Modify response body before res.send() executes in ExpressJS

Question

In application which I currently develop, it's using Express. In my case I want to get response before it's been sent and modify it (for purpose of JWT). In this application, there is a dozen of endpoints and I don't want to create my own function like sendAndSign() and replace res.send() everywhere in code. I heard there is option to override/modify logic of res.send(...) method.

I found something like this example of modifying, but in my case this doesn't work. Is there any other option (maybe using some plugin) to manage this action?

Answer 1

You can intercept response body in Express by temporary override res.send:

function convertData(originalData) {
  // ...
  // return something new
}

function responseInterceptor(req, res, next) {
  var originalSend = res.send;

  res.send = function(){
    arguments[0] = convertData(arguments[0]);
    originalSend.apply(res, arguments);
  };
  next();
}

app.use(responseInterceptor);

I tested in Node.js v10.15.3 and it works well.