startWith vs defaultIfEmpty with combineLatest operator

index.js

const { rxObserver } = require('api/v0.3');
const { timer, combineLatest } = require('rxjs');
const { startWith, filter, defaultIfEmpty, skip } = require('rxjs/operators');

// our value to compare to
// toString is needed only for vizualisation
const NO_VALUE = { toString: _=>'-' };


const a$ = timer(5).pipe(
    // skip(1) // <-- uncomment this
  );

const b$ = timer(10).pipe(
    // skip(1) // <-- uncomment this
  );


// vizualise a$ and b$
a$.subscribe(rxObserver('a$'));
b$.subscribe(rxObserver('b$'));

// startWith
combineLatest(
  a$.pipe(  startWith(NO_VALUE)  ),
  b$.pipe(  startWith(NO_VALUE)  )
)
.pipe(filter(([a, b]) => a !== NO_VALUE || b !== NO_VALUE))
.subscribe(rxObserver('result for startWith'));


// startWith
combineLatest(
  a$.pipe(  defaultIfEmpty(NO_VALUE)  ),
  b$.pipe(  defaultIfEmpty(NO_VALUE)  )
)
.pipe(filter(([a, b]) => a !== NO_VALUE || b !== NO_VALUE))
.subscribe(rxObserver('result for defaultIfEmpty'));