# What is JavaScript? JavaScript (JS) is a **high-level, interpreted, dynamically-typed programming language** used to make web pages interactive. It is the only programming language natively understood by browsers. The three pillars of the web: | Technology | Role | |---|---| | HTML | Structure (skeleton) | | CSS | Style (appearance) | | JavaScript | Behaviour (logic + interactivity) | --- ## History - Created by **Brendan Eich** in **1995** (at Netscape) in just 10 days. - Standardised as **ECMAScript (ES)** by ECMA International. - Modern version: **ES6+** (2015 onwards) — most of what you write today. --- ## Two Environments ### 1. Browser (Client-side) - JS is **built into every browser** — no installation needed. - Runs on the **user's machine**. - Has access to the **DOM** (Document Object Model) — can read and change HTML/CSS. What you can do: - React to user events (clicks, typing, scrolling) - Validate forms before submission - Fetch data from APIs without reloading the page - Animate elements ```html ``` ### 2. Node.js (Server-side) - A **runtime environment** released in **2009** by Ryan Dahl. - Runs JS **outside the browser**, on a server or your local machine. - Built on Chrome's **V8 engine**. What you can do: - Build web servers and REST APIs - Read/write files on disk - Connect to databases - Run JS from the terminal ```bash # Run a JS file with Node.js node app.js ``` > **Key point:** Same JavaScript language — different built-in APIs. > Browser gives you `document`, `window`. Node.js gives you `fs`, `http`, `process`. --- ## How JavaScript Runs ``` Source code (.js) ↓ JS Engine parses & compiles (JIT) ↓ Executes line by line ``` **JS Engines by browser:** | Browser | Engine | |---|---| | Chrome / Edge / Node.js | V8 | | Firefox | SpiderMonkey | | Safari | JavaScriptCore | --- ## Key Characteristics | Property | Meaning | |---|---| | **Interpreted / JIT compiled** | No manual compile step — engine handles it | | **Dynamically typed** | Variables have no fixed type; type is determined at runtime | | **Single-threaded** | One task at a time, but handles async via the **Event Loop** | | **Multi-paradigm** | Supports OOP, functional, and procedural styles | | **Garbage collected** | Memory is managed automatically | --- ## Three Ways to Add JS to a Webpage ### 1. Inline (avoid) ```html ``` ### 2. Internal — ` ``` ### 3. External — separate `.js` file (best practice) ```html ``` --- ## Your First JavaScript Program ```js console.log("Hello, World!"); ``` - `console.log()` prints output to the **browser console** or **terminal**. - Open browser console: `F12` → Console tab. --- ## Browser vs Node.js | Feature | Browser | Node.js | |---|---|---| | Environment | Client (user's device) | Server / local machine | | Access to DOM | Yes | No | | File system access | No | Yes | | How to run | Open HTML in browser | `node filename.js` in terminal | | Global object | `window` | `global` | | Primary use | UI & interactivity | Backend, scripts, tools |

Technology	Role
HTML	Structure (skeleton)
CSS	Style (appearance)
JavaScript	Behaviour (logic + interactivity)

What is JavaScript? (browser + Node.js)

Introduction

What is JavaScript?