Edit the upvote() function to take parameters

// models/Post.js mongoose schema for mongoDB
PostSchema.methods.upvote = function(cb) {
  this.upvotes += 1;
  this.save(cb);
};
// Now we create the route in our index.js:
router.put('/posts/:post/upvote', function(req, res, next) {
  req.post.upvote(function(err, post){
    if (err) { return next(err); }

    res.json(post);
  });
});
// source https://thinkster.io/mean-stack-tutorial#getting-user-input-enable-upvoting