Detect if a request in Express is for HTML or JSON

wantsJSON.js

/*
  Usage:
  var jsoncheck = require('./wantsJSON') // path to file with this middleware function
  app.use(jsoncheck)
  
  app.get('/', function(req,res,next){
    if (req.wantsJSON) {
      // serve JSON
    }
    if (req.wantsHTML) {
      // serve HTML
    }
  })
  
  But feel free to just scan the code and the bits you need
  
  The Middleware below is super simple, but Google was letting me down
  
  Here's what I was after: sometimes, clients, including browsers, would be requesting HTML, 
  and other times JSON, and I needed a simple way to detect between them. 
  
  This was surprisingly tough!

  Some folks recommended using req.xhr http://expressjs.com/api.html#req.xhr
  However, req.xhr only checks if the request’s “X-Requested-With” header field is “XMLHttpRequest”
  In that situation, there's a solid chance the client wants JSON, but that's not a guarantee!
  
  Maybe you'd think, "I could look for 'html' in the Accepts header!"
  Solid theory!
  Well, here's Internet Explorere's Accept header when requesting a web page:
  >  image/jpeg, application/x-ms-application, image/gif,
  >  application/xaml+xml, image/pjpeg, application/x-ms-xbap,
  >  application/x-shockwave-flash, application/msword, * /*
  Note that "text/html" isn't there. 
  Yes, a web browser doesn't specify that it might be keen on HTML ( ._.)
  
  Other things I found recommended using req.accepts('json')
  However, if the client indicates they accept * / *, that says "hey JSON is OK!", 
    so req.accepts('json') will return "json" instead of undefined
  You'll see this with most browsers: they have * / * in the Accepts header when requesting a web page.
  
  The trick is to use req.accepts(), but specify both 'html' and 'json' IN THAT ORDER.
  
  req.accepts('html','json') says, more or less:
    Given the Accepts headers on this request (which take order into account), 
    AND given a choice between matching 'html' or 'json', 
    AND I prefer 'html', (based on the order of arguments)
    which would be the best match?
  
  That covers you: 
    browsers that accept text/html return "html"
    IE that accepts * return "html"
    If a request explicitly puts application/json before text/html or *, it'll get "json"

  And that's probably what you want
*/
module.exports = function(req, res, next){
  req.wantsJSON = req.accepts('html','json') === 'json';
  req.wantsHTML = req.accepts('html','json') === 'HTML';
}

Looks like you might have a typo on line 57 – you've got res in there twice. I'm guessing the first one is supposed to be req?