map 

Turn emitted value into another value.

const { rxObserver } = require('api/v0.3');
const { timer } = require('rxjs');
const { map, take } = require('rxjs/operators');

timer(0, 10)
  .pipe(
    take(5)
  )
  .subscribe(rxObserver('timer(0, 5)'));

timer(0, 10)
  .pipe(
    map(i => i + ' 🦆'),
    take(5)
  )
  .subscribe(rxObserver('map(i => i + 🦆)'));

Try comparing map to pluck