Subscribe multiple FormControl(s) only once

The best option for you is combineLatest that emits after all source Observables emitted at least one value and then on every emission from any source Observable:

Observable.combineLatest(controlA.valueChanges, controlB.valueChanges)
  .subscribe(...);

Eventually you can chain each source with startWith(null) to guarantee that every source will emit one item immediately and filter what you want in the subscriber.

Observable.combineLatest(
    controlA.valueChanges.startWith(null),
    controlB.valueChanges.startWith(null)
  )
  .subscribe(([val1, val2]) => ...);

Btw, about the other operators:

• zip - emits only when all sources emitted the same number of items
• forkJoin - emits when all sources emitted at least one item and completed (never happens with valueChanges)
• merge - just merges Observables but you can't know which one emitted what value
• join - I don't think that's part RxJS 5

For those getting combineLatest as deprecated, try combining the valueChanges observables in an array. Also, for those getting startWith as deprecated, try not using null.

combineLatest([
    this.typeSelected.valueChanges.pipe(startWith('')),
    this.checkedFC.valueChanges.pipe(startWith(false))
  ]
).subscribe(([type, checked]) => {
   console.log([type, checked]);
});