RxJS observable which emits both previous and current value starting from first emission

Actually, it was as easy as pairing pairwise() with startWith() operators:

subject
    .startWith(null) // emitting first empty value to fill-in the buffer
    .pairwise()
    .subscribe([previousValue, currentValue] => {
        if (null === previousValue) {
            console.log('Probably first emission...');
        }
    })
;