Intro

• There are both a written and accompanying video version of this tutorial. There are timestamps under the headings that align with the video version of the topic.
• Read it: For each topic heading, first read the topic. 
• Watch it: Then watch the associated section of the video.
• Do it: Then follow the instructions to replicate the steps on your computer.
• The Node.js CheatSheet has a major category that aligns with this tutorial.
• Review it: When you are done with the whole tutorial, open the CheatSheet and review the Events category from this tutorial. Make sure you understand everything, and can refer back to the CheatSheet when you are working on your own projects in the future. 

The Event Loop

The Events Module

• Node core modules such as fs, HTTP, and Process.stdin/stdout.
• It's also used in third party npm packages that involve Input/Output tasks such as querying a database.
• AND it can be used in your own modules if you have I/O tasks.

const EventEmitter = require('events');

// Create an instance of the EventEmitter class.
const emitter = new EventEmitter();

// Listen for the event, then execute a callback function.
emitter.on('logIt', () => {
  console.log('LogIt event occurred');
});

// Emit the logIt event
emitter.emit('logIt');

1. Create an instance of the EventEmitter class:

const EventEmitter = require('events');
const emitter = new EventEmitter();

2. Register one or more Listeners on an event:

1. The first argument is the event name that it is listening for. 
2. The second argument is the callback function that gets executed when the event is emitted.

3. Emit an event

Execute the file:

Event example with arguments

nodejs-tutorial/8-events/eventWithArgs.js

const EventEmitter = require('events');

// Create an instance of the EventEmitter class.
const emitter = new EventEmitter();

// Listen for the event, then execute a callback function.
emitter.on('logIt', (msg) => {
  console.log(msg);
});

// Emit event, passing in a message as the second argument.
emitter.emit('logIt', 'Log message 1');
emitter.emit('logIt', 'Log message 2');

Execute the file:

Inherit EventEmitter

nodejs-tutorial/8-events/inheritEventEmitter.js

// 1) Create instance of a class that inherits from EventEmitter.
const EventEmitter = require('events');
class Chat extends EventEmitter {
  constructor(topic) {
    super();
    this.topic = topic;
    this.status = 'inactive';
  }
}
const session = new Chat('Event Loop');

// 2a) Register listener for first occurrence of the begin event.
session.once('begin', () => {
  session.status = 'active';
  console.log(`Chat Session topic: ${session.topic}`);
});

// 2b) Register listener for the send event.
session.on('send', (participant, msg) => {
  if (session.status === 'inactive') { 
    session.emit('error', new Error('Chat is not active'));
  } else {
    console.log(`${participant}: ${msg}`);
  }
});

// 2c) Register listener for first occurrence of the end event.
session.once('end', () => {
  session.status = 'inactive';
  console.log(`Chat Session has ended.`);
});

// 2d) Register listener for the error event.
session.on('error', (err) => {
  console.error(err.message);
});

// 3) Emit events
session.emit('begin'); 
session.emit('send', 'Sheena', 'Can you explain the Event Loop?'); 
session.emit('send', 'Joey', 'Yes'); 
session.emit('send', 'Sheena', 'Will you?'); 
session.emit('send', 'Joey', 'No'); 
session.emit('end');
session.emit('send', 'Sheena', 'I have another question.'); 

1. Create an instance of the EventEmitter class:

• At the top we import the EventEmitter class as usual.
• Then we create a Chat class that inherits from EventEmitter by using the extends keyword.
• In the constructor function when we inherit from a class we have to include the super() method before setting our own properties.
• Our Chat class has two properties, the topic and the status.
• We set the status value initially to "inactive".

• Then we instantiate a chat object.
• We are assigning it to a variable named session,
• And passing in the topic property value: "Event loop".

2. Register one or more Listeners on an event:

• The second step is to register listener functions. This time we have four events: begin, send, end, and error.

• Once method: We haven't used the once method yet. The once method is like the on method but it executes the callback only the first time the event is called.

• Begin event: We use the once method with the "begin" event.

In the callback we change the session.status property to "active",

Then log a message.

• Send event: The on method listens for the send event. The callback has parameters for the participant name and message.
• You can emit events from within an event listener function.
• In the send event callback/listener function, we have an if statement to check if the status is inactive.
• If so it emits an error event.
• Then the second argument creates a new error object with an error message that stays "Chat is not active".

• If the status is not inactive (i.e, if it's active), 

then we log the two arguments passed in with the send event:

the participant's name and the chat message.

• Error event: The listener for the error event logs the error message to the console.

• End event: We also register a listener for the end event using the once method so it only executes one time. 
• The callback function sets the session status to inactive and logs a message.

3. Emit an event

• The third event step is at the bottom where we emit all of our events.
• We'll first  emit a begin event to make the session active.
• Then several send events which log the chat messages.
• The send event passes two arguments. The participant name and the message.
• The end event will set the session status to "inactive".
• Then we emit another send event which will trigger an error event since the chat session status is now inactive.

Execute the file: