rxjs execute tap only at the first time

Question

I want to execute tap() only when i get the first emitted value

Something like:

Observable
  .pipe(
     tap(() => { /* execute only when I get th         


        

Answer 1

You can use the index in map operators like concatMap. Unlike other approaches this is totally flexible about the chosen index. Let's say if you want tap on 2nd emission index === 1 or any predicate like index % 2 === 0

// these are because of using rxjs from CDN in code snippet, ignore them
const {of, interval} = rxjs;
const {take, tap, concatMap} = rxjs.operators;


// main code
const stream = interval(250).pipe(take(4))

stream.pipe(
  concatMap((value, index) => index === 0
    ? of(value).pipe(
        tap(() => console.log('tap'))
      )
    : of(value)
  )
)
.subscribe(x => console.log(x));