Node.js is a single-threaded application, but it can support concurrency via the concept of event and callbacks. Every API of Node.js is asynchronous and being single-threaded, they use async function calls to maintain concurrency. Node uses observer pattern. Node thread keeps an event loop and whenever a task gets completed, it fires the corresponding event which signals the event-listener function to execute.

Event-Driven Programming

Node.js uses events heavily and it is also one of the reasons why Node.js is pretty fast compared to other similar technologies. As soon as Node starts its server, it simply initiates its variables, declares functions and then simply waits for the event to occur.
In an event-driven application, there is generally a main loop that listens for events, and then triggers a callback function when one of those events is detected.

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Although events look quite similar to callbacks, the difference lies in the fact that callback functions are called when an asynchronous function returns its result, whereas event handling works on the observer pattern. The functions that listen to events act as Observers. Whenever an event gets fired, its listener function starts executing. Node.js has multiple in-built events available through events module and EventEmitter class which are used to bind events and event-listeners as follows −

// Import events module
var events = require(‘events’);

// Create an eventEmitter object

var eventEmitter = new events.EventEmitter();

Following is the syntax to bind an event handler with an event −

// Bind event and event handler as follows
eventEmitter.on(‘eventName’, eventHandler);

We can fire an event programmatically as follows −

// Fire an event
eventEmitter.emit(‘eventName’);

Example

Create a js file named main.js with the following code −


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

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

// Create an event handler as follows
var connectHandler = function connected() {
console.log('connection succesful.');

// Fire the data_received event 
eventEmitter.emit('data_received');
}

// Bind the connection event with the handler
eventEmitter.on('connection', connectHandler);

// Bind the data_received event with the anonymous function
eventEmitter.on('data_received', function() {
console.log('data received succesfully.');
});

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

console.log("Program Ended.");

Now let’s try to run the above program and check its output −
$ node main.js
IT should produce the following result −
connection successful.
data received successfully.
Program Ended.

How Node Applications Work?

In Node Application, any async function accepts a callback as the last parameter and a callback function accepts an error as the first parameter. Let’s revisit the previous example again. Create a text file named input.txt with the following content.
Tutorials Point is giving self learning content
to teach the world in simple and easy way!!!!!
Create a js file named main.js having the following code −

var fs = require("fs");

fs.readFile('input.txt', function (err, data) {
if (err) {
console.log(err.stack);
return;
}
console.log(data.toString());
});
console.log("Program Ended");

Here fs.readFile() is a async function whose purpose is to read a file. If an error occurs during the read operation, then the err object will contain the corresponding error, else data will contain the contents of the file. readFile passes err and data to the callback function after the read operation is complete, which finally prints the content.
Program Ended
Tutorials Point is giving self learning content
to teach the world in simple and easy way!!!!!

Node.js – Event Emitter

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

EventEmitter Class

As we have seen in the previous section, EventEmitter class lies in the events module. It is accessible via the following code −

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

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

When an EventEmitter instance faces any error, it emits an ‘error’ event. When a new listener is added, ‘newListener’ event is fired and when a listener is removed, ‘removeListener’ event is fired.
EventEmitter provides multiple properties like on and emit. on property is used to bind a function with the event and emit is used to fire an event.

Methods

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.

Class Methods

Sr.No.

Method & Description

listenerCount(emitter, event)

Returns the number of listeners for a given event.

Events

Sr.No.

Events & Description

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.

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 named 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
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.

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• GraphQL
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• Functions with ES6
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• Async/Await
• Introduction to Babel
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• Processing Query String
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User Authentication and Application Security

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• JSON Web Tokens
• Bcrypt
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