title: Typescripts type 'string | string[]' is not assignable to type 'string', what is the 'string | string[]' type? how to cast them to string?
body: <p>When I do TypeScript:</p>
  
  <pre><code>let token = req.headers['x-access-token'] || req.headers['authorization'] as string;
  </code></pre>
  
  <p>I have fellow error:</p>
  
  <pre><code>Argument of type 'string | string[]' is not assignable to parameter of type 'string'
  </code></pre>
  
  <p>Any one know what is 'string | string[]' type? I mean if I want use logical 'or' of two string in typescript. How to do it?</p>
  
  <p>And How to cast 'string | string[]' type to string type?</p>