var users = require('./users');

var category = mongoose.model('categories', new mongoose.Schema({
    _id:     String,
    name:    String,
    ownerId: { type: String, ref: 'users' }
}));

router.post('/settings/addCategory', function(req, res, next) {
  console.log(req.body);
  var category_toAdd = new category();
  category_toAdd._id = mongoose.Types.ObjectId();
  category_toAdd.name = req.body.categoryName;
  category_toAdd.ownerId = req.body.ownerId;

  category.findOne({
    name: req.body.categoryName,
    ownerId: req.body.ownerId
  }, function(error, result) {
     if(error) console.log(error);
     else {
       if(result === null) {
         category_toAdd.save(function(error) {
           if(error) console.log(error);
           else {
             console.log("Added category: " + category_toAdd);
<<<<<<<<<<<<<<<<<<<THE CONSOLE LOG WORKS GOOD
             users.categories.push(category_toAdd);
           }
         });
       }
     }
  });

var categories = require('./dashboard');

var user = mongoose.model('users', new mongoose.Schema({
    _id:          String,
    login:        String,
    password:     String,
    email:        String,
    categories:   [{ type: String, ref: 'categories' }]
}));

users.categories.push(category_toAdd);

Cannot read property "push" of undefined.