I have two observables

• load$ is a stream of load events, emitted on startup or when a reload button is clicked
• selection$ is a stream of selected list items, emitted when a list item is selected

I'm wondering if there is an operator that lets me (A) get the latest value emitted by selection$ whenever load$ emits, while (B) ignoring the value emitted by load$.

I was able to acplish (A) by using withLatestFrom but that doesn't satisfy (B) because withLatestFrom does not ignore the source observable.

load$.withLatestFrom(selection$).subscribe(([_, latestSelection) => {
  // _ is not needed and I don't like to write code like this
  // I only need the latest selection
});

I've looked at switchMapTo which satisfies (B) but not (A) without also using first() or take(1).

load$.switchMapTo(selection$.take(1)).subscribe(latestSelection => {
  // I would like to avoid having to use take(1)
});

I'm looking for a hypothetical switchMapToLatestFrom operator.

load$.switchMapToLatestFrom(selection$).subscribe(latestSelection => {
  // switchMapToLatestFrom doesn't exist :(
});

Unfortunately it doesn't exist and I don't like the two other ways that I came up with because it's not immediately obvious why the code contains take(1) or unused arguments.

Are there any other ways to bine two observables so that the new observable emits only the latest value from the second observable whenever the first observable emits?

I have two observables

• load$ is a stream of load events, emitted on startup or when a reload button is clicked
• selection$ is a stream of selected list items, emitted when a list item is selected

I'm wondering if there is an operator that lets me (A) get the latest value emitted by selection$ whenever load$ emits, while (B) ignoring the value emitted by load$.

I was able to acplish (A) by using withLatestFrom but that doesn't satisfy (B) because withLatestFrom does not ignore the source observable.

load$.withLatestFrom(selection$).subscribe(([_, latestSelection) => {
  // _ is not needed and I don't like to write code like this
  // I only need the latest selection
});

I've looked at switchMapTo which satisfies (B) but not (A) without also using first() or take(1).

load$.switchMapTo(selection$.take(1)).subscribe(latestSelection => {
  // I would like to avoid having to use take(1)
});

I'm looking for a hypothetical switchMapToLatestFrom operator.

load$.switchMapToLatestFrom(selection$).subscribe(latestSelection => {
  // switchMapToLatestFrom doesn't exist :(
});

Unfortunately it doesn't exist and I don't like the two other ways that I came up with because it's not immediately obvious why the code contains take(1) or unused arguments.

Are there any other ways to bine two observables so that the new observable emits only the latest value from the second observable whenever the first observable emits?

• javascript
• rxjs
• reactive-programming

Share Improve this question Follow asked Jul 13, 2017 at 9:35 Steven LiekensSteven Liekens 14.1k99 gold badges6666 silver badges9494 bronze badges

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2 Answers 2

Sorted by: Reset to default 11

withLatestFrom takes an optional project function that's passed the values.

You can use it to ignore the value from load$ and emit only the latest selection:

load$
  .withLatestFrom(selection$, (load, selection) => selection)
  .subscribe(selection => {
    // ...
  });

use "sample" operator In your case:

selection$.pipe(sample(load$)).subscribe(
   // the code you want to run
)