Node.js | Event Emitter

Hello folks! welcome back to a new edition of our tutorial on Node.js. In this section of our tutorial on Node.js, we will be studying about Node.js Event Emitter.

Numerous objects in Node emits events, for example, a net.Server emits an event every time a peer connects to it, an fs.readStream emit an event when the file is opened. Every objects which emit events are the instances of events.EventEmitter.

EventEmitter Class

As we have seen in our previous tutorial, the EventEmitter class lies in the events module. It is accessible using the below code -

// Import events module
var events = require('events');

// Create an eventEmitter object
var eventEmitter = new events.EventEmitter();

When an EventEmitter instance encounters any error, it outputs an 'error' event. When a new listener is added, 'newListener' event is tripped and when a listener is removed, then 'removeListener' event is fired.

EventEmitter provides numerous properties like on and emit. on property is used to bind a function with the event and emit is used to fire an event.

READ: Node.js | Event Loop

EventEmitter Methods

Following below is the list of EventEmitter methods available in Node.js -

Sr.No.	Method & Description
1	addListener(event, listener) Adds a listener at the end of the listeners array for the specified event. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of event and listener will result in the listener being added multiple times. Returns emitter, so calls can be chained.
2	on(event, listener) Adds a listener at the end of the listeners array for the specified event. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of event and listener will result in the listener being added multiple times. Returns emitter, so calls can be chained.
3	once(event, listener) Adds a one time listener to the event. This listener is invoked only the next time the event is fired, after which it is removed. Returns emitter, so calls can be chained.
4	removeListener(event, listener) Removes a listener from the listener array for the specified event.Caution − It changes the array indices in the listener array behind the listener. removeListener will remove, at most, one instance of a listener from the listener array. If any single listener has been added multiple times to the listener array for the specified event, then removeListener must be called multiple times to remove each instance. Returns emitter, so calls can be chained.
5	removeAllListeners([event]) Removes all listeners, or those of the specified event. It's not a good idea to remove listeners that were added elsewhere in the code, especially when it's on an emitter that you didn't create (e.g. sockets or file streams). Returns emitter, so calls can be chained.
6	setMaxListeners(n) By default, EventEmitters will print a warning if more than 10 listeners are added for a particular event. This is a useful default which helps finding memory leaks. Obviously not all Emitters should be limited to 10. This function allows that to be increased. Set to zero for unlimited.
7	listeners(event) Returns an array of listeners for the specified event.
8	emit(event, [arg1], [arg2], [...]) Execute each of the listeners in order with the supplied arguments. Returns true if the event had listeners, false otherwise.

EventEmitter Class Method

Following below is the list of class methods supported by Node.js -

Sr.No.	Method & Description
1	listenerCount(emitter, event) Returns the number of listeners for a given event.

READ: Node.js | Callbacks Concept

Events

Following is the list of events supported by Node.js -

Sr.No.	Events & Description
1	newListener event − String: the event name listener − Function: the event handler function This event is emitted any time a listener is added. When this event is triggered, the listener may not yet have been added to the array of listeners for the event.
2	removeListener event − String The event name listener − Function The event handler function This event is emitted any time someone removes a listener. When this event is triggered, the listener may not yet have been removed from the array of listeners for the event.

Example

Create a js file, main.js with the following Node.js code -

var events = require('events');
var eventEmitter = new events.EventEmitter();

// listener #1
var listner1 = function listner1() {
   console.log('listner1 executed.');
}

// listener #2
var listner2 = function listner2() {
   console.log('listner2 executed.');
}

// Bind the connection event with the listner1 function
eventEmitter.addListener('connection', listner1);

// Bind the connection event with the listner2 function
eventEmitter.on('connection', listner2);

var eventListeners = require('events').EventEmitter.listenerCount
   (eventEmitter,'connection');
console.log(eventListeners + " Listner(s) listening to connection event");

// Fire the connection event 
eventEmitter.emit('connection');

// Remove the binding of listner1 function
eventEmitter.removeListener('connection', listner1);
console.log("Listner1 will not listen now.");

// Fire the connection event 
eventEmitter.emit('connection');

eventListeners = require('events').EventEmitter.listenerCount(eventEmitter,'connection');
console.log(eventListeners + " Listner(s) listening to connection event");

console.log("Program Ended.");

Now run the main.js to see the result -

$ node main.js

Output

Verify the output -

2 Listner(s) listening to connection event
listner1 executed.
listner2 executed.
Listner1 will not listen now.
listner2 executed.
1 Listner(s) listening to connection event
Program Ended.

READ: Node.js | REPL Terminal

Alright guys! This is where we are going to be rounding up for this tutorial. In our next tutorial, we will be studying about Node.js Buffers.

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