startWith 

Prepend a stream with a value

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

interval(10)
  .pipe(
    take(5)
  )
  .subscribe(rxObserver('interval(10)'));

interval(10)
  .pipe(
    startWith('🐶'),
    take(5)
  )
  .subscribe(rxObserver('startWith(🐶)'));