How does the Node.js event loop work, and what are its phases?

Node runs your JavaScript on one thread and hands I/O to the operating system through libuv. The event loop is what decides which completed callback runs next, and it cycles through fixed phases:

• timers — callbacks scheduled by setTimeout and setInterval whose time has come.
• pending callbacks — some system-level callbacks deferred from the previous cycle.
• idle, prepare — internal use only.
• poll — the important one. Node retrieves new I/O events and runs their callbacks, and will block here waiting for work if there is nothing else to do.
• check — setImmediate callbacks.
• close callbacks — 'close' events, such as a socket being destroyed.

Between every phase, and between individual callbacks, Node drains two queues completely: process.nextTick first, then the promise microtask queue.

Note: The consequence worth stating is that any CPU-heavy synchronous work blocks the entire loop — no other request is served while it runs. That is why hashing, image processing, or large JSON parsing belong in a worker thread or a separate process.