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Sung M. Kim
Sung M. Kim

Posted on • Originally published at slightedgecoder.com on

Promise.race vs. Promise.any And Promise.all vs. Promise.allSettled

                    Photo by Ryan Franco on Unsplash

What’s new in JavaScript (Google I/O ’19) on May 8, 2019 showed what’s coming/available for static Promise combinator methods, Promise.allSettled and Promise.any.

There are already two methods available in modern browsers, Promise.all and Promise.race.

Let’s take a look at differences and how each method works.

🚀 Prerequisite

🔆 Promise Definition

I will skip on what a promise is and jump straight into static methods and will discuss differences.

A gist is that, a promise is JavaScript’s way of promising you that a work will be done (or might fail if the work could not be completed).

If you are familiar with C#, it’s analogous Task class.

For more info, refer to following documentations.

🔆 Promise State Definitions

  • Fulfilled – When a promise is resolved successfully.
  • Rejected – When a promise failed.
  • Pending – When a promise is “neither fulfilled nor rejected“.
  • Settled – Not really a state but an umbrella term to describe that a promise is either fulfilled or rejected.
    • This term will be used to describe characteristics of new methods later.

For more detailed explanation of states & fates, please refer to States and Fates.

There are other static Promise methods such as Promise.reject, Promise.resolve but I will cover only “combinator” methods, which takes in an iterable object as an argument.

🚀 Differences

Let’s first take a look at difference between existing & new combinator methods.

🔅 Promise.all vs. Promise.allSettled

Both accepts an iterable object but

  • Promise.all rejects as soon as a promise within the iterable object rejected.
  • Promise.allSettled resolves regardless of rejected promise(s) within the iterable object.

🔅 Promise.race vs. Promise.any

Both accepts an iterable object but

  • Promise.race short-circuits on the first settled (fulfilled or rejected) promise within the iterable object.
  • Promise.any short-circuits on the first fulfilled promise and continues to resolve regardless of rejected promises unless all within the iterable object reject.

🚀 Comparison Table

Now let’s take a look at existing/upcoming combinator methods.

Now let’s move on to learn more about each method.

Note that all “Characteristics” are taken from TC39 proposal README.

🚀 Promise.all

  • What is this? Resolve all promises passed as an iterable object.
  • Idiom – One bad 🍏 spoils the bunch (“all”).
  • Characteristic – short-circuits when an input value is rejected

🔆 Example

When Promise.all fulfilled(promisesWithoutReject), all apples are returned.

The latter example using promisesWithOneReject shows that one rejected promise results in rejecting all promises.

🚀 Promise.allSettled

  • What is this? all promises regardless of settled (fulfilled/rejected) status.
  • Idiom – Let’s “wait and see” 🤔.
  • Characteristic – Does not short-circuit unlike Promise.all/race
  • Note – Available in Chrome 76.

🔆 Example

Regardless of settled (fulfilled or rejected) state, all promises resolve without short-circuiting to catch.

To differentiate if resolved values were successful, they are returned as an array of objects of following shape.

  • Fulfilled promise is returned as {status: 'fulfilled', value}
  • Rejected promise is returned as {status: 'rejected', reason}

🚀 Promise.race

  • What is this? The first fulfilled promise or reject the whole promise when even one promise rejects.
  • Idiom – A race between Good 😇](https://s.w.org/images/core/emoji/12.0.0-1/72x72/1f607.png) (Fulfilled) and Evil ![😈 (Rejected)
    • Not really an idiom though 😅
  • Characteristic – Short-circuits when an input value is settled

🔆 Example

In promiseWillFulfill example, the first promise fulfilled within 1 millisecond and thus the humanity survived.

But the second example using promiseWillReject had a promise rejecting in 1 millisecond and thus the humanity is doomed.

And the last example (promisesWithOUTReject) fulfilled without rejection thus the first fulfilled promise value of ”

three” was returned.

From these examples, you can see that the first settled state (fulfilled or reject) short circuited the promise.

