About JS Mastery Lab

An interactive JavaScript course you operate rather than watch: 18 modules, 98 lessons, 16 visualisers and 7 labs.

How it works

• Your code runs in a sandbox. Every snippet executes in an iframe on an opaque origin, so it cannot reach your progress or the page, and a runaway loop costs one click.
• Exercises are graded by tests. Each one ships a hidden assertion suite that checks behaviour rather than how you wrote it.
• No accounts. Progress lives in your browser and exports as JSON.
• No build step. Native ES modules and hand-written CSS, so the source is readable as reference material.

Browse the curriculum.