Error in stateless observable service: Type ‘Observable‘ is not assignable to type ‘Observable

I’m learning rxjs and I got stuck with this error.

I have the following method

  getCourseById(id: number): Observable<Course> {
    return this.http.get<Course[]>('the-api').pipe(
      map(courses =>
          courses.filter(course => course.id === id)
        )
    )
  }

I make a call to a particular API endpoint, grab the entire response, then use map to filter against that collection and get the course with id equals to the parameter id.

The function returns an Observable of type Course.

Yet, in the console I see:

error TS2322: Type 'Observable<Course[]>' is not assignable to type 'Observable<Course>'.
  Type 'Course[]' is missing the following properties from type 'Course': id, name, url, description

 27     return this.http.get<Course[]>('the-api').pipe(
        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 28       map(courses =>
    ~~~~~~~~~~~~~~~~~~~~~
... 
 30         )
    ~~~~~~~~~
 31     )
    ~~~~~

I don’t know how to fix it. I’d appreciate help. Thanks

Answer

You should use Array.find() instead of Array.filter(). This will only return the matching object:

getCourseById(id: number): Observable<Course> {
    return this.http.get<Course[]>('the-api').pipe(
      map(courses =>
          courses.find(course => course.id === id)!
        )
    )
}