2. The event loop

Node runs your synchronous code first, then repeatedly processes queued work in phases: timers (`setTimeout`/`setInterval`), pending I/O callbacks, `setImmediate`, and close callbacks. Between phases it drains microtasks — resolved Promises (`.then`) and `process.nextTick` — before moving on.

This ordering explains a classic surprise: a `Promise.resolve().then(...)` scheduled after a `setTimeout(fn, 0)` still runs first, because microtasks are drained before the next macrotask phase.

Understanding the loop matters once you write anything that mixes timers, network callbacks and promises — which is essentially every real Node service.