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Introduction to JavaScript

What is JavaScript?​

JavaScript is a programming language that was originally designed to make web pages interactive. However, JavaScript is much more than "the browser language", it's a full-featured programming language that runs in many environments.

Today, JavaScript powers:

  • Web browsers — making websites interactive and dynamic
  • Node.js — running JavaScript on servers and in command-line tools
  • Mobile apps — through frameworks like React Native
  • Desktop applications — through Electron and similar technologies

Despite its name, JavaScript is not related to Java. The name was a marketing decision made in the 1990s. JavaScript is its own language with its own syntax, rules, and ecosystem.

Where JavaScript runs​

In the browser​

When JavaScript runs in a web browser, it has access to browser-specific features like the Document Object Model (DOM), which lets you manipulate web pages. However, this guide focuses on core JavaScript not browser APIs. The browser API will be covered in the next section.

In Node.js​

Node.js is a JavaScript runtime built on Chrome's V8 JavaScript engine. It lets you run JavaScript on your computer outside of a browser, similar to how you run Python or other languages. You can use Node.js to:

  • Build web servers
  • Write command-line tools
  • Create scripts to automate tasks

Other environments​

JavaScript also runs in database systems (like MongoDB), IoT devices, and many other places. The core language features are the same across all these environments.

JavaScript vs Python: key differences​

If you're coming from Python, here are some important mental model differences:

Statements vs expressions​

Both JavaScript and Python have statements and expressions, but JavaScript often uses expressions where Python uses statements:

// JavaScript - conditional expression
const result = x > 5 ? "big" : "small";

// Python - conditional statement
result = "big" if x > 5 else "small"

JavaScript makes heavy use of expressions for control flow, which can make code more compact (but sometimes less readable).

Indentation vs braces​

Python uses indentation to define code blocks:

if x > 5:
    print("big")
else:
    print("small")

JavaScript uses curly braces {}:

if (x > 5) {
  console.log("big");
} else {
  console.log("small");
}

Dynamic typing (with differences)​

Both languages are dynamically typed, but they behave differently:

  • Python treats None as distinct from other falsy values
  • JavaScript has both null and undefined, and they behave slightly differently
  • JavaScript has some type coercion quirks that Python doesn't have (e.g., "5" + 3 vs "5" - 3)

JavaScript allows you to omit semicolons in many cases (automatic semicolon insertion), but adding them explicitly prevents subtle bugs and is generally recommended for beginners.

What this guide covers​

This guide will teach you:

  • Core JavaScript language features (variables, types, functions, objects)
  • Control flow (conditionals, loops)
  • Functions and scope
  • Objects and arrays
  • Object-oriented programming in JavaScript
  • Asynchronous programming (promises, async/await)
  • Modules and code organization

What these guides intentionally do not cover​

These guides focus on JavaScript the language, not JavaScript in specific environments:

  • Browser APIs (DOM manipulation, fetch, events) — these belong in a Web Development guide
  • Node.js-specific features — file system APIs, require (though we cover ES6 import/export)
  • Frameworks — React, Vue, Angular, etc. are built on JavaScript but are separate topics
  • Build tools — webpack, Babel, etc. are ecosystem tools, not language features

The goal is to give you a solid foundation in JavaScript itself, so you can then learn browser APIs, Node.js, or frameworks with confidence.

How to use this guide​

Work through these guides in order. Each builds on previous concepts:

  1. Start with the basics — variables, types, and running JavaScript
  2. Learn data structures — strings, arrays, objects
  3. Master control flow — conditionals and loops
  4. Understand functions — how to write and organize code
  5. Explore advanced topics — OOP, async, modules

You don't need to master everything at once. Focus on understanding each concept before moving forward, and feel free to revisit earlier sections as you learn more.