What Is Node.js? Understanding the JavaScript Runtime

Category: Backend Development

Reading Time: 6–8 minutes

Introduction

JavaScript is widely known as a programming language used to make websites interactive. But did you know that JavaScript can also run outside a web browser to build backend applications, APIs, and server-side services?

This is possible with Node.js, a JavaScript runtime environment.

Node.js allows developers to use JavaScript on the server side, making it possible to build complete applications using JavaScript for both frontend and backend development.

Whether you want to develop REST APIs, real-time chat applications, authentication systems, or database-driven websites, Node.js is an important technology to understand.

In this blog, we will explore what Node.js is, how it works, its key features, and how beginners can start learning it.

What Is Node.js?

Node.js is an open-source, cross-platform JavaScript runtime environment that allows JavaScript code to execute outside a web browser.

It uses the V8 JavaScript engine, the same engine used by Google Chrome, to execute JavaScript code.

Node.js also provides additional APIs that allow JavaScript programs to interact with the operating system, read and write files, create network servers, and communicate with databases and external services.

For example, you can execute a JavaScript file directly from your terminal:

console.log("Welcome to JobJunction!");

Save this code in a file named app.js and run it using:

node app.js

Output:

Welcome to JobJunction!

Unlike browser JavaScript, this code runs directly in the Node.js environment without requiring a webpage.

What Is a JavaScript Runtime?

To understand Node.js, you first need to understand the meaning of a runtime environment.

A runtime environment provides the components required to execute a program.

For JavaScript, the runtime environment includes a JavaScript engine and additional APIs and services that support program execution.

For example:

• Browser runtime: Provides JavaScript execution along with browser APIs such as the DOM, document, and window.
• Node.js runtime: Provides JavaScript execution along with APIs for files, networking, processes, and server-side development.

Both environments execute JavaScript, but they provide different capabilities.

For example, this code works in a browser:

document.querySelector("h1");

It accesses an HTML heading through the browser's DOM API.

However, document is not available by default in Node.js because Node.js does not provide a browser DOM.

In Node.js, you can instead work with files using built-in modules:

const fs = require("node:fs");

fs.writeFileSync("message.txt", "Hello from Node.js!");

This example creates a text file containing a message.

How Does Node.js Work?

Node.js is designed to handle many concurrent operations efficiently, particularly when applications perform tasks such as reading files, querying databases, or making network requests.

Its architecture includes several important components.

1. V8 JavaScript Engine

V8 executes JavaScript code and compiles it into machine code where appropriate.

It is responsible for executing the JavaScript portion of a Node.js application.

2. Event Loop

The event loop coordinates the execution of JavaScript callbacks and helps Node.js handle asynchronous operations.

For example, a server can continue handling other work while a database query or network request is waiting to complete.

3. Asynchronous I/O

Node.js provides asynchronous APIs for operations such as file access and network communication.

Rather than requiring JavaScript to wait synchronously for every operation to finish, many I/O operations can be initiated and completed asynchronously.

4. libuv

Node.js uses a library called libuv to support its event loop, asynchronous I/O, and thread pool.

Some operations use operating-system facilities, while certain tasks use the libuv thread pool.

These components work together to help Node.js handle multiple connections without creating a separate JavaScript execution thread for every request.

What Is the Event Loop in Node.js?

The event loop is one of the most important concepts in Node.js.

Consider this example:

console.log("First");

setTimeout(() => {
  console.log("Second");
}, 1000);

console.log("Third");

Output:

First
Third
Second

Why does this happen?

1. Node.js executes console.log("First").
2. It schedules the timer callback.
3. It executes console.log("Third") without waiting for the timer.
4. When the timer is ready and the callback can run, Node.js executes console.log("Second").

This illustrates how asynchronous operations can allow other JavaScript code to continue running.

The event loop is especially important for backend applications that handle multiple requests and network operations.

What Is npm in Node.js?

npm (Node Package Manager) is the default package manager distributed with Node.js installations.

It allows developers to install and manage third-party packages, define project scripts, and maintain dependencies.

For example, to initialise a Node.js project, run:

npm init -y

This creates a package.json file containing project metadata and configuration.

To install Express.js, a popular Node.js web framework, run:

npm install express

Your project can then import Express and use it to build a web server.

The Node.js runtime and npm serve different purposes: Node.js executes JavaScript, while npm helps manage packages and project dependencies.

What Is Express.js?

Express.js is a web framework commonly used with Node.js to build web servers and APIs.

Node.js provides the runtime and low-level networking capabilities, while Express adds convenient tools for routing, middleware, and handling HTTP requests.

Here is a simple Express server:

const express = require("express");

const app = express();

app.get("/", (req, res) => {
  res.send("Welcome to JobJunction!");
});

app.listen(3000, () => {
  console.log("Server running on port 3000");
});

After installing Express and running the file, you can visit http://localhost:3000 to see the welcome message.

This is a simple example of how Node.js and Express work together to create a backend application.

What Can You Build With Node.js?

Node.js is used in many areas of software development.

1. REST APIs

Build backend APIs that allow frontend applications to retrieve, create, update, and delete data.

2. Real-Time Applications

Develop chat applications, live notifications, collaborative tools, and other applications that exchange data in real time.

3. Authentication Systems

Implement login systems, session handling, token-based authentication, and role-based access control.

4. Backend Services

Create services that communicate with databases, process requests, integrate with third-party APIs, and implement business logic.

5. Streaming Applications

Build applications that process or deliver data in chunks rather than waiting for an entire resource to load.

6. AI-Powered Applications

Integrate AI APIs into applications, build chatbots, process user requests, and connect frontend interfaces to AI services.

Node.js can act