express_json_monkey_patch.js

var express = require('express')
var responseTime = require('response-time');

var app = express();

var monkeyPatch = function(req, res, next) {
  // only monkey patched json method , you get the idea
  var oldJson = res.json;
  res.json = function(obj) {
    res.resData = obj;
    console.log("monkey patched json method!");
    oldJson.apply(this, arguments);
  }
  next();
}

app.use(monkeyPatch);
app.use( responseTime(function(req, res, time) {
    var log = {
      "date" : new Date(),
      "os" : req.headers['user-agent'],                 
      "requestUrl" : req.originalUrl,
      "ipAddress" : req.ip,
      "requestMethod" : req.method,
      "statusCode" : res.statusCode,
      "statusMessage" : "",
      "timeTaken" : Math.floor(time),
      "data" : JSON.stringify(req.body)
    };
  // response is in res.resData
  console.log(log, res.resData);
}))

app.get('/', function (req, res) {
  res.send('hello, world!')
})

app.get('/error', function (req, res) {
  res.status(400).json({
    error: "it's error"
  });
});


app.listen(3000, function () {
  console.log('Example app listening on port 3000!');
});