JavaScript Practice

Twenty-one short, hands-on JavaScript lessons: start with your first variable, build up through closures and classes, debug a real thrown error from its stack trace, revisit arrays and objects with curried functions, set operations, and a combined search, work through a few classic katas (roman numerals, a Caesar cipher, a smallest-common-multiple calculator), then climb into the code that runs clean and is still wrong. Each lesson gives you a task and a Run button - your code is checked against real tests the moment you run it. Everything runs in your browser.

The last six are the ones that bite at work. A thrown error tells you where to look; these do not get you that. sort() putting 10 before 9 because it compares text, || overwriting a deliberate 0, a spread copy that still shares its nested objects, map(parseInt) returning NaN out of nowhere, a cart total that drifts by a cent, and a capstone where an API sends some amounts as numbers and some as strings, so + glues them together instead of adding them up.

Start with lesson 1. You can leave and come back any time - your code is saved locally.

JavaScript Practice

Lessons