Node.js
Event loop phases
Learn the event loop phases (poll, check, close).
Event loop phases
The Node.js event loop consists of six phases: timers, pending callbacks, idle/prepare, poll (I/O events), check (setImmediate) and close. Each turn the loop visits these phases in order.
An eventLoopPhases() function models this order: it returns ['timers', 'pending', 'idle', 'poll', 'check', 'close']. In the timers phase setTimeout callbacks run. In the poll phase I/O events are processed. In the check phase setImmediate calls run. Knowing the phase order is the key to understanding when a callback runs. The output is timers -> pending -> idle -> poll -> check -> close.
Phase order
The event loop visits phases in a fixed order each turn. It starts with timers and ends with close; then a new turn begins. This order sets the priority of different callback types.
Example: eventLoopPhases() returns ['timers', 'pending', 'idle', 'poll', 'check', 'close']. .join(' -> ') turns it into one string: timers -> pending -> idle -> poll -> check -> close. Knowing this order explains why a setTimeout callback runs before setImmediate (at the same delay): the timers phase comes before the check phase. The phase order is the skeleton of the event loop.
Exercise Write the phase order
Write function eventLoopPhases(). Return ['timers', 'pending', 'idle', 'poll', 'check', 'close']. Print console.log(eventLoopPhases().join(' -> ')).
Show example solution
function eventLoopPhases() {
return ['timers', 'pending', 'idle', 'poll', 'check', 'close'];
}
console.log(eventLoopPhases().join(' -> '));
Test yourself
-
In which phase do setTimeout callbacks run?
Show answer
timers — setTimeout callbacks run in the timers phase.
-
Write the first phase name
function eventLoopPhases() { return ['____', 'pending', 'idle', 'poll', 'check', 'close']; }
Show answer
function eventLoopPhases() { return ['timers', 'pending', 'idle', 'poll', 'check', 'close']; }
-
Match each phase to its role
- timers
- setTimeout callbacks
- poll
- I/O events
- check
- setImmediate
- close
- Close callbacks
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