mergeMap

mergeMap is a combination of map + mergeAll.

const fetchMovie = (name) => {
  return new Prmose((resolve) => {
    setTimeout(() => {
      resolve({
        name
      })
    }, 2000);
  })
}

a$.pipe(
  mergeMap((a) => from(fetchMovie(a)))
).subscribe((b) => {
  console.log(b);
})

// is exactly the same as 

a$.pipe(
  map((a) => from(fetchMovie(a))),
  mergeAll()
).subscribe((b) => {
  console.log(b);
})