Promise.resolve().then vs setImmediate vs nextTick

javascript node.js promise es6-promise
Benjamin Gruenbaum picture Benjamin Gruenbaum · Dec 25, 2014 · Viewed 10.8k times · Source

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

vkurchatkin picture vkurchatkin · Dec 25, 2014

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.

Related questions

Is Node.js native Promise.all processing in parallel or sequentially?

I would like to clarify this point, as the documentation is not too clear about it; Q1: Is Promise.all(iterable) processing all promises sequentially or in parallel? Or, more specifically, is it the equivalent of running chained promises like …

Read More
Promise callbacks returning promises

With regard to these great two sources: NZakas - Returning Promises in Promise Chains and MDN Promises, I would like to ask the following: Each time that we return a value from a promise fulfillment handler, how is that value …

Read More
How to throttle Promise.all() to 5 promises per second?

I have a couple of items that I need to query a 3rd party API for and said API has a call limit of 5 calls per second. I need to somehow throttle my calls to the API to a maximum …

Read More