Describe the phases of the Node.js event loop.

Each loop iteration goes through six phases in order. Each phase has its own queue of callbacks:

• timers runs setTimeout and setInterval callbacks whose time has passed.
• pending callbacks runs some system-level callbacks delayed from the previous iteration, such as certain TCP errors.
• idle, prepare is for Node's internal use.
• poll gets new I/O events and runs their callbacks. If nothing else is scheduled, the loop waits here for I/O, but only until the next timer is due.
• check runs setImmediate callbacks.
• close callbacks runs handlers like a socket's close event.

Two queues sit outside the phases. After every single callback, Node empties the process.nextTick queue first. Then it empties the promise microtask queue. So these callbacks always run before the loop moves on.

The process exits when nothing keeps the loop alive. That means no timers, no open handles (like sockets or servers) and no pending requests.