Beginner → Advanced
JavaScript Mastery — The Complete Interactive Course
From your first console.log to closures, prototypes and the event loop. Every example runs in a real JavaScript engine in your browser, and every exercise is graded by executing your code against hidden tests — sandboxed in a worker, so nothing you write can touch the page.
The course
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Common questions
Do I need to install anything to take this course?
No. Your code runs in a sandboxed Web Worker inside your own browser. Nothing is installed, nothing is sent anywhere, and an endless loop is stopped after four seconds rather than freezing the page.
How much of JavaScript does it cover?
It covers 12 modules — from your first program through types and coercion, strings, control flow, arrays, objects, functions and closures, then this, classes and prototypes, the event loop and async/await, errors, JSON, Map and Set, and finally iterators, generators, functional patterns and modules.
Is the JavaScript course free?
Yes. Every lesson, the graded exercises and the timed drill are free and need no account.
Are the exercises actually graded?
Yes. Your function is executed against 166 assertions — some shown, some hidden and aimed at edge cases. The tests check behaviour, never the text of your code, so any correct solution passes.
Does it cover JavaScript interview questions?
The topics interviews lean on hardest get a lesson each: the difference between let, const and var, why typeof null is "object", how this is determined by the call site, what a closure captures, the order the event loop runs things in, and debounce versus throttle.