JavaScript

Learn JavaScript from your first line to classes and async code, then bring a web page to life with it.

Each lesson has runnable examples and exercises. JavaScript is real (Node.js 24), running in a small Linux machine inside this tab: no install, no server running your code. The first visit downloads about 30 MB, of which the first 5 MB before you can start; later visits start from your browser's copy.

Linux + Node.js 24 in your browser

Chapter 1 · Foundations

What runs your program; numbers, text and errors

1. 1 Hello, JavaScript

   Your first program, the command that runs it, and console.log.

2. 2 Inside the computer

   What a computer is made of, what happens when your program runs, and where its values live in memory.

3. 3 Values and variables

   let and const, numbers and arithmetic, typeof, and the numbers that surprise.

4. 4 Text

   Strings, + with text, template literals, Number(), toFixed and your first methods.

5. 5 Reading errors

   Syntax errors, stack traces, and the mistakes that print no error.

Chapter 2 · Decide, repeat, reuse

Decisions, loops, functions, modules and input

1. 6 Decisions and loops

   if, else if and else, for and while, the classic FizzBuzz, and a test of it all.

2. 7 Functions

   Your own functions, return, a frame per call, arguments, scope and arrow functions.

3. 8 Modules and input

   Programs in several files with import and export, what Node gives you, random numbers, files and input.

Chapter 3 · Data

Text, arrays, objects, callbacks, maps and recursion

1. 9 Working with text

   Characters by index, slices, methods that make new text, split and join, and columns that line up.

2. 10 Arrays

   Lists of values in order, loops over them, two names for one array, copies, and grids.

3. 11 Objects

   Values by name, missing values, every key and value, records, shared and copied objects, and JSON.

4. 12 Callbacks: functions on arrays

   forEach, map, filter, find, reduce and sort, each given a function to call.

5. 13 Maps and sets

   Look values up by key with a Map, count with it, and keep each value once with a Set.

6. 14 Recursion

   Functions that call themselves, the base case that stops them, and the stack they fill.

Chapter 4 · Real programs

Closures, prototypes, classes, exceptions, async code and a console tool

1. 15 Closures

   Functions that keep their variables after the call that made them has returned.

2. 16 Prototypes: objects that share

   Methods and this, the hidden link to a prototype, and where push and map really live.

3. 17 Classes

   A class builds lesson 16's links for you; private fields, extends and super.

4. 18 Exceptions

   Throw an error, catch it with try and catch, make your own kind, and survive bad input and missing files.

5. 19 Waiting: the event loop

   Timers, the event loop, promises, async and await, and files read and written without stopping the program.

6. 20 A console project of your own

   Finish a to-do tool of four modules that reads commands, keeps tagged tasks in a JSON file and prints a report.

Chapter 5 · The web page

HTML, CSS, the DOM, events and a page of your own

1. 21 A web page

   HTML elements, CSS rules, and a script that finds, changes and builds the page's elements.

2. 22 Events

   Handlers for clicks, typing and forms; state in variables, and a render() that redraws the page from it.

3. 23 A page of your own

   Lesson 20's to-do list as a page, run by the same tasks.js: render, add, mark done, filter and count.