What Is JavaScript? Beginner Guide, Uses & Examples

What is JavaScript?

JavaScript is a lightweight, interpreted, multi-paradigm programming language created by Brendan Eich in 1995 at Netscape. Originally named Mocha, then LiveScript, it was renamed JavaScript as a marketing decision. Today it is the world's most widely used programming language - the only language that runs natively in web browsers.

Client-side: Runs in the browser - manipulates DOM, handles events, validates forms
Server-side: Runs on Node.js - builds REST APIs, web servers, CLI tools
Mobile: React Native, Ionic - cross-platform mobile apps
Desktop: Electron - VS Code, Slack, Discord are built with it

JavaScript Versions (ECMAScript)

JavaScript is standardized by ECMA International as ECMAScript (ES). Since ES2015 (ES6), new versions are released annually.

Version	Year	Key Features
ES5	2009	strict mode, JSON, Array methods (forEach, map, filter)
ES6 / ES2015	2015	let/const, arrow functions, classes, modules, promises, template literals
ES2016	2016	Array.includes(), exponentiation operator (**)
ES2017	2017	async/await, Object.entries(), Object.values()
ES2018	2018	rest/spread for objects, Promise.finally(), async iteration
ES2019	2019	Array.flat(), Array.flatMap(), Object.fromEntries()
ES2020	2020	Optional chaining (?.), nullish coalescing (??), BigInt, Promise.allSettled()
ES2021	2021	String.replaceAll(), Promise.any(), logical assignment (&&=, \|\|=, ??=)
ES2022	2022	Array.at(), Object.hasOwn(), class fields, top-level await
ES2023	2023	Array.findLast(), Array.toSorted(), Array.toReversed()
ES2024	2024	Promise.withResolvers(), Object.groupBy(), Map.groupBy()

Your First JavaScript Program

You can run JavaScript in three common places: inside the browser console, inside an HTML page using a <script> tag, or outside the browser with Node.js. Beginners usually start in the browser because they can immediately see JavaScript interact with the page.

Hello World

// In browser - open DevTools Console (F12) and type:
console.log('Hello, World!');

// In HTML file
// <script src="script.js"></script>
// or inline:
// <script>alert('Hello!');</script>

// Variables
let name = 'Alice';        // mutable
const age = 25;            // immutable
var old = 'avoid var';     // function-scoped (legacy)

// Template literals
console.log(`My name is ${name} and I am ${age} years old.`);

// Modern features
const user = { name: 'Bob', address: { city: 'Delhi' } };
console.log(user?.address?.city);  // Optional chaining: 'Delhi'
console.log(user?.phone ?? 'N/A'); // Nullish coalescing: 'N/A'

How JavaScript Runs in the Browser

When a browser opens a web page, it reads the HTML, builds a document object model called the DOM, applies CSS styles, and then runs JavaScript. JavaScript can read the DOM, change text, add or remove elements, respond to button clicks, validate forms, and request data from APIs.

Browser Example

// HTML: <button id="saveBtn">Save</button>
const button = document.querySelector("#saveBtn");

button.addEventListener("click", function () {
  button.textContent = "Saved";
  button.classList.add("is-success");
});

This is why JavaScript is called the language of the web. HTML gives the page structure, CSS gives it style, and JavaScript adds behavior.

JavaScript Execution Model

JavaScript runs on a single main thread in the browser. That means one piece of JavaScript code runs at a time. Slow work such as timers, network calls, and user events is handled asynchronously through the event loop, so the browser can stay responsive while waiting for those tasks to finish.

Event Loop Idea

console.log("First");

setTimeout(function () {
  console.log("Third");
}, 0);

console.log("Second");

// Output:
// First
// Second
// Third

The timer callback runs later, even with a delay of 0, because callbacks wait until the current synchronous code finishes. This same idea is used by events, promises, fetch(), and async/await.

JavaScript vs Java

Despite the similar name, JavaScript and Java are completely different languages. JavaScript is dynamically typed, interpreted, and runs in browsers. Java is statically typed, compiled to bytecode, and runs on the JVM. The name similarity was purely a marketing decision in 1995.

Key Takeaways

JavaScript was created in 10 days by Brendan Eich in 1995 - originally called Mocha, then LiveScript.
ECMAScript is the official standard. ES6 (2015) was the biggest update - introduced classes, modules, arrow functions.
JavaScript is the only language that runs natively in all web browsers.
Node.js (2009) brought JavaScript to the server - enabling full-stack JS development.
Use let and const (ES6+) - avoid var which has confusing function-scoping behavior.

What Is JavaScript? Beginner Guide, Uses & Examples