title: How to use @types/express-session?
body: <p>when I write this: </p>
  
  <pre><code>import { Request } from 'express-serve-static-core';
  
  router.post((req: Request, res, next) =&gt; {
    req.session.user = user;
  }
  </code></pre>
  
  <p><code>tsc</code> gives me an error: </p>
  
  <blockquote>
    <p>'Object is possibly 'undefined'.</p>
  </blockquote>
  
  <p>I know the original <code>Request</code> type does not have the <code>session</code> field.</p>
  
  <p>I check <code>@types/express-session</code> <code>index.d.ts</code> file, found this: </p>
  
  <pre><code>declare global {
    namespace Express {
      interface Request {
        session?: Session;
        sessionID?: string;
      }
    //....
  }
  </code></pre>
  
  <p>So I want to add extra field <code>session</code> and <code>sessionID</code> type to the <code>req</code>.</p>
  
  <p>How can I use ? like this: <code>req: Request &amp; ExpressSessionRequest</code>.</p>
  
  <p>So the <code>req</code> will have both original <code>Request</code> type and extra fields type which <code>@types/express-session</code> add.</p>