What are the main event-loop phases?

Definition:

The event loop is the scheduling mechanism that decides when ready callbacks can run. Node organizes several categories of callbacks into phases, with separate processing for promise microtasks and next-tick callbacks. A timer delay is a minimum waiting threshold, not a promise of an exact execution time.

Key points:

• Timers handle eligible timer callbacks; poll handles I/O; check handles setImmediate.
• Pending callbacks and close callbacks have their own phases too.
• Promise microtasks and process.nextTick callbacks run at checkpoints, outside this phase list.

Reference: Node event loop.

• Repeatedly scheduling microtasks or next-tick callbacks can delay I/O even though each callback is short.

Interview point:

• Do not promise a universal ordering between top-level setTimeout(0) and setImmediate; their context matters.