javascript - Promise.resolve().then vs setImmediate vs nextTick

Question

NodeJS 0.11 as well as io.js and the Node 0.12 branch all ship with native promises.

Native promises have a .then method which always executes on a future event loop cycle.

So far I've been using setImmediate to queue things to the next iteration of the event loop ever since I switched from nextTick:

setImmediate(deferThisToNextTick); // My NodeJS 0.10 code
process.nextTick(deferThisToNextTick); // My NodeJS 0.8 code

Since we now have a new way to do this:

Promise.resolve().then(deferThisToNextTick); 

Which should I use? Also - does Promise.resolve.then act like setImmediate or like nextTick with regards to code running before or after the event loop?

Answer 1

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.