RxJS / Observable flatMap可以返回Observable或数组

Question

Can someone explain to me why the .flatMap operator can accept a function which returns an Observable , or an array ?

The official docs say:

The FlatMap operator transforms an Observable by applying a function that you specify to each item emitted by the source Observable, where that function returns an Observable that itself emits items.

Why can it also return an array ?

For example, these are both valid:

obs$.flatMap((data) => {
    return [];
});

obs$.flatMap((data) => {
    return new Observable<string>();
});

But this does not work:

obs$.flatMap((data) => {
    return 1;
});

Answer 1

The official docs are not relevant because they refer to RxJS 4 and not RxJS 5.

mergeMap projection function returns not just Observable but ObservableInput interface , which applies to miscellaneous values that can be converted to observables :

Arrays can be interpreted as observables that emit all values in array one by one, from left to right, and then complete immediately.

This means that

obs$.flatMap((data) => arr)

is basically a shorter version of

obs$.flatMap((data) => Observable.from(arr))