Frontend Development › JavaScript & TypeScript

JavaScript

The programming language of the web.

Also known as: JS, JavaScript, ECMAScript language

JavaScript is the programming language of the web. It runs in every browser, where it makes pages interactive, and with Node.js (and similar runtimes) it also runs on servers and in command-line tools.

const prices = [10, 25, 8];
const total = prices.reduce((sum, p) => sum + p, 0);

document.querySelector("#total").textContent = `$${total}`;

document.querySelector("button").addEventListener("click", () => {
  alert("Clicked");
});

What it does in the browser

• Reads and changes the page (DOM).
• Reacts to events: clicks, typing, scrolling (event handling).
• Talks to servers without reloading (fetch).
• Stores data in the browser (browser storage).

Outside the browser: web servers and APIs, build tools, scripts, desktop and mobile apps.

Key features

• Dynamically typed: variables hold any type, and types are checked at runtime. TypeScript adds static types.
• First-class functions: functions are values you can pass around.
• Prototype-based objects, with class syntax on top.
• Single-threaded with an event loop: asynchronous work uses callbacks, promises and async/await (event loop, promises).
• Standardized as ECMAScript, with new features every year.

Quirks you’ll meet

• == converts types, so use ===.
• 0.1 + 0.2 isn’t exactly 0.3 (floating point).
• this depends on how a function is called.
• null and undefined are both “nothing” in different ways.
• typeof null is "object".

None of these are reasons to avoid it, but they explain why linters and TypeScript are popular.

Not the same as Java

The name was marketing. They are unrelated languages.

It’s a good first language for the web: you can run it in the browser console right now.