What is Node.js? JavaScript on the Server Explained

Node.js is a runtime environment that allows JavaScript to run outside the browser.

Before Node.js, JavaScript was mainly used only for frontend browser interactions like:

• button clicks

• DOM manipulation

• form handling

With Node.js, JavaScript can now also run on:

• servers

• backend applications

• command-line tools

• APIs

---

Why JavaScript Was Originally Browser-Only

JavaScript was created for browsers to make web pages interactive.

Browsers already had:

• JavaScript engines

• APIs like DOM and events

So JavaScript depended on the browser environment to run.

Without a browser:

• there was no engine to execute JavaScript

• no system APIs for file handling or networking

That is why JavaScript was originally browser-only.

---

How Node.js Made JavaScript Run on Servers

Node.js took the Chrome V8 JavaScript engine and combined it with C++ features and system-level APIs.

Main parts of Node.js:

• V8 Engine → executes JavaScript

• C++ bindings → connect JS with system operations

• libuv → handles async operations and event loop

libuv provides:

• event loop

• thread pool

• async file system handling

• networking support

This allowed JavaScript to:

• access files

• create servers

• handle requests

• work outside browsers

---

V8 Engine Overview (High Level)

V8 is Google Chrome’s JavaScript engine.

Its job is:
convert JavaScript into machine code so the computer can execute it quickly

Node.js uses V8 internally to run JavaScript code.

Example:

console.log("running with node");

V8 compiles and executes this code.

---

Event-Driven Architecture Idea

Node.js follows an event-driven architecture.

This means:

• actions happen based on events

• functions run when something occurs

Examples of events:

• timer completed

• file loaded

• request received

• API response returned

When an event occurs:

• its callback/task is placed into a queue

• the event loop later executes it when the call stack becomes empty

---

Example

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

What happens:

1. Timer starts in background using libuv

2. Main thread continues execution

3. Timer completes → event occurs

4. Callback enters queue

5. Event loop pushes callback to call stack

6. Callback executes

So:

• event = something happened

• callback/task = code waiting to execute

---

Role of libuv

libuv is one of the core parts of Node.js.

It provides:

• event loop

• async I/O handling

• thread pool

Without libuv, Node.js would not be able to efficiently handle asynchronous operations.

It helps Node.js remain:

• non-blocking

• scalable

• efficient

---

Real-World Use Cases of Node.js

Node.js is commonly used for:

APIs and Backend Servers

const http = require("http");

http.createServer((req, res) => {
  res.end("Hello");
}).listen(3000);