RxJS5 operator similiar to .combineLatest but fire whenever a single observable emits

Question

I am looking for a way to combine multiple Observables into a flat tuple of scalar values - similar to .combineLatest() - but with the exception that it should emit a new value tuple even when no value has been emitted on one of the source observables - yieldung "undefined" in the tuple for those observables that did not yet emit.

Example:

const s1 = new Subject<string>();
const s2 = new Subject<string>();

Observable.combineWithUndefined(s1, s2).subscribe( ([t1, t2]) => {
    console.log(t1.toString() + " " + t2.toString());
});

s1.next("Hello");
s2.next("John");

// expected output:
// Hello undefined
// Hello John

Answer 1

使两个主题开始时具有未定义的值，因此当其中一个主题发出第一个值时，combinateLatest也将发光，并将其与另一个主题的起始值合并。