express.js - how to intercept response.send() / response.json()

Question

Lets say I have multiple places where I call response.send(someData). Now I want to create a single global interceptor where I catch all .send methods and make some changes to someData. Is there any way in express.js? (hooks, listeners, interceptors, ...)?

Answer 1

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);

Answer 2

for those finding on google, based off the top answer:

app.use((req, res, next) => {
    const oldSend = res.send
    res.send = function(data) {
        console.log(data) // do something with the data
        res.send = oldSend // set function back to avoid the 'double-send'
        return res.send(data) // just call as normal with data
    }
    next()
})

Answer 3

Yes this is possible. There are two ways to do this, one is to use a library that provides the interception, with the ability to run it based on a specific condition: https://www.npmjs.com/package/express-interceptor

The other option is to just create your own middleware (for express) as follows:

function modify(req, res, next){
  res.body = "this is the modified/new response";

  next();
}
express.use(modify);