What is JavaScript?

JavaScript is a lightweight, interpreted programming language with first-class functions. It is the only programming language that runs natively in all major web browsers, making it the language of the web. JavaScript adds interactivity, animations, and dynamic behaviour to HTML pages. Thanks to Node.js, it also runs on servers, making it the only language that covers both front-end and back-end development.

History

JavaScript was created by Brendan Eich at Netscape in just 10 days in May 1995. It was originally called Mocha, then LiveScript, before being renamed JavaScript — a marketing decision to capitalise on the popularity of Sun's Java language. Despite the name similarity, the two languages are unrelated. JavaScript was standardised as ECMAScript (ES) by Ecma International in 1997. Major milestones include ES5 (2009), ES6/ES2015 (2015 — the biggest update in language history), and annual releases from ES2016 onwards.

Basic Syntax

JavaScript is dynamically typed: variables can hold any type of value and types are checked at runtime. Modern JavaScript uses let and const for variable declarations. Functions are first-class values and can be assigned to variables, passed as arguments, and returned from other functions.

// Variables
const name = "Alice";
let age = 30;

// Function
function greet(person) {
  return `Hello, ${person}!`;
}

// Arrow function
const square = (n) => n * n;

// Array methods
const numbers = [1, 2, 3, 4, 5];
const doubled = numbers.map(n => n * 2);   // [2, 4, 6, 8, 10]
const evens   = numbers.filter(n => n % 2 === 0); // [2, 4]

// Async / Await
async function fetchUser(id) {
  const response = await fetch(`/api/users/${id}`);
  const data = await response.json();
  return data;
}

Data Types

JavaScript has eight data types — seven primitives and one object type:

• String — Text: "hello", 'world', `template ${literal}`
• Number — All numeric values (integer and float): 42, 3.14, NaN, Infinity
• BigInt — Arbitrary-precision integers: 9007199254740991n
• Boolean — true or false
• Undefined — A variable declared but not assigned a value
• Null — Intentional absence of a value: null
• Symbol — Unique, immutable identifiers
• Object — Collections of key-value pairs, including arrays, functions, and dates

Core Concepts

• DOM manipulation — JavaScript can read and modify HTML elements, attributes, styles and content in the browser in real time via the Document Object Model
• Events — JavaScript responds to user actions (clicks, key presses, form submissions) through event listeners
• Asynchronous programming — Callbacks, Promises, and async/await allow non-blocking operations like API calls and timers
• Prototypal inheritance — Objects can inherit properties from other objects through the prototype chain
• Closures — Functions remember the scope in which they were created, enabling data encapsulation
• Modules — ES Modules (import / export) allow splitting code into reusable files

Common Use Cases

• Interactive web pages — Form validation, dropdown menus, image carousels, modal dialogs, live search
• Single-page applications (SPA) — React, Vue.js and Angular let you build full web apps that update without page reloads
• Server-side development — Node.js runs JavaScript on the server for APIs, web servers, and microservices
• Mobile apps — React Native lets you build iOS and Android apps in JavaScript
• Desktop apps — Electron powers apps like VS Code, Slack, and Figma using web technologies
• Browser extensions — Chrome, Firefox and Edge extensions are written in JavaScript

JavaScript vs Java

Despite the similar name, JavaScript and Java are completely different languages:

• Typing — Java is statically typed; JavaScript is dynamically typed
• Execution — Java compiles to bytecode and runs on the JVM; JavaScript is interpreted by the browser's JS engine (V8, SpiderMonkey)
• Paradigm — Java is class-based OOP; JavaScript is multi-paradigm (prototypal OOP, functional, procedural)
• Use case — Java is used for enterprise backends and Android; JavaScript runs in the browser and on Node.js

JavaScript vs TypeScript

TypeScript is a superset of JavaScript developed by Microsoft that compiles to plain JavaScript:

• TypeScript adds optional static typing, catching type errors at compile time rather than runtime
• TypeScript supports interfaces, generics, enums, and decorators
• All valid JavaScript is valid TypeScript — migration can be incremental
• TypeScript is the language of choice for large React and Angular projects

Rules to Remember

• Use const by default; use let when the variable needs to be reassigned. Avoid var.
• JavaScript is single-threaded — use async/await for non-blocking I/O
• === checks both value and type; == performs type coercion — prefer ===
• typeof null returns "object" — a historical bug in the language
• Arrays are objects: typeof [] returns "object"; use Array.isArray() to check
• Functions are first-class objects — they can be stored in variables and passed as arguments