The page and the network. Your code runs in a sandboxed worker with no access to this page, so document, fetch and localStorage aren't available. Everything else — variables, loops, functions, classes, promises, generators — runs for real.
Run some code to see execution steps here.
JavaScript Visualizer Lab — write some code and run it
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Write some code to see execution
Write JavaScript → click Run. Every line, loop, and function call is animated in real time.
Ready. Write some code to begin.
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Free accounts get 7 Visualizer runs every day — no card needed.
Your code, progress and streaks are saved as you go.