title: Typescript array type for values for a key from an array of objects
body: <p>I have the following array:</p>
  <pre><code>export const columns: GridColumns /* from mui-x-datagrid */ = [
    { field: 'boost', headerName: 'Boost' },
    { field: 'totalCash', headerName: 'Total Cash' },
    { field: 'totalUsers', headerName: 'Total Users' },
    { field: 'overCash', headerName: 'Over Cash' },
    { field: 'underCash', headerName: 'Under Cash' },
  ];
  </code></pre>
  <p>I want a type that takes all the values of the <code>field</code> property:</p>
  <pre><code>type Fields&lt;typeof columns&gt; // 'boost' | 'totalCash' | 'totalUsers' | 'overCash' | 'underCash'
  </code></pre>
  <p>How do I do it?</p>