Setimmediate VS. Nexttick

setImmediate vs. nextTick

Use setImmediate if you want to queue the function behind whatever I/O event callbacks that are already in the event queue. Use process.nextTick to effectively queue the function at the head of the event queue so that it executes immediately after the current function completes.

So in a case where you're trying to break up a long running, CPU-bound job using recursion, you would now want to use setImmediate rather than process.nextTick to queue the next iteration as otherwise any I/O event callbacks wouldn't get the chance to run between iterations.

Promise.resolve().then vs setImmediate vs nextTick

Using Promise.resolve().then has no advantages over nextTick. It runs on the same queue, but have slightly higher priority, that is, promise handler can prevent next tick callback from ever running, the opposite is not possible. This behaviour is an implementation detail and should not be relied on.

Promise.resolve().then is obviously slower (a lot, I think), because it creates two promises which will be thrown away.

You can find extensive implementation info here: https://github.com/joyent/node/pull/8325

The most important part: Promise.resolve().then is like nextTick and not like setImmediate. Using it n place of setImmediate can change your code behaviour drastically.

nextTick vs setImmediate, visual explanation

Consider the following two examples:

setImmediate

setImmediate(function A() {
  setImmediate(function B() {
    log(1);
    setImmediate(function D() { log(2); });
    setImmediate(function E() { log(3); });
  });
  setImmediate(function C() {
    log(4);
    setImmediate(function F() { log(5); });
    setImmediate(function G() { log(6); });
  });
});

setTimeout(function timeout() {
  console.log('TIMEOUT FIRED');
}, 0)

// 'TIMEOUT FIRED' 1 4 2 3 5 6
// OR
// 1 'TIMEOUT FIRED' 4 2 3 5 6

nextTick

process.nextTick(function A() {
  process.nextTick(function B() {
    log(1);
    process.nextTick(function D() { log(2); });
    process.nextTick(function E() { log(3); });
  });
  process.nextTick(function C() {
    log(4);
    process.nextTick(function F() { log(5); });
    process.nextTick(function G() { log(6); });
  });
});

setTimeout(function timeout() {
  console.log('TIMEOUT FIRED');
}, 0)

// 1 4 2 3 5 6 'TIMEOUT FIRED'

setImmediate callbacks are fired off the event loop, once per iteration in the order that they were queued. So on the first iteration of the event loop, callback A is fired. Then on the second iteration of the event loop, callback B is fired, then on the third iteration of the event loop callback C is fired, etc. This prevents the event loop from being blocked and allows other I/O or timer callbacks to be called in the mean time (as is the case of the 0ms timeout, which is fired on the 1st or 2nd loop iteration).

nextTick callbacks, however, are always fired immediately after the current code is done executing and BEFORE going back to the event loop. In the nextTick example, we end up executing all the nextTick callbacks before ever returning to the event loop. Since setTimeout's callback will be called from the event loop, the text 'TIMEOUT FIRED' will not be output until we're done with every nextTick callback.

nextTick vs setImmediate, visual explanation

Consider the following two examples:

setImmediate

setImmediate(function A() {
  setImmediate(function B() {
    log(1);
    setImmediate(function D() { log(2); });
    setImmediate(function E() { log(3); });
  });
  setImmediate(function C() {
    log(4);
    setImmediate(function F() { log(5); });
    setImmediate(function G() { log(6); });
  });
});

setTimeout(function timeout() {
  console.log('TIMEOUT FIRED');
}, 0)

// 'TIMEOUT FIRED' 1 4 2 3 5 6
// OR
// 1 'TIMEOUT FIRED' 4 2 3 5 6

nextTick

process.nextTick(function A() {
  process.nextTick(function B() {
    log(1);
    process.nextTick(function D() { log(2); });
    process.nextTick(function E() { log(3); });
  });
  process.nextTick(function C() {
    log(4);
    process.nextTick(function F() { log(5); });
    process.nextTick(function G() { log(6); });
  });
});

setTimeout(function timeout() {
  console.log('TIMEOUT FIRED');
}, 0)

// 1 4 2 3 5 6 'TIMEOUT FIRED'

setImmediate callbacks are fired off the event loop, once per iteration in the order that they were queued. So on the first iteration of the event loop, callback A is fired. Then on the second iteration of the event loop, callback B is fired, then on the third iteration of the event loop callback C is fired, etc. This prevents the event loop from being blocked and allows other I/O or timer callbacks to be called in the mean time (as is the case of the 0ms timeout, which is fired on the 1st or 2nd loop iteration).

nextTick callbacks, however, are always fired immediately after the current code is done executing and BEFORE going back to the event loop. In the nextTick example, we end up executing all the nextTick callbacks before ever returning to the event loop. Since setTimeout's callback will be called from the event loop, the text 'TIMEOUT FIRED' will not be output until we're done with every nextTick callback.

Significance of using setImmediate() and process.nextTick() in Node.js

Callbacks deferred with process.nextTick() run before any other I/O event is fired.

With setImmediate(), the execution is queued behind any I/O event that is already in the queue.

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