Node's Event-driven Architecture

• Callbacks
• Promises
• async/await
• The EventEmitter module
• Arguments
• errors
• order of listeners
• Practical Example: Task List Manager

Callback

fs.readFile('/foo.txt', function(err, data) {
  // If an error occurred, handle it (throw, propagate, etc)
  if(err) {
    console.log('Unknown Error');
    return;
  }
  // Otherwise, log the file contents
  console.log(data);
});

Callbacks !== Asynchrony

Async

async.map(['file1','file2','file3'], fs.stat, function(err, results) {
    // results is now an array of stats for each file
});

async.filter(['file1','file2','file3'], function(filePath, callback) {
  fs.access(filePath, function(err) {
    callback(null, !err)
  });
}, function(err, results) {
    // results now equals an array of the existing files
});

async.parallel([
    function(callback) { ... },
    function(callback) { ... }
], function(err, results) {
    // optional callback
});

async.series([
    function(callback) { ... },
    function(callback) { ... }
]);

Callback HELL!!!

fs.readdir(source, function (err, files) {
  if (err) {
    console.log('Error finding files: ' + err)
  } else {
    files.forEach(function (filename, fileIndex) {
      console.log(filename)
      gm(source + filename).size(function (err, values) {
        if (err) {
          console.log('Error identifying file size: ' + err)
        } else {
          console.log(filename + ' : ' + values)
          aspect = (values.width / values.height)
          widths.forEach(function (width, widthIndex) {
            height = Math.round(width / aspect)
            console.log('resizing ' + filename + 'to ' + height + 'x' + height)
            this.resize(width, height).write(dest + 'w' + width + '_' + filename, function(err) {
              if (err) console.log('Error writing file: ' + err)
            })
          }.bind(this))
        }
      })
    })
  }
})

var form = document.querySelector('form')
form.onsubmit = function (submitEvent) {
  var name = document.querySelector('input').value
  request({
    uri: "http://example.com/upload",
    body: name,
    method: "POST"
  }, function (err, response, body) {
    var statusMessage = document.querySelector('.status')
    if (err) return statusMessage.value = err
    statusMessage.value = body
  })
}

How do I fix callback hell?

1. Keep your code shallow

document.querySelector('form').onsubmit = formSubmit

function formSubmit (submitEvent) {
  var name = document.querySelector('input').value
  request({
    uri: "http://example.com/upload",
    body: name,
    method: "POST"
  }, postResponse)
}

function postResponse (err, response, body) {
  var statusMessage = document.querySelector('.status')
  if (err) return statusMessage.value = err
  statusMessage.value = body
}

2. Modularize

module.exports.submit = formSubmit

function formSubmit (submitEvent) {
  var name = document.querySelector('input').value
  request({
    uri: "http://example.com/upload",
    body: name,
    method: "POST"
  }, postResponse)
}

function postResponse (err, response, body) {
  var statusMessage = document.querySelector('.status')
  if (err) return statusMessage.value = err
  statusMessage.value = body
}

var formUploader = require('formuploader')
document.querySelector('form').onsubmit = formUploader.submit

3. Handle every single error

var fs = require('fs')

 fs.readFile('/Does/not/exist', handleFile)

 function handleFile (error, file) {
   if (error) return console.error('Uhoh, there was an error', error)
   // otherwise, continue on and use `file` in your code
 }

Promise

BlueBird

Async/Await

--harmony-async-await

--harmony_async_await

To work with async/await

> node --harmony-async-await app.js

EventEmitter

Practical Example: Task List Manager

Task List Manager

Available Commands