The EventEmitter Pattern and the Event Loop -- a timeline

The EventEmitter pattern is a derivative of the Observer pattern: an object can notify a set of observers when a change in its state occurs. Consider the following example using the EventEmitter pattern:

const EventEmitter = require('events').EventEmitter
const fs = require('fs')

const findPattern = (files, regex) => {
  const emitter = new EventEmitter()

  files.forEach(file => {
    fs.readFile(file, 'utf8', (err, content) => {
      if (err) return emitter.emit('error', err)

      emitter.emit('fileread', file)

      let match
      if (match = content.match(regex)) {
        match.forEach(elem => {
          emitter.emit('found', file, elem)
        })
      }
    })
  })
  return emitter
}

// fileA.txt contains the words "hello blah blah blah"
// fileB.json is empty
// fileC.md does not exist
const files = ['fileA.txt', 'fileB.json', 'fileC.md']

findPattern(files, /hello \w+/g)
  .on('fileread', file => {console.log(`${file} was read`)})
  .on('found', (file, match) => {console.log(`Matched ${match} in file ${file}`)})
  .on('error', err => {console.log(`Error emitted: ${err.message}`)})

Some of the script is executed synchronously, but the real work is done asynchronously. The EventEmitter pattern takes advantage of the event loop to work efficiently.

Keep the following schematic in mind as you parse through the execution of this script:

image credit: Mario Casciaro and Luciano Mammino from Node.js Design Patterns. Colour notations are mine.

The first set of operations happen synchronously:

• call findPattern with the file list and regex pattern
• create a new event emitter
• submit readFile I/O request for fileA.txt to the event demultiplexer
• submit readFile I/O request for fileB.json to the event demultiplexer
• submit readFile I/O request for fileC.md to the event demultiplexer
• return the event emitter
• register fileread listener on the event emitter
• register found listener on the event emitter
• register error listener on the event emitter

Once the I/O operations are submitted to the event demultiplexer, they will return in the order in which the requests are fulfilled. In this example, fileC.md does not exist, so the event demultiplexer submits the result along with the handler to the Event loop.

• The event loop runs the result through the callback for fileC.md, which emits an error event (line 9)! The EventEmitter adds the error handler to the event loop.

In the meantime, the event demultiplexer has fulfilled the I/O request for fileB.json, and added the result and the callback to the event loop.

• The event loop runs the result through the callback for fileB, which emits a fileread event (line 11). The EventEmitter adds the fileread handler to the event loop.

The event demultiplexer also fulfills the I/O request for fileA.txt and adds the result and the callback to the event loop.

• The event loop runs the result through the callback for fileA, which emits a fileread event (line 11) AND a found event (line 16). The EventEmitter adds the fileread and found handlers to the event loop.

The event loop is busily processing all of these events in the order of arrival:

• run the listener callback for the error event (fileC)
• run the listener callback for the fileread event (fileB)
• run the listener callback for the fileread event (fileA)
• FINALLY — run the listener callback for the found event (fileA)

The result of running this script on the command line:

$ node --use_strict events.js
Error emitted: ENOENT: no such file or directory, open 'fileC.md'
fileB.json was read
fileA.txt was read
Matched hello blah in file fileA.txt

Side note: set the flag --use-strict to run scripts using ES6 syntax

Why did the files process in what seems to be reverse order? The event demultiplexer adds work to the event loop as it fulfills I/O requests. When readfile() is given three files — a file with content (fileA), an empty file (fileB), and a non-existent file (fileC) — it is reasonable to assume that the I/O request for a non-existent file will return first, as there is no file to read. It is also reasonable that an empty file will return next, as there is very little work to be done reading an empty file. Finally, the I/O request for the file with content returns — it requires more work to read a file with content than an empty file.

For more about the event emitter pattern in Node, check out Node.js Design Patterns by Mario Casciaro and Luciano Mammino.