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.