How the Node.js event loop works step by step

The Event Loop Phases

The event loop is a continuous cycle that processes pending work in six phases. Most application code runs in poll (I/O callbacks) and timers (setTimeout, setInterval).

Execution Order

console.log('1 - synchronous');
setTimeout(() => console.log('4 - timer'), 0);
Promise.resolve().then(() => console.log('3 - microtask'));
process.nextTick(() => console.log('2 - nextTick'));
console.log('1b - still synchronous');
// Output order: 1, 1b, 2, 3, 4

Priority order: the current call stack runs to completion first. Then process.nextTick callbacks drain before Promise microtasks. Only then does the event loop advance to the next phase (timers, I/O).

Why This Matters

If you flood process.nextTick recursively, you starve the event loop — I/O callbacks never run. Use setImmediate when you want to yield to I/O between chunks of work. setImmediate fires in the check phase, after the poll phase, giving pending I/O a chance to execute first.