Event loop phases and timer execution in Node.js - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the event loop in Node.js?

The event loop is a process that handles asynchronous callbacks in Node.js. It allows Node.js to perform non-blocking operations by offloading tasks and executing callbacks when ready.

intermediate

Name the main phases of the Node.js event loop.

The main phases are: timers, pending callbacks, idle/prepare, poll, check, and close callbacks. Each phase has a specific role in processing callbacks and events.

beginner

What happens during the 'timers' phase of the event loop?

During the timers phase, callbacks scheduled by setTimeout() and setInterval() are executed if their time has expired.

intermediate

How does the 'poll' phase work in the event loop?

The poll phase waits for new I/O events and executes their callbacks. If there are no timers to run, it will wait for callbacks or new events to arrive.

intermediate

Why might a timer callback be delayed in Node.js?

Timer callbacks can be delayed if the event loop is busy running other callbacks or if the poll phase is waiting for I/O. Timers only run after their delay and when the event loop reaches the timers phase.

Which event loop phase executes callbacks from setTimeout()?

A check

B poll

C timers

D close callbacks

The timers phase runs callbacks scheduled by setTimeout() and setInterval().

What does the poll phase do in the Node.js event loop?

A Executes close callbacks

B Executes setImmediate callbacks

C Runs timer callbacks

D Waits for and processes I/O events

The poll phase waits for new I/O events and executes their callbacks.

When are setImmediate() callbacks executed?

A During the check phase

B During the poll phase

C During the timers phase

D During the close callbacks phase

setImmediate() callbacks run in the check phase, which happens after the poll phase.

If the event loop is busy, what happens to timer callbacks?

A They are delayed until the event loop is free

B They are skipped

C They run immediately

D They run during the poll phase

Timer callbacks wait until the event loop reaches the timers phase and is free to run them.

Which phase runs callbacks for socket or handle close events?

A timers

B close callbacks

C check

D poll

The close callbacks phase runs callbacks for closed sockets or handles.

Explain the sequence of event loop phases and how timer callbacks are executed in Node.js.

Think about how the event loop cycles through phases and when timers get their turn.

Describe why a setTimeout callback might not run exactly after its delay time in Node.js.

Consider what else the event loop might be doing when the timer expires.