Conflicting TypeScript return types when using RxJS combineLatest

In my TypeScript project, I am working with RxJS and trying to implement the combineLatest function. Below is the method I have defined:

myMethod(): Observable<{ one: string }> {
  return combineLatest({
    one: of('one'),
    two: of('two'),
  });
}

I was expecting a TypeScript compilation error because the return type of my method specifies 

Observable<{ one: string }>
, but the combineLatest function actually returns an observable that includes both one and two properties, which does not align with the return type of the method.

If anyone can provide an explanation or some guidance on this issue, I would greatly appreciate it.

typescriptrxjsrxjs-observablescombinelatest

Answer №1

The issue with the code arises from using Observable<T>. TypeScript functions correctly without the Observable, but encounters problems with it.
myMethod(): Observable<{ one: string }> {
  return of({
    one: 'one',
    two: 'two',
  });
}

Link to Stackblitz project

To resolve this issue, one can simply utilize typing in the combineLatest function. However, implementing it in a similar manner causes an error regarding a missing property.

export interface MyMethodType {
  // [key: string]: Observable<string>;
  one: string;
  two: string;
}
type TypedCombineLatest = {
  [K in 'one']: {
    one: Observable<string>;
    two: Observable<string>;
  }[K];
};
...

...
myMethod(): Observable<MyMethodType> {
  return combineLatest<TypedCombineLatest>({
    one: of<string>('one'),
    two: of<string>('two'),
  });
}

Check out the Stackblitz demo here

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