I am new to node.js and little bit confused on understanding the event-loop. As far as i know from https://github.com/nodejs/node/blob/master/doc/topics/event-loop-timers-and-nexttick.md, the event-loop phases only process setTimeout, setInterval, setImmediate, process.nextTick, promises and some I/O callbacks.
My question is, if i have following code:
for (var i = 0; i < 100000000; i++) ;
in which phase the above code will get executed ?
Regular JavaScript code, like the for loop in your example, is executed before the queues are cleared. The first thing node will do is run your code, and will only call callbacks, timeout results, I/O results, and so on after your code finishes.
As an example, you could try this code:
fs.open('filename', 'r', () => {
console.log('File opened.');
});
for (var i = 0; i < 100000000; i++);
console.log('Loop complete.');
No matter how big or small your loop variable, 'Loop complete' will always appear before 'File opened'. This is because with only one thread, node can't run the callback you've supplied to the fs.open function until the loop code has finished.
Remember that there isn't a "main" thread that node keeps going back to. Most long-running node programs will run through the code in main.js pretty quickly, and subsequent code is all going to come from callbacks. The purpose of the initial execution is to define how and when those callbacks happen.