Practicing with Node.js Event Emitters

event_emitters.js

// 10/25/2018

// Node practice from docs
//----- Experimenting with event emitters!

const EventEmitter = require('events');
class MyEmitter extends EventEmitter {}

const myEmitter = new MyEmitter();
myEmitter.on('myevento', () => {
  console.log('an evento occurred!');
});

// goes second
setTimeout(()=>{
  console.log(2.5)
  myEmitter.emit('myevento');
}, 3000);

// goes first
process.nextTick( () => {
  console.log("next loop iteration 2")
  myEmitter.emit('myevento');

  // goes third
  setTimeout(()=>{
    console.log(3)
    myEmitter.emit('myevento');
  }, 3000);
})



//----- passing args to event.emit
const myEmitter2 = new MyEmitter();
myEmitter2.on('event', function(a, b) {
  console.log(a, b, this === myEmitter2, this);
  // Prints:
  //   a b true MyEmitter {
  //     domain: null,
  //     _events: { event: [Function] },
  //     _eventsCount: 1,
  //     _maxListeners: undefined }
});
myEmitter2.emit('event', 'a', 'b');

class myEmitter3 extends EventEmitter {};
console.log("\n" + Object.getPrototypeOf(myEmitter3)) //[Function: EventEmitter]


// ----- Async Emitters 
// up until this point I've used sync emitters, sure I called them async by wrapping the invokation within aysnc fn's
// but to avoid race conditions or logic errors here is an async emitter

const myEmitter4 = new MyEmitter();
myEmitter4.on("evt4", (a, b) => {
  setImmediate(()=>{
    console.log("this happens async")
  });
});

myEmitter4.emit("evt4");


// ----- Handling events only once
const myEmitterr = new MyEmitter();
let m = 0;
myEmitterr.once('event', () => {
  console.log("m value: ", ++m);
});
myEmitterr.emit('event');
// Prints: 1
myEmitterr.emit('event');
// Ignored



// ----- Error handling
//const myEmitteer = new MyEmitter();
//myEmitteer.emit('error', new Error('whoops!'));
// Throws and crashes Node.js


// As a best practice, listeners should always be added for the 'error' events.
// keeps app running w/out crashing
const myEmittter = new MyEmitter();
myEmittter.on('error', (err) => {
  console.error('whoops! there was an error');
});
myEmittter.emit('error', new Error('whoops!'));
// Prints: whoops! there was an error