Tell me about the event loop mechanism in Node.js.

The Event Loop allows Node.js to perform non-blocking input/output operations despite JavaScript being single-threaded. It continuously polls the event queue and executes callbacks.

The main phases of the Event Loop (in execution order):

• timers: Executes callbacks for setTimeout() and setInterval().
• pending callbacks: Handles operations scheduled for the next iteration of the cycle.
• idle, prepare: Used by Node.js internal logic.
• poll: The main phase. Waits for new input/output events, executes callbacks for them. It can block if there are no active timers or setImmediate() callbacks.
• check: Executes callbacks for setImmediate().
• close callbacks: Handles close event callbacks, e.g., socket.on('close', ...).

After each event cycle phase, Node.js checks microtasks:

1. process.nextTick(): Callbacks scheduled with process.nextTick() are executed before any other microtasks or moving to the next Event Loop phase.
2. Promise callbacks: Promise .then(), .catch(), .finally() callbacks and await are executed after process.nextTick() and before the next Event Loop phase.

Example of Event Loop operation:

// Scheduled in the timers phase of the next cycle
setTimeout(() => {
  console.log('Timer (timers phase)');
}, 0);

// Scheduled in the check phase of the next cycle
setImmediate(() => {
  console.log('Immediate (check phase)');
});

// Executed immediately in the current microtask queue
process.nextTick(() => {
  console.log('Next Tick (microtask)');
});

// Promise also adds callbacks to the microtask queue
Promise.resolve().then(() => {
  console.log('Promise (microtask)');
});

console.log('Start of execution (immediate)');

Expected output:

Start of execution (immediate)
Next Tick (microtask)
Promise (microtask)
Timer (timers phase) // May appear earlier or later "Immediate", depends on load
Immediate (check phase) // May appear earlier or later "Timer", depends on load

The Event Loop is managed by the libuv library, which provides an abstraction over low-level input/output operations and task scheduling mechanisms in the operating system.