Understanding Events in Node.js

Readers can also refer to some other Node.js Learning articles available on Mytour to get a better understanding of Node.js installation and how to create Node.js applications. Keep reading the content below from Mytour to explore Events in Node.js.

Exploring the Concept of Events in Node.js

Understanding Node.js Events

In Node.js applications, the concepts of Events and Callbacks are utilized to support concurrent processing. Since Node.js applications are Single Threaded and Node's APIs are asynchronous, it employs async functions to maintain concurrency.

Node.js employs the Observer Pattern. Threads in Node.js maintain an Event Loop, and whenever a task is completed, it triggers the corresponding Event to notify the ready-to-execute Event Listener function.

Event-Driven Programming Model

Node.js utilizes event-driven programming; immediately upon server startup, it initializes variables, declares functions, and waits for events to occur. This explains why Node.js is faster than other similar technologies.

The main loop in event-driven model applications listens for events, then triggers a callback function upon event detection.

Distinguishing between Event and Callback in Node.js

While Event and Callback share many similarities, in reality, they also have several differences. Callback functions are invoked when an asynchronous function returns a result, whereas Event Handlers are processed within the Observer Pattern.

Whenever an Event is triggered, the Listener function begins execution. Node.js integrates numerous events through event modules, and the EventEmitter class is utilized to link events and event listeners.

To associate the EventEmitter class with an Event and Event Listener, use the syntax:

// Import events module

var events = require('events');

// Initialize an eventEmitter object

var eventEmitter = new events.EventEmitter();

To link an Event Handler with an Event, use the following syntax:

// Bind event and event handler as follows

eventEmitter.on('eventName', eventHandler);

To trigger an Event, use the following syntax:

// Fire an event

eventEmitter.emit('eventName');

An Example of Event in Node.js

Create a file named main.js containing the following code:

Next, open the Node.js command prompt and enter the following command to run the program and check the output:

node main.js

The returned result appears as follows:

Connection successful.

Data received successfully.

Program completed.

How does a Node application work?

In a Node application, any async function will accept a callback as its last parameter, and the callback function will accept an error as its first parameter. Let's review the previous example, create a text file named input.txt containing the following content:

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Next, create a JavaScript file named main.js, containing 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');

Where fs.readFile() is an asynchronous function used to read files. If an error occurs during the reading process, the err object will contain the corresponding error, or the data will contain the content of the readFile file. Then the function will pass err and data into the callback function after the file reading process ends, and finally print the content.

Program Ended

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Above, readers have just explored What Event is in Node.js with Mytour. Additionally, if you wish to delve deeper into NPM or Callback in Node.js, readers can refer to some other articles already available on Mytour for more information.

In the next articles, Mytour will further introduce you to the EventEmitter class in Node.js.