Free JavaScript Playground Online

Run JavaScript instantly in your browser. No sign-up, no setup. Practice interview snippets, output questions, promises, closures, event loop and more.

JavaScript editor

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Output

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Practice JavaScript Interview Topics

Event Loop
Mix setTimeout, Promise.then and queueMicrotask, then predict the log order.
Closures
Build a counter or a loop with var vs let and inspect captured values.
Promises
Chain, reject and combine with Promise.all / allSettled to see what resolves.
async / await
Await in sequence vs in parallel and log timing with sleep(ms).
this
Compare regular functions, arrow functions, call/apply/bind and methods.
Arrays and Objects
Practise map, filter, reduce, spreading and reference vs copy.
Output Prediction
Write down the expected output first, then press Run to check.
Common Interview Scenarios
Debounce, deep clone, flatten, group-by and memoize in a few lines.