⏱ Node.js – Event Loop & Async Code Execution

01📌 What is Event Loop?

02💡 Key Points

• User-written synchronous code takes priority over async code.
• Async callbacks are executed only after the call stack is empty.
• setTimeout() and setInterval() callbacks are prioritized as timers.
• Timer callbacks run before I/O callbacks, even if both are ready simultaneously.

03🔹 Event Loop Execution Order

• 1. Callbacks in micro task queues: nextTick queue → promise queue
• 2. All callbacks in the timer queue
• 3. Micro task queues (if present) executed again
• 4. All callbacks in the I/O queue (Poll phase)
• 5. Micro task queues (if present)
• 6. All callbacks in the check queue
• 7. Micro task queues (if present)
• 8. All callbacks in the close queue
• 9. Final micro task queues execution: nextTick → promise queue

04🔹 What is the Event Loop?

• An endless loop that waits for tasks, executes them, and sleeps until new tasks arrive.
• Allows programs to remain responsive while handling long-running tasks asynchronously.
• Ensures callback functions are executed only when the call stack is empty.
• Keeps the loop alive if more callbacks are pending; exits when all callbacks are processed.

05📄 Example – Event Loop Demonstration

06🔹 Notes

• Synchronous functions have higher priority than asynchronous functions.
• process.nextTick() executes before any other microtasks.
• setImmediate() runs after I/O events and timers, in the check phase.