Online JavaScript Compiler
Write JavaScript and run it immediately. console.log output appears below. Because this runs server-side rather than in your page, there is no DOM — it is the right tool for logic and algorithm practice.
Running code needs a free account. Write and edit here as much as you like — pressing Run asks you to sign in. It is free, takes about a minute, and covers all five languages plus graded exercises.
Create free accountHow this JavaScript compiler works
Executed in a QuickJS engine compiled to WebAssembly and sandboxed on our servers, so your code cannot touch the page or the network.
Frequently asked questions
Can I use DOM APIs like document or window?
No. This runs server-side, so there is no page to manipulate. It is aimed at logic, algorithms and interview practice rather than front-end work.
How do I read input?
Use readline(), which returns one line of the Input box at a time and null once there is nothing left — the usual convention in competitive programming.
Is this JavaScript compiler really free?
Yes — free, with no card and no trial. It needs a free account, because running code costs real server time and an account is how we keep that fair for everyone. Signing up takes about a minute.
Do I need to install anything?
No. Everything runs on our servers and the results come back to your browser. Nothing to download, no setup, works on a phone.
Can I use this for interview practice?
Yes, and there are graded exercises with hidden test cases if you want feedback rather than just output — those need a free account.
Want feedback, not just output?
A free account unlocks graded exercises — you write a solution, it runs against hidden test cases, and you are told exactly which ones fail and why. Plus structured courses in JavaScript and 17 other paths.
Create a free account