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 – JavaScript on MDN
JavaScript Promises: an Introduction on Google Developers

🔆 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?

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