title: Request header field Access-Control-Allow-Headers is not allowed by itself in preflight response
body: <p>I have come across CORS issues multiple times and can usually fix it but I want to really understand by seeing this from a MEAN stack paradigm. </p>
  
  <p>Before I simply added middleware in my express server to catch these things, but it looks like there is some kind of pre-hook that is erroring out my requests. </p>
  
  <blockquote>
    <p>Request header field Access-Control-Allow-Headers is not allowed by Access-Control-Allow-Headers in preflight response</p>
  </blockquote>
  
  <p>I assumed that I could do this:</p>
  
  <pre class="lang-js prettyprint-override"><code>app.use(function(req, res, next) {
    res.header("Access-Control-Allow-Headers","*")
  })
  </code></pre>
  
  <p>or the equivalent but this doesn't seem to fix it. I also of course tried</p>
  
  <pre class="lang-js prettyprint-override"><code>app.use(function(req, res, next) {
    res.header("Access-Control-Allow-Headers","Access-Control-Allow-Headers")
  })
  </code></pre>
  
  <p>Still no luck.</p>