Skip to content
Guide Docs

Node.js Event Loop

Node.js Event Loop

The Event Loop is what allows Node.js to perform non-blocking I/O operations, despite JavaScript being single-threaded.

How it Works

The Event Loop handles external events and converts them into callback invocations. It operates in several phases:

  1. Timers: Executes callbacks scheduled by setTimeout() and setInterval().
  2. Pending Callbacks: Executes I/O callbacks deferred to the next loop iteration.
  3. Idle, Prepare: Used only internally.
  4. Poll: Retrieves new I/O events; executes I/O related callbacks.
  5. Check: Executes setImmediate() callbacks.
  6. Close Callbacks: Executes close events, e.g., socket.on('close', ...).

Visualizing the Loop

Imagine a loop that constantly checks if there’s any work to do:

  • If a timer expires -> Run its callback.
  • If a file finished reading -> Run its callback.
  • If nothing is happening -> Wait for new events.

[!IMPORTANT] Don’t Block the Event Loop! Heavy computational tasks (like complex math or large JSON parsing) will stop the loop from processing other requests.

process.nextTick() vs setImmediate()

  • process.nextTick(): Fires immediately after the current operation, before the next phase of the event loop.
  • setImmediate(): Fires on the next iteration of the event loop (Check phase).