Type cannot be used as an index type : <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>
