Select element from Observable array Angular

Question

I'm querying an API and trying to get a specific element out of the array it returns.

I query the API and get a JSON response back. I send the response via a .map call to a function that maps each object in the response to a T.

getOne(num: number): Observable<T> {
      let t$ = this.http
      .get(`${this.API_URL}?${this.getParams()}`)
        .map(mapToTypeT)
        .filter(x => Number.parseInt(x.rn) === num)

      return t$;
}

I want to call filter to find the one element that has a value rn that matches the num parameter. I know that's not right, as I'm getting two errors back:

Property 'rn' does not exist on type 'T[]'

Type 'T[]' is not assignable to type 'T'

Is there a way I can reach into the T[ ] array to check the properties of each element, and then return the element that matches the given predicate? As it stands I'm working with an Observable array of T's but I want to return an Observable of just one T.

For reference, the other two functions:

function mapToTypeT(response:Response): T[]{
  return response.json().map(t => toT(t))
  }


  function toT(r:any): T{
  let t = <T>({
    staId:    r.staId,
    stpId:    r.stpId,
    staNm:    r.staNm,
    stpDe:    r.stpDe,
    rn:       r.rn,
  });
  return t;
}

Answer 1

I see something wrong here.

• Return type should be:

Observable<Array<T>>

• Add 'any' into filter function:

.filter(x => Number.parseInt((<any>x).rn) === num)