🚀 Promise.any

  • What is this? Returns the first fulfilled promise regardless of other rejected promises. If all promises reject, then reject by providing errors for all rejects.
  • Idiom – All’s well that ends well.
  • Characteristic – Short-circuits when an input value is fulfilled.
  • Note – Not yet implemented in any browsers and it is in Stage 1.

🔆 Example

First example has promises that rejects right away but did not short-circuit because of a fulfilled promise, thus you win at life.

Second example has promises resolving after a certain period. The first fulfilled promise was resolved after a series of rejects but didn’t short-circuit. And you were able to get a job.

When all promises reject, then that’s when Promise.any rejects and you didn’t get any job offers.

👋 Conclusion

How I understood was that the new Promise.allSettled/any are introduced for Promise to try its best to resolve promises to fulfill unlike existing ones that fails on first encounter of reject.

Promise.all & Promise.race has been available in modern browsers (this exclude IE ;p) and Promise.allSettled will be available in Chrome 76.

Promise.any is still in stage 1 and not available in any browsers (but available in Bluebird or using polyfills – for the demo I used promise-any NPM library for demo.)

I’d love to hear where you would (have) use(d) each method to solve a problem.

And would you please kindly let me know if you find any mistakes and/or how I can improve the example?

The post Promise.race vs. Promise.any And Promise.all vs. Promise.allSettled appeared first on Sung's Technical Blog.

Top comments (14)

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vulpcod3z profile image
vulpz

Great post and explanation! I personally have dreaded not having an alternative to Promise.allSettled() as it has required me to rewrite much of my applications to still work with Promise.all(). It usually happens anytime there's a need to gather finalized data AFTER running async methods from an array.

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dance2die profile image
Sung M. Kim

Thank you, vulpz 😃

rewrite much of my applications to still work with Promise.all()

How did you get around the issue of current short-circuit behavior of .all()? (write a custom .allSettled or used many try/catch blocks? Those are only two I can think of 🤔)

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vulpcod3z profile image
vulpz

Possibly bad form, but I went the route of "misusing" resolve and reject. If there was a bad case that had issues, yet needed to be caught by Promise.all(), I returned an object that used the try/catch but searched for a specific flag.

Not the most efficient, but it worked and was easier to implement than rewriting the majority of the project!

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dance2die profile image
Sung M. Kim

Getting things done FTW~

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bingocaller profile image
Jesper N. Jakobsen

Great article! I wasn't even aware of the new Promise features but you did a good job of explaining them.
Also, it looks like you need to swap line 2 and 3 in your Promise.any example output gist so they correspond to the example order. :)

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mayassalman profile image
mayas salman

I think that they correspond to the example order....
It is because of setTimeout(....

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bingocaller profile image
Jesper N. Jakobsen

I agree that they correspond to what output you would get from the code (because of setTimeout()), but I don't agree that they correspond to the example order.
Maybe what I should have said was that to make the example and output more readable, one would swap example 2 and 3. That way the output Gist corresponds to the example order and the actual output of the code.

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dance2die profile image
Sung M. Kim • Edited

Thanks Jesper for the suggestion.
I've updated the "output" with comments.

The reason I left the order as it was because I initially expected people to "copy/paste" the code and see the result.
(as opposed to copy/paste each example one at a time)

updated

Behold the power of Gist, now all my x-posts are updated too 😀

I will take notes of your suggestion when I post any types of examples next time.

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iammpp94 profile image
idevthings101@gmail.com

Good post, looking forward to seeing these implemented in all browser vendors

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dance2die profile image
Sung M. Kim

Thanks.
These methods will rid the need for boilterplate codes for sure :)

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wormss profile image
WORMSS

So, what is the purpose of extractApples method? It seem to be be defined, but never used?

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dance2die profile image
Sung M. Kim

Thank you @wormss for catching the vestigial code and I apologize for getting back late.

I removed extractApples from the code snippet as it's "never used" :)

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davidokonji profile image
David Okonji

This was an awesome article, 👍🏼 really helped understand the different static methods of a promise.

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dance2die profile image
Sung M. Kim

Thanks for the kind words, David. Glad that you liked it :)