JavaScript single-threaded model

JavaScript in the browser is single-threaded: one call stack, one heap, one event loop per tab. Async work feels parallel — but execution is still interleaved on that one thread.

Why one thread?

Browsers originally ran JS for small UI tweaks. A single stack keeps the DOM model simple: no data races on document from two threads at once.

Heavy work (network, timers) is delegated to the browser or runtime. When ready, callbacks return via the event loop.

The classic snippet

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

Output: A, D, C, B — not alphabetical order.

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

Team Radar connection

When metrics “jump” on the dashboard, you’re watching the loop drain queues — not a second JS thread.

Teach-back prompt

Why can a long synchronous loop block SSE updates from painting?