title: Typescript "Cannot assign to property, because it is a constant or read-only" even though property is not marked readonly
body: <p>I have the following code </p>
  
  <pre><code>type SetupProps = {
      defaults: string;
  }
  
  export class Setup extends React.Component&lt;SetupProps, SetupState&gt; {
      constructor(props: any) {
          super(props);
          this.props.defaults = "Whatever";
  }
  </code></pre>
  
  <p>When trying to run this code the TS compiler returns the following error:</p>
  
  <blockquote>
    <p>Cannot assign to 'defaults' because it is a constant or a read-only
    property.</p>
  </blockquote>
  
  <p>How is <code>deafualts</code> a readonly property, when its clearly not marked this way. </p>