map

map turns each emitted value into another value, using mapping function

Also check out pluck operator and try comparing map to pluck

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

// 5 values from a timer
timer(0, 10)
  .pipe(
    take(5)
  )
  .subscribe(rxObserver('timer(0, 5)'));

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

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NOTE: if your mapping function returns an Observable or a Promise — you'll need one of *Map operators:
mergeMap, exhaustMap, switchMap, concatMap

NOTE 2: I've created a package rxjs/proxy {👓} to simplify subproperty access:
It turns Observables of Objects into Objects of Observables
And rxjs/autorun package will let you easily map a single stream or combine multiple.