Why does understanding event loop phases matter?
Because it explains the order in which async callbacks run. Knowing that timers run before I/O callbacks, and setImmediate runs after I/O, helps you reason about and debug async behavior in Node.js, which is essential for backend development.
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More FAQs in libuv Event Loop Phases Explained: Timers, Poll, Check, and Close
Timers (setTimeout and setInterval callbacks), pending callbacks (deferred I/O callbacks), idle/prepare (internal), poll (new I/O events and I/O callbacks), check (setImmediate callbacks), and close callbacks (cleanup for closed resources).
Callbacks scheduled by setTimeout and setInterval run. The event loop checks if any timer's threshold has been reached and runs those callbacks. This is the first phase of each loop iteration.
The event loop retrieves new I/O events and executes I/O-related callbacks. If there are no timers scheduled and no pending callbacks, it blocks here waiting for events. This is where most I/O callbacks run.
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