title: Type Program[] is not assignable to string[]
body: <p>I have the class below.  What I'd like to do is filter for the active programs and inactive programs and then get the program names, as an array of strings, for those programs.</p>
  
  <p>Here is the Program class:</p>
  
  <pre><code>export class Program {
    ID: number;
    ProgramDescription: string;
    ProgramName: string;
    ProgramStatus: string;
    FiscalYear: string;
    SupervisorID: string;
    DBN: string;
    IsActive: string;
  }
  </code></pre>
  
  <p>Here are the methods I wrote in the service:</p>
  
  <pre><code>getPrograms(): Observable&lt;Program[]&gt; {
  return this.httpClient.get&lt;Program[]&gt;(environment.apiBaseURL + 'api/Program')
                 .pipe(
                   map(data =&gt; data)
                 );
  }
  
  getActivePrograms(): Observable&lt;Program[]&gt; {
  return this.getPrograms()
             .pipe(
               map(data =&gt; data.filter(program =&gt; program.IsActive === 'Y'))
             );
  }
  
  getActiveProgramNames(): Observable&lt;string[]&gt; {
  return this.getActivePrograms()
             .pipe(
               map(data =&gt; data.filter(program =&gt; program.ProgramName.toString())
             );
  }
  </code></pre>
  
  <p>In getActiveProgramNames(), I'm getting the following error:</p>
  
  <pre><code>Type Program[] is not assignable to type string[]
  </code></pre>