NodeJS - setTimeout(fn,0) vs setImmediate(fn)

Question

What is the difference between those two, and when will I use one over the other?

Answer 1

use setImmediate() for not blocking the event loop. The callback will run on the next event loop, as soon as the current one is done.

use setTimeout() for controlled delays. The function will run after the specified delay. The minimum delay is 1 millisecond.

Answer 2

always use setImmediate, unless you are really sure that you need setTimeout(,0) (but I can't even imagine, what for). setImmediate callback will almost always be executed before setTimeout(,0), except when called in first tick and in setImmediate callback.

Answer 3

setImmediate() is to schedule the immediate execution of callback after I/O events callbacks and before setTimeout and setInterval .

setTimeout() is to schedule execution of a one-time callback after delay milliseconds.

This is what the documents say.

setTimeout(function() {
  console.log('setTimeout')
}, 0)

setImmediate(function() {
  console.log('setImmediate')
})

If you run the above code, the result will be like this... even though the current doc states that "To schedule the "immediate" execution of callback after I/O events callbacks and before setTimeout and setInterval." ..

Result..

setTimeout

setImmediate

If you wrap your example in another timer, it always prints setImmediate followed by setTimeout.

setTimeout(function() {
  setTimeout(function() {
    console.log('setTimeout')
  }, 0);
  setImmediate(function() {
    console.log('setImmediate')
  });
}, 10);