What are the main event-loop phases?

Understand the concept

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.

Important ideas

• 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.