hoping for a little confirmation on understanding of node.js execution model. I understand that when node.js process starts, this is the sequence of executions:
(from Jonas Schmedtmann's Udemy node.js course)
With the main takeaway being that top-level code is always executed first before any callbacks.
Then, in the event-loop, this is the sequence of the 'phases':
After some digging, I also confirmed why a setTimeout and a setImmediate called in the main module has 'arbitrary' execution order, but when called from the I/O phase, the setImmediate will always execute first, based on this post: https://github.com/nodejs/help/issues/392#issuecomment-274032320.
(Reason: assuming the timer threshold has already passed, since we are currently in the I/O phase, and the next phase after that is the check-handles phase where setImmediate callbacks are executed, immediate always executes before timer.)
Now, when timer and immediate callbacks are called from a phase such that the next phase is the due-timers phase (such as from main module), if the top-level code took long enough that the timer is due, the timer callback will always execute first, correct? I've tested this with the following code, and it seems to be true (everytime I've run it, timer executes first, even though it has a full second delay compared to the immediate callback)
setTimeout(() => {
console.log('timer completed');
}, 1000);
setImmediate(() => {
console.log('immediate completed');
});
for (let i = 0; i < 5000; i++) {
console.log(`top-level code: ${i}`);
}
So here is my question: shouldn't an I/O operation callback also be executed before the immediate's callback due to the event-loop, assuming that the top-level code takes long enough that the I/O operation completes by the time we start the event-loop?
However, this code below suggests otherwise, as the execution order is always: top-levels->timer->immediate->io
Even though based on the model above I should be expecting: top-levels->timer->io->immediate (?)
setTimeout(() => {
console.log('timer completed');
}, 1000);
fs.readFile('test-file.txt', 'utf-8', () => {
console.log('io completed');
});
setImmediate(() => {
console.log('immediate completed');
});
for (let i = 0; i < 5000; i++) {
console.log(`top-level code: ${i}`);
}
Thank you!
