Explain the event loop. What happens when a Promise resolves?

JavaScript is single-threaded: one call stack runs code at a time. The event loop coordinates the stack with async work handled by the host environment (browser Web APIs or Node.js libuv).

Components (browser)

• Call stack — currently executing functions.
• Web APIs — setTimeout, DOM events, fetch (run outside the JS thread).
• Microtask queue — Promise callbacks, queueMicrotask, MutationObserver.
• Macrotask queue — setTimeout/setInterval callbacks, I/O.

When a Promise resolves, its .then / .catch / .finally handlers are queued as microtasks — not run immediately.

One event loop turn

1. Execute synchronous code until the call stack is empty.
2. Drain all microtasks (including ones scheduled during draining).
3. Render UI if needed (browser).
4. Dequeue and run the next macrotask.

So a resolved Promise always runs before a setTimeout(..., 0) scheduled in the same synchronous block — microtasks have priority over macrotasks.