Kyle Nazario

RxJS 7 has shipped! For us Angular developers, it unfortunately did not ship in time for Angular 12.

I’ve summarized key takeaways from Ben Lesh’s talk from RxJS Live Asia and his slides below. Lesh is a member of the RxJS core team and formerly worked at Google on the Angular team.

Big feature: smaller bundle sizes

Lesh said while RxJS 7 was “a bit faster,” the big improvement for the new version is its bundle size. RxJS 7 is 53% the size of RxJS 6. If your app used every operator in version 6, that would require 52 KB, but the same thing in RxJS 7 requires just 19 KB.

“This was done via a refactor, a hundred-point improvement of going around and individually moving around code, keeping the same tests, keeping the same code, and moving things around slowly but surely until we got to a place where we could see daylight and we were able to refactor larger portions of the code,” Lesh said in his talk.

See this chart of operator sizes in RxJS 6:

And this chart of the same operator sizes in RxJS 7:

Consolidating sharing operators

Lesh’s talk includes a long discussion about how many ways RxJS lets you share a stream (multicast, shareReplay, refCount, etc).

RxJS 7 deprecates multicast, publish, publishReplay, publishLast, and refCount. shareReplay was too popular to deprecate in 7, but Lesh said it’s next because it is “full of footguns.” Long term, the only sharing operators will be share, connect and connectable. He recommends moving to share now.

share is picking up some new features as the single solution operator. It takes an optional config object as a parameter, where you can define custom behavior for the stream.

share({
  connector: () => new ReplaySubject(),
  resetOnRefCountZero: true,
  resetOnComplete: true,
  resetOnError: true
});

Better TypeScript typings

RxJS 7 requires TypeScript 4.2, Lesh said, because it contains features that enable more accurate, stricter types. One example he gave in his slides involved Subject:

// allowed in RxJS 6, errors in 7 because next() must be called with a number
const subject = new Subject<number>();
subject.next();

For teams that are unable to upgrade to TypeScript 4.2, Lesh recommended staying on RxJS 6, which the RxJS team will continue to support.

toPromise() deprecated

The problem with toPromise(), Lesh explained, was that it didn’t make sense with Observables. Should a promise created by toPromise() resolve with the first or last value emitted from the source Observable?

So, toPromise() is deprecated in favor of lastValueFrom() and firstValueFrom(). These new functions still convert Observables to Promises, but in a way that clarifies that value the Promise will resolve with.

const source = from([1, 2]);

const firstVal = await firstValueFrom(source);

console.log(firstVal); // 1

const lastVal = await lastValueFrom(source);
console.log(lastVal); // 2

If an Observable completes without emitting a value, the Promise created by lastValueFrom or firstValueFromrejects. If that is not desired behavior, you can configure the new Promise to resolve with a defaultValue.

const emptyVal = await firstValueFrom(source, { defaultValue: 'empty' });
console.log(emptyVal); // 'empty'

AsyncIterable support

Anywhere you can pass an Observable, RxJS 7 also lets you pass an AsyncIterable.

async function* ticket(delay: number) {
  let n = 0;
  while (true) {
    await sleep(delay);
    yield n;
  }
}

Other updates

• finalize() operators now run in the order in which they are written in pipe(). In contrast, RxJS 6 ran them in reverse.
• subscription.add(someSubscription) now returns void so people will stop writing add() chains, which Lesh says never worked.

// add() returns void, cannot be chained

subscription.add(subOne).add(subTwo); // errors

• animationFrames() creates Observables to do animation logic reactively
• switchScan() operator, aka switchMap with an accumulator
• throwError() requires a callback, not an error, as the error captures the current stack at the moment of its creation

Your with is my command

• combineLatest operator renamed to combineLatestWith
• merge operator renamed to mergeWith
• zip operator renamed to zipWith
• race operator renamed to raceWith
• concat operator renamed to concatWith

Bitovi recommendations for migrating to RxJS 7

If your project can be upgraded to RxJS 7, we would recommend doing so. The speed and bundle size improvements offer tangible, immediate benefits to end users.

Important points to remember:

• Replace your toPromise calls with firstValueFrom and lastValueFrom
• Replace your shareReplay calls with share
• Stop using .add chains to manage your subscriptions. Lesh recommends takeUntil