Node.js Event Loop

Event loop representation

MicroTask Queue

MicroTask Queue has 2 built in queues inside of it:

1. NextTick Queue
2. Promise Queue

The rule of MicroTask Queue is that first nextTick callbacks are executed first then the Promise callbacks The process.nextTick should be discouraged because it can cause Event Loop starvation.

Timer Queue

To start timer queue you can use 2 methods:

1. setTimeout - This will delegate the time you specified before it is called to Queue.

    setTimeout(() => {
	    console.log("This is a setTimeout \n");
    }, 1000);

2. setInterval - This will call the callback function in time interval in loop in the time you specified.

    setInterval(() => {
	    console.log("This is a setInterval \n");
    }, 1000);

I/O Queue

Most of the 'async' methods from build in modules are from I/O Queue

    fs.readFile()

Check Queue

Check queue is just straightforward it has method setImmediate that is dispatching sometimes before I/O Queue

setImmediate(() => {
	console.log("This is a setImmediate #1\n");
});

The logic why Check Queue will sometimes dispatch before I/O Queue is because of I/O polling. In the first iteration of code I/O queue is empty because of Node.js internals fs for this example needs to poll to check if the task is completed before calling the callback method.