Making a function an express middleware in a route

So I have a route and I have some ugly code in the route that I'd like to make a middleware. The only problem is that Express's documentation isn't clear and my code just 404s.

How can I accomplish this?

Route:

router.get('/product/:slug', function(req, res) {
//route params
  var slug = req.params.slug;
  var productResp; //scope up api response to pass to render()
  console.log(slug);
//api call
  Prismic.api("https://domeha.prismic.io/api").then(function(api) {
    return api.getByUID('product' , slug);
  }).then(function(response) {

   app.use(markedHtml)
  })
  .catch(function(error) {
    res.render('404');
  })
});

Function:

  var markedHtml = function(req, res, next) {
      var md_col_1 = response.data["product.markdown-col-one"].value[0].text;
      var md_col_1_1 = response.data["product.markdown-col-one"].value[1].text;
      var md_col_2 = response.data["product.markdown-col-two"].value[0].text;
      var md_col_2_1 = response.data["product.markdown-col-two"].value[1].text;
      var md_col_3 = response.data["product.markdown-col-three"].value[0].text;
      var md_col_3_1 = response.data["product.markdown-col-three"].value[1].text;


      var html_col_1 = marked(md_col_1);
      var html_col_1_1 = marked(md_col_1_1);
      var html_col_2 = marked(md_col_2);
      var html_col_2_1 = marked(md_col_2_1);
      var html_col_3 = marked(md_col_3);
      var html_col_3_1 = marked(md_col_3_1);



    res.render('product-template', {
      product: response,
      md_one: html_col_1,
      md_one_1: html_col_1_1,
      md_two: html_col_2,
      md_two_1: html_col_2_1,
      md_three: html_col_3,
      md_three_1: html_col_3_1,
    })

      next();
    }

via Quesofat