How to Write Express.js Middleware

What is Express Middleware?

Before we can write a middleware, we need to know what it is.

If you have written Express app, you’ve probably seen this before routes in app.js:

app.use(logger('dev'));
app.use(bodyParser.json());
app.use(bodyParser.urlencoded({ extended: false }));
app.use(cookieParser());

This is how middleware functions are pluggled into your Express app. To setup middleware, we use app.use(), a method provided by the Express API.

When working with Express.js, you should be familiar with the req, res and next objects. Put simply, middleware is a function that have access to the request(req) objects, response(res) objects and the next(next) middleware function in the stack.

What can you do with Middleware?

• Modify the request or response objects before passing them to the next middleware.
• Execute any extra code before request, response objects.
• When some conditions are not met, deliberate end the request or response cycle.
• Call the next middleware in the stack using next object.

Writing Middleware

A simple midddleware function that logs the remote IP for each request looks like this:

Create a file name app.js

var express = require('express');
var app = express();
var PORT = 3000;

function logger(req, res, next){
    console.log(req.connection.remoteAddress);
    next();
}

app.use(logger);

app.get('/', function (req, res){
  res.json({
    message: {humans: [{"name": "John"}, {"name": "Jane"}]}
  });
});

app.listen(PORT, function(){
    console.log('Server listening on port ' + PORT);
})

In this particular example, a logger prints remote IP address of these request to the server console, and then continues the following function in the stack by calling next();

Since middleware is a amazing pattern that allows developers to reuse code (DRY) within applications and even distribute it through NPM modules, I am going to show you how to create a middleware module that can be easily use and publish it to the NPM.

Create a file name logger.js:

modules.export = function () {
    return function(req, res, next) {
        console.log(req.connection.remoteAddress);
        next();
    }
};

Change the app.js code :

var express = require('express');
var app = express();
var PORT = 3000;
var logger = require('./logger');

app.use(logger);

app.get('/', function (req, res){
  res.json({
    message: {humans: [{"name": "John"}, {"name": "Jane"}]}
  });
});

app.listen(PORT, function(){
    console.log('Server listening on port ' + PORT);
})

Now instead of having the function written in app.js, you can import logger module and use it with app.use(logger).

Pitfalls

Some of the pitfalls to take note when using Express middleware:

Not ending response

If you want to continue with the next middleware function use next object. If the function is the end of the middleware function chain, you must end the response by using res.end(), if not Express app will just hang.

Ordering of the middleware is important

Express.js run through middleware as a chain, and it keeps going to the next function until a respose is ended (app.end()). For this reason, the order in which you register(app.user()) the middleware is important.