Example showing the pitfalls of using an observable for init code

test.ts

import { Observable, Observer } from 'rxjs';

function runTest(share: boolean, delay: boolean) {

  let observable = Observable.create((observer: Observer<boolean>) => {
    setTimeout(() => {
      console.log('I did my expensive thingy');
      observer.next(true);
    }, 1000);
  });
  
  if(share) {
    observable = observable.shareReplay(1);
  }

  observable.first().subscribe(() => {
    console.log('Sub 1 done');
  });
  if(delay) {
    setTimeout(() => {
      observable.first().subscribe(() => {
        console.log('Delayed sub done');
      });
    }, 2000);
  } else {
    observable.first().subscribe(() => {
      console.log('Sub 2 done');
    });
  }

  return observable.first().toPromise();
};

console.log('==No sharing==');
runTest(false, false).then(() => {
  console.log('==Sharing==');
  return runTest(true, false);
}).then(() => {
  console.log('==Sharing but delayed==');
  return runTest(true, true);
});