Connect or Express middleware to modify the response.body

Question

I would like to have a middleware function which modifies the response body.

This is for an express server.

Something like:

function modify(req, res, next){
  res.on('send', function(){
    res.body = res.body + "modified"
  });

  next();
}

express.use(modify);

I don't understand what event to listen for. Any help or documentation would be appreciate.

Answer 1

You don't need to listen to any events. Just make it

function modify(req, res, next){
  res.body = res.body + "modified";

  next();
}

And use it after you use the router. This way after all your routes have executed you can modify the body

Answer 2

I believe the OP actually wants to modify the response stream once a middleware has handled the request. Look at the bundled Compress middleware implementation for an example of how this is done. Connect monkey patches the ServerResponse prototype to emit the header event when writeHead is called, but before it is completed.

Answer 3

express-mung is designed for this. Instead of events its just more middleware. Your example would look something like

const mung = require('express-mung')

module.exports = mung.json(body => body.modifiedBy = 'me');