What's the Event Loop?

The Event Loop is a fundamental concept in Node.js that enables non-blocking I/O operations, allowing Node.js to perform asynchronous tasks such as reading from the filesystem, making network requests, and more. Despite JavaScript being single-threaded, the Event Loop facilitates concurrent operations by offloading operations to the system kernel whenever possible.

How the Event Loop Works

1. Phases of the Event Loop:

   • Timers: Executes callbacks scheduled by setTimeout() and setInterval().
   • I/O Callbacks: Executes almost all callbacks with the exception of close callbacks, those scheduled by timers, and setImmediate().
   • Idle, Prepare: Only used internally.
   • Poll: Retrieves new I/O events; node will block here when appropriate.
   • Check: setImmediate() callbacks are invoked here.
   • Close Callbacks: Executes callbacks like socket.on('close', ...).

   Each phase has a FIFO queue of callbacks to execute. The Event Loop enters a given phase, performs any operations specific to that phase, then executes callbacks in that phase's queue until the queue is exhausted or the maximum number of callbacks has executed[1][2][3][4][5][6].

2. Execution Order:

   • The Event Loop executes tasks from the event queue only when the call stack is empty, meaning there are no ongoing tasks.
   • It allows Node.js to perform non-blocking I/O operations by offloading operations to the system kernel, which can handle multiple operations executing in the background[2].

3. Role of process.nextTick() and Microtasks:

   • process.nextTick() all...