Explain event loop in Node.js

The event loop has 6 phases:

Phases (in order)

1. timers - Execute setTimeout/setInterval callbacks
2. pending callbacks - Execute I/O callbacks deferred from previous cycle
3. idle, prepare - Internal use only
4. poll - Retrieve new I/O events
5. check - Execute setImmediate callbacks
6. close callbacks - Handle close events (socket.on('close'))

setTimeout(() => console.log('timeout'), 0);
setImmediate(() => console.log('immediate'));
process.nextTick(() => console.log('nextTick'));
Promise.resolve().then(() => console.log('promise'));

// Output:
// nextTick
// promise
// timeout (or immediate)
// immediate (or timeout)

Priority

1. process.nextTick() - runs between phases
2. Microtasks (Promises) - after nextTick
3. Macrotasks (setTimeout, setImmediate)