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Event Loop: The Engine

Understand the heart of Node.js concurrency and how it handles thousands of connections on a single thread.

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The Loop

The core mechanism driving Node.js concurrency.

To master Node.js, you must master the Event Loop—the continuous process that manages asynchronous execution.

1Non-Blocking I/O

Unlike blocking systems where the thread 'pauses' to wait for data, Node.js uses Non-Blocking I/O. It delegates the waiting to the Operating System or a worker pool (libuv) and continues executing your script. When data is ready, a notification is sent back.

2The Phases of the Loop

1. Timers: setTimeout() and setInterval().

2. Pending Callbacks: Deferred I/O.

3. Poll: The critical phase where I/O is retrieved.

4. Check: setImmediate() callbacks.

5. Close: Cleanups like socket.on('close').

3Microtasks vs Macrotasks

Promises and process.nextTick are handled as Microtasks. They have higher priority and run immediately after the current operation finishes, before the Event Loop moves to the next phase.