Question 294063 in Javascript Question posted in
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Javascript – Combine two subjects in RxJS

In RxJS, how can I achieve this: I have two subjects. I want an observables that combines them in this way: When both have emitted values, emit the latest of their values. Then, both need to emit at least once again, then emit their latest emitted values, etc.

Both subjects:

1 ---------- 2 ----- 3 -- 4 ---------------- 5 ------ 6 -----------------

------- a ------------------ b ------ c --------------------- d --------

Goal observable:

------- 1a ----------------- 2b ----- 3c ------------------- 4d -------
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2

Answers


  1. 0

    This can be done with the zip function:

    import { zip } from 'rxjs'

zip(subject1$, subject2$)
  .subscribe(([val1, val2]) => {
    console.log(`${val1}${val2}`);
  });

    The term "zip" comes from zippers. The analogy is that a zipper pairs up successive teeth from each side of the zipper, so too the zip function pairs up successive values from the observables.

    https://rxjs.dev/api/index/function/zip

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  2. 0

    To achieve the desired behavior using RxJS, you can use the combineLatest operator along with the startWith operator. Here’s an example using TypeScript:

    private subject1 = new BehaviorSubject<number>(1);
private subject2 = new BehaviorSubject<string>('a');


combineLatest([
      this.subject1.pipe(startWith(this.subject1.value)),
      this.subject2.pipe(startWith(this.subject2.value)),
    ]).subscribe(([value1, value2]) => `${value1}${value2}`)

    This example uses combineLatest to combine the emissions from both subjects and startWith to emit the initial values.

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