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
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].
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].
