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setTimeout vs setImmediate vs process.nextTick

Mark on April 22, 2019

What's the difference between setTimeout(callback, 0) and process.nextTick(callback)? How about Node's setImmediate(callback)? On the surface it a...

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Andrei Gatej • Apr 24 '19

Thanks for pointing out the difference between them!

One thing confuses me:

Finally, we have setTimeout. /* ... */, but it does run after I/0 events and after setImmediate.

So if it runs after setImmediate, why is timeout printed before immediate?

Mark • Apr 24 '19 • Edited

Clearly my model of how this works is wrong! I did some research in writing this, but the timeout is running first here.

Time to do some more investigation and revise this. Thanks!

Edit: setImmediate callbacks go into the "close" queue! I've updated the post.

Mark • Apr 22 '19

The output is...

[Running] node "/Users/logicmason/timeouts.js"
nextTick
nextTick
nextTick
timeout
timeout
timeout
immediate
immediate
immediate

[Done] exited with code=0 in 0.355 seconds

Andreas Jakof • May 16 '19

which of the next ticks will be first? Is it FIFO or LIFO?
According to your previous explanation I‘d say LIFO.

Sachin Goel • May 16 '19 • Edited

No It will be FIFO

Andreas Jakof • May 16 '19

So in your example it will be
nextTick (racer1)
nextTick (racer2)
nextTick (racer3)
...?

Sachin Goel • May 16 '19

This is not going to affect the result anyway.

Andreas Jakof • May 16 '19

Not in this case, but if there are three different callbacks set this way, it might be. That's why I'm asking.
Or are they all called at the same time, if you have enough cores? Then they will be started one after another in FIFO order, but they might end in a different order, even if they are doing exactly the same, because of different workloads on different cores.

As long as they are independent it should not matter ... all hail a functional approach. But if you work on the same object with three different callbacks, this might be a question, that could become very important.

Or is all of this dependent on the browser and its JS implementation?

I clearly (luckily?) don't have enough experience with JS, as I am working mostly in the backend.

Mark • May 19 '19

I think they're FIFO, but I definitely wouldn't rely upon that behavior.

Re: using multiple cores, in that case you're probably running an instance of Node on each and using PM2.

Rishabh jaishwal • Jul 22 '20

code 1

setTimeout(function cb1() {
console.log("getting")
}, 0);

setImmediate(cb1);

code 2

setImmediate(cb1);
setTimeout(function cb1() {
console.log("getting")
}, 0);

code 3

function cb1() {
console.log("call second");
}
setImmediate(cb1);
setTimeout(function cb1() {
console.log("getting")
}, 0);
process.nextTick(cb);

code 3

function cb1() {
console.log("call second");
}

setTimeout(function cb1() {
console.log("getting")
}, 0);
setImmediate(cb1);
process.nextTick(cb);

SEE UNEXPECTED BEHAVIOUR

nirmalpatel59 • Apr 21 '22

Not sure I am understanding your explanation of setImmediate rightly.
Consider following code. setImmediate will be executed before the timer.

const fs = require('fs');
fs.readFile('raw.githubusercontent.com/komljen/... => {
racer();
});
let racer = function() {
setTimeout(() => console.log("timeout"), 0);
setImmediate(() => console.log("immediate"));
process.nextTick(() => console.log("nextTick"));
console.log("current event loop");
}

Shibiru A Ararso • Feb 22 '22

nextTick
nextTick
nextTick
timeout
timeout
timeout
immediate
immediate
immediate
//this what i get from the out put, but i thought like this
nextTick
timeout
immediate
nextTick
timeout
immediate
nextTick
timeout
immediate