params without specifying type

[devpato]

enroll(classId): Observable<any> {
    if (!this.currentUser)
      return Observable.throw('User not signed in');

    if (this.currentUser.classes.includes[classId])
      return Observable.throw('Already enrolled');

    this.currentUser.classes.push(classId);

    return Observable.empty().delay(1000);
  }

I noticed in several parts of you code you don't specify the param type. for example in the code above classId should be string or number