📌 What is Event Loop?

💡 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.

🔹 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

🔹 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.

📄 Example – Event Loop Demonstration

const fs = require('fs');

console.log('First ...');

setTimeout(() => {
    console.log("Time out Callback..");
}, 0);

setTimeout(() => {
    console.log("Time out Callback with delay..");
}, 2000);

Promise.resolve().then(() => {
    console.log("Promise Callback.."); 
});

fs.readFile('info.txt', (err, data) => {
    console.log("Read file content..");
});

setImmediate(() => {
    console.log("Immediate Callback..");
});

process.nextTick(() => {
    console.log("Next tick Callback");
});

console.log("Third..");

🔹 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.