What are the scenarios for implementing the Event Loop in Node.js?

The Event Loop in Node.js operates in iterations (ticks) and processes the following phases:

1. Timers: handles callbacks for setTimeout() and setInterval().
2. Pending callbacks: executes callbacks for I/O operations deferred in the previous iteration.
3. Idle, prepare: internal use of Node.js.
4. Poll:
   • Executes I/O callbacks (mostly).
   • If no ready I/O callbacks, it may wait for new connections/events or transition to the Check phase.
5. Check: executes callbacks for setImmediate().
6. Close callbacks: executes close event callbacks, e.g., for sockets ('close').

Between main phases of the Event Loop, and after each phase, Node.js checks and executes the following microtask queues:

• process.nextTick() queue: has the highest priority and executes before all other microtasks and Event Loop phases.
• Promise queue: executes resolved promises.

Example:

// Executes before setTimeout, setImmediate, and I/O callbacks
process.nextTick(() => {
  console.log('nextTick');
});

// Executes in the Timers phase
setTimeout(() => {
  console.log('timeout');
}, 0);

// Executes in the Check phase
setImmediate(() => {
  console.log('immediate');
});

// Example of an asynchronous I/O operation
const fs = require('fs');
fs.readFile(__filename, () => {
  // Executes in the Poll phase
  console.log('read file');
});

// Example of a promise
Promise.resolve().then(() => {
  // Executes in the promise microtask queue
  console.log('promise');
});

The output order in the console will be approximately:

nextTick
promise
timer
read file
immediate

(The order of 'read file' and 'immediate' may vary depending on the I/O operation execution).