title: Add a property with .map() in rxjs
body: <p>If I had an object <em>car</em>:</p>
  
  <pre><code>{
      make: "Chevy",
      model: "Sonic"
      type: {
          id: 1,
          name: "Tiny"
      }
  }
  </code></pre>
  
  <p>And I wanted to manipulate the object to add a typeId property:</p>
  
  <pre><code>{
      make: "Chevy",
      model: "Sonic"
      type: {
          id: 1,
          name: "Tiny"
      },
      typeId: 1
  }
  </code></pre>
  
  <p>Which rxjs operator would you use, assuming you had an array of these objects? This is what I'm trying to avoid:</p>
  
  <pre><code>someArray.map(item =&gt; ({
      make: item.make,
      model: item.model,
      type: item.type,
      typeId: item.type.id
  }));
  </code></pre>