Increasing delay over a stream? - rxjs

Question

I have the following, and it does work, it keeps increasing the delay and eventually timing out which is what I wanted.
But because I am using Concatmap i lose the original value from the interval.
let x = 1
let source2$ = interval(500)
.pipe(
concatMap(() => {
x++
let newtime = x * 500
console.log("newtime ", newtime)
return of(5).pipe(delay(newtime))
}),
timeout(3000),
map((data) => {
return 'Source 2: ' + data
})
)
so it prints Source 2: 5.. where as i want it to print the value of the interval.
I got working what i wanted using the concatmap but i think its the wrong operator as I lose the original value.
Can somebody help?
More info
TO summarize, all i would like to do is emit values using the interval and after each emit increase the delay time - eventually it hits the timeout of 3000 ms and errors out.

Answer 1

I've mentioned in comments that you can use concatMap for this that receives ever increasing index from interval:
concatMap(index => {
let newtime = index * 500
console.log("newtime ", newtime)
return of(index).pipe(delay(newtime))
}),
Notice, that I'm returning the value back to the stream by of(index).
I think I understand what were you concerned about returning another Observable. Since you want to emit items in sequence (emit one only after the previous one completes) then you have to use concatMap with another inner Observable. There isn't a special operator only for this functionality because this is "composable behavior" which means you can achieve this behavior by combining existing operators.

Answer 2

const source2$ = interval(500)
.pipe(
map(x => x * 500),
switchMap(x => timer(x)),
timeout(3000),
map(data => 'Source 2: ' + data)
)
UPDATE:
https://stackblitz.com/edit/rxjs-iywcm6?devtoolsheight=60
const source2$ = interval(500)
.pipe(
tap(x => console.log('Tick before delay', x)),
concatMap(x => timer((x + 1) * 500).pipe(mapTo(x))),
tap(x => console.log('Tick after delay', x)),
map(data => 'Source 2: ' + data),
timeout(3000)
).subscribe(
(data) => console.log(data),
e => console.error('Timeout', e))