RXJS on first emit

Question

I'd like to map the value, but only the first time it's emitting. The other times it can pass through. Any ideas?

this.sourceStream$.takeUntil(this.onDestroy$).someFunc((data) => {
  return data['some tranformation']
}).subscribe(data => {

If there any way to do this without a variable to track if the first value has been received?

Thanks!

Answer 1

The map operator has a second argument index that is automatically incremented:

this.sourceStream$.map((value, index) =>
  index === 0 ? whatever(value) : value
);

Answer 2

What about using a combination of take skip and merge.

Something along these lines should do the trick, even if a bit verbose

const obs = Observable.interval(1000).take(4);

const obs1 = obs.take(1).map(val => 'I am the fist '+ val);
const obs2 = obs.skip(1);

merge(obs1, obs2).subscribe(console.log)