You can define a middleware as below (taken and modified from this answer)
function modifyResponseBody(req, res, next) {
var oldSend = res.send;
res.send = function(data){
// arguments[0] (or `data`) contains the response body
arguments[0] = "modified : " + arguments[0];
oldSend.apply(res, arguments);
}
next();
}
app.use(modifyResponseBody);