res.status(500).send(err)

    fs.writeFile(outputFile, data, onWriteFile)
  }
    
  function onWriteFile(err) {
    if (err) return res.status(500).send(err)

    res.status(200).send('processed successfully using callback hell')
  }
}