Lab · Runtime

Event Loop Visualizer

Why does a Promise callback run before a setTimeout(…, 0)? Step through real snippets and watch the call stack, microtask queue, and macrotask queue move — the exact ordering the runtime guarantees.

demonstrates: deep runtime understanding

1console.log('A');
2setTimeout(() => console.log('B'), 0);
3Promise.resolve().then(() => console.log('C'));
4console.log('D');

console

—

Call stack

main()

Microtasks

empty

Macrotasks

empty

Script enters the call stack and begins executing top to bottom.

step 1/10