title: Type cannot be used as an index type
body: <p>Go to <a href="https://www.typescriptlang.org/play/index.html" rel="noreferrer">https://www.typescriptlang.org/play/index.html</a> and paste:  </p>
  
  <pre><code>let userTypes = {};
  let keys = Object.keys[userTypes];
  </code></pre>
  
  <p>receive error:</p>
  
  <pre><code>error TS2538: Type '{}' cannot be used as an index type.
  </code></pre>
  
  <p>Why?</p>