Node EventEmitter example

events.js

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
}

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}`)})

/*
$ 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
*/

fileA.txt

hello blah blah blah

fileB.json

{}