Express.js: Route Model Binding

I’ve been using express.js for a while but until the other day I was unaware of the nifty router.param method. It allows you to execute a callback if a certain param is present in the route.

const express = require("express");
const app = express(); 

const router = express.Router(); 

route.param('user', function(req, res, next) { 
    // if ":user" placeholder in any of the router's route definitions 
    // it will be intercepted by this middleware 
    const user = { id: 1, name: 'Mirko' }; 
    req.user = user; 
    next();
}); 

router.get("/:user", function(req, res) { 
    // req.user will be populated with { id: 1, name: 'Mirko' } 
    return res.json({ result: req.user });
}); 

app.use("/api/users", router); app.listen(3000);

This is a pretty usefull feature as is because often times you will have a router that constantly fetches a model from a database for further actions. If nothing else it really cleans the code up.

But what if we got a little bit creative with this feature. First thing that came to my mind is to have some sort of “binding registration process” and then dynamically bind params accross the app. With a framework like Laravel (btw Laravel supports RMB and was the inspiration for this post - credit where credit’s due) there are certain conventions about models and their location. We will rely on configuration over convention and specify model fetching functions.

End result looks something like this:

const express = require("express");
const app = express();
const assert = require("assert"); 

const router = express.Router(); 

// if you use your imagination getUser and getPost execute a select
// query towards our database and get real data
function getUser(id) { 
    return Promise.resolve({ id: 1, name: "Mirko" });
}

function getPost(id) { 
    return Promise.resolve({ id: 1, title: "Express.js is cool" });
} 

const bindings = [ 
  { param: "user", handler: getUser }, 
  { param: "post", handler: getPost }
]; 

function handleParam({ param, handler }) { 
    // just a sanity check to make sure we have what we need 
    assert(param, "Binding mush have a param"); 
    assert(handler, "Binding must have a handler"); 
    
    // second argument to `route.param` must be a function 
    // of similar signature to a normal middleware with exception of 
    // having an additional parameter which represents the value of placeholder 	
        
    return function(req, res, next, id) { 
    	return handler(id)
          .then(model => { 
            	// we assign the db model to request object for future use 					
            	req[param] = model; 
                next(); 
            })
            .catch(err => { 
                // any errors thrown by handler will be passed to express error handler 
                next(err); 
            }); 
       };
} 

bindings.forEach(function(binding) { 
  router.param(binding.param, handleParam(binding));
}); 

router.get("/:user/posts/:post", function(req, res) { 
  return res.json({ user: req.user, post: req.post });
}); 

router.get("/:user", function(req, res) { 
  return res.json({ result: req.user });
}); 

app.use("/api/users", router); 

app.listen(3000);

If you're executing this locally navigate to http://localhost:3000/api/users/1/posts/1 in your browser and check out the result.