title: ReactJs map user and append to existing array
body: <p>I'm receiving a JSON response array that I need to map into a specific format before appending to an existing array. I've tried various approaches including below but each time I do the compiler fails to read the map property</p>
  
  <p>code:</p>
  
  <pre><code>componentWillMount() {
  fetch('http://localhost:8090/v1/user')
  .then(results =&gt; {
    return results.json();
  })
  .then(data =&gt; {
    data.results.map((user) =&gt; ( userList.concat({label: user.title, value: user.id})))
    console.log(data);
  })
  }
  </code></pre>
  
  <p>My intended result is an array of user that are in the following format `{label: "Jerry", value: "7425398538"}
  Can this not be done from within the .then()?</p>
  
  <p>The error I get is: <code>Unhandled Rejection (TypeError): Cannot read property 'map' of undefined</code></p>
  
  <p>Thanks</p>