Summary - Event loop internals

Combine phases, microtasks and I/O to write an event loop visualizer.

Summary - Event loop internals

In this group we saw the internals of the event loop: the six phases, microtasks, the timers queue, I/O callbacks and setImmediate. Each determines when callbacks run.

Phases: timers, pending, idle, poll, check, close. Microtasks run between phases; nextTick first. The timers queue orders by delay. I/O callbacks run in the poll phase. setImmediate runs in the check phase. In the final lesson we combine them in a report.

The report function

The final task combines event loop concepts into one report. The loopReport() function joins the phase count and execution order with ' | '. Output: Phases: 6 | Order: timer -> io -> immediate.

This report summarizes two aspects of the event loop: how many phases (6) and the execution order in one turn (timer -> io -> immediate). Inside the function you keep the phases array and the order string in separate variables, then join them with +. phases.length gives the phase count. This lets you see what you learned on the console; you model the structure without the uncertainty of running the real event loop.

Exercise Write the event loop report

Write function loopReport(). Define const phases = ['timers', 'pending', 'idle', 'poll', 'check', 'close']. Define const order = ['timer', 'io', 'immediate'].join(' -> '). Return 'Phases: ' + phases.length + ' | Order: ' + order. Print console.log(loopReport()).

Show example solution

function loopReport() {
  const phases = ['timers', 'pending', 'idle', 'poll', 'check', 'close'];
  const order = ['timer', 'io', 'immediate'].join(' -> ');
  return 'Phases: ' + phases.length + ' | Order: ' + order;
}
console.log(loopReport());

Test yourself

When do microtasks run?
- Between phases
- Only at the start
- Inside the phases
- Sadece sonda
Show answer

Between phases — Microtasks run between phases.
Match each concept to its role

timers phase

setTimeout
poll phase

I/O callbacks
check phase

setImmediate
microtask

nextTick and Promise

Next lesson: Heap and stack

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies