higher order error handling

higher-order-error-handling.js

/* Turn this */
router.get("/api/products/:id", function(req, res) {
  Product.getProduct(req.params.id, function(err, products) {
    if (err) {
      res.status(404).json({"message": "Not Found"});
    }
    else {
      res.status(200).json(products);
    }
  });
});
 
router.get("/api/users/:id", function(req, res) {
  User.getUserById(req.params.id, function(err, user) {
    if (err) {
      res.status(404).json({"message": "Not Found"});
    }
    else {
      User.updateUser(id, req.body, function(err, user) {
        if (err) {
          res.status(400).json({"message": "Invalid data provided"});
        }
        else {
          res.status(200).json(user);
        }
      });
    }
  });
}

/* Into this */
function ensure(notOkCode, notOkMessage, response, okFx) {
  return function(err) {
     if (err) {
       response.status(notOkCode).json({"message": notOkMessage});
     }
     else {
       okFx.apply(okFx, Array.prototype.slice.call(arguments, 1));
     }
  };
}
 
var ensureFound = _.partial(ensure, 404, "Not Found"),
    ensureValid = _.partial(ensure, 400, "Bad Request");
 
router.get("/api/products/:id", function(req, res) {
  Product.getProduct(req.params.id, ensureFound(res, function(product) {
    res.status(200).json(product);
  }));
});
 
router.get("/api/users/:id", function(req, res) {
  User.getUserById(req.params.id, ensureFound(res, function(user) {
    User.updateUser(user, req.body, ensureValid(res, function(updated) {
      res.status(200).json(updated);
    }));
  }));
});