Summary  
This chapter explains the phases of the Node.js event loop and how it sequentially processes callbacks—timers, I/O callbacks, poll, check, and close callbacks—to manage asynchronous operations efficiently and predictably.

General domain of usage  
Network and file I/O operations

To understand how Node.js handles asynchronous operations, you need to know the main phases of the event loop. Each phase has a specific role in managing different types of events and callbacks. The event loop phases are:

- **Timers**: executes callbacks scheduled by `setTimeout()` and `setInterval()`;
- **I/O Callbacks**: handles callbacks for some system operations, such as errors from network or file operations;
- **Idle/Prepare**: internal phase used by Node.js, not accessible for user code;
- **Poll**: retrieves new I/O events, executes their callbacks (excluding those handled by other phases), and may wait for new events;
- **Check**: executes callbacks scheduled by `setImmediate()`;
- **Close callbacks**: handles cleanup for closed resources, like sockets or file streams.

The event loop always moves through these phases in the same order. At each turn, it processes all eligible callbacks for that phase before moving on to the next. This structure ensures that asynchronous operations are handled efficiently and predictably. Understanding which phase your code runs in can help you reason about callback timing and performance.

Imagine the event loop as a repeating cycle, with each phase passing control to the next in a fixed order. Here's a step-by-step walkthrough of how the event loop processes tasks during each cycle:

1. Start with the **Timers** phase, where Node.js checks if any timer callbacks (from `setTimeout()` or `setInterval()`) are ready to execute;
2. Move to the **I/O Callbacks** phase to handle deferred callbacks for some system operations;
3. Enter the **Idle/Prepare** phase, which is reserved for Node.js internals;
4. Proceed to the **Poll** phase, where the event loop waits for new I/O events and executes their callbacks if any are ready;
5. Continue to the **Check** phase, where callbacks scheduled by `setImmediate()` are executed;
6. Finish with the **Close Callbacks** phase, where cleanup callbacks for closed resources are run.

After completing all phases, the event loop starts the cycle again, constantly checking for new events and callbacks. This sequential, phase-by-phase progression is what allows Node.js to manage asynchronous tasks efficiently, ensuring that each type of callback is handled at the right time in the cycle.

Promises and other microtasks (like `process.nextTick`) are not part of the main event loop phases. Instead, Node.js processes them between each phase—after the current phase's callbacks finish but before the event loop moves to the next phase. This ensures that microtasks run as soon as possible, giving them higher priority than normal callbacks like those from `setTimeout()` or `setImmediate()`.

Note

Which of the following lists shows the correct order of the main phases in the Node.js event loop?

Understand how Node.js manages asynchronous operations and concurrency through its event loop. Explore callbacks, Promises, and async/await to control complex asynchronous flows with clarity. Apply modern patterns and best practices to write efficient, non-blocking, and resilient JavaScript for real-world Node.js applications.

Explore how Node.js handles multiple tasks without blocking execution. Examine the event loop, its phases, and the mechanisms that enable asynchronous behavior, including timers, nextTick, and setImmediate.

Work with Node.js asynchronous APIs and understand how they interact with the event loop. Transition from traditional callbacks to Promises, managing asynchronous operations more predictably and with cleaner code.

Simplify asynchronous workflows with async/await syntax and handle errors gracefully. Avoid common pitfalls like callback hell, manage unhandled rejections, and apply proven strategies for writing reliable, maintainable asynchronous code.

Phases of the Event Loop

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Phases of the Event Loop