Writing Express Middleware to Modify the Response

I recently had a need for some Express middleware that would track any change to a variable, and send the details of any changes in the response, but only if the response is JSON data. This turned out to be rather more interesting delve into ExpressJS than I expected, and I thought the details of it were worth a post detailing how to do it, and my findings.

The first thing you notice when writing Express middleware functions is that they take three arguments (I’m skipping error-handling middleware deliberately), the request, the response and a next() function that triggers the next step in the processing chain. The next function doesn’t typically take any arguments, instead you’re expected to modify the request and response objects to expose or capture any changes you want.

This post will show you how to capture any changes to a variable (we’re going to pretend all our users are playing a game and we have a leader-board), we want to track any changes that happen during the request and report both the changes and the current state of the variable in every response. I’m going to assume some other middleware has injected a user property onto the request. First off, this is what the code will look like:

const leaderboard = require('./leaderboard');
function leaderboardTracking(req, resp, next) {
  const user = req.user;
  const startingPosition = leaderboard.getUserPosition(user);
  const json_ = resp.json; // capture the default resp.json implementation

  resp.json = function(object) {
    const endPosition = leaderboard.getUserPosition(user);
    object['leaderboard_info'] = {
      'delta':    endPosition - startPosition,
      'position': endPosition,
      'score':    user.score
    };

    json_.call(resp, object);
  };

  next();
}

So in this example we swap out the json function with our own delegate implementation, but you’ll notice we leave the send function alone. The send function will delegate to the json function if it’s given a plain JavaScript object, and json in-turn uses send after stringifying the object.

You’ll also notice that the code doesn’t just invoke the captured json_ function, it uses the call function and specifies the response as this. The json function in Express expects to be invoked within the context of the response object, so we need to keep it that way.

That’s it really, it’s not a terribly complicated pattern, but it can be extremely powerful because it bounds the request / response cycle end-to-end.