Logging Express Request & Response Content

Introduction

While Logging the Request object in Express is amazingly easy, doing so with the response is not quite as straight forwards.

We need to utilize a couple of tricks and some patching to get things working smoothly and get all the information we are sending back to the client.

We can also use this technique to perform some other manipulation rather than just logging, some use cases might be:

1. To Observe the response (Logging, Tracing)
2. To Amend the response (Redacting information by origin for example or modify response for target audience)

The Middleware

Logging the Request

Firstly we need to create a basic middleware which can log our req, nothing new here other than our middleware is a higher order function which takes a logger making it more re-usable.

const requestLoggerMiddleware = ({ logger }) => (req, res, next) => {
  logger("RECV <<<", req.method, req.url, req.hostname);
  ...
  next();
};

Logging the Response

Next we need to log our response. As we can see we have access to the response under the reference res. However, this does not have the information we need yet.

Our middleware runs BEFORE our router, so if we call for any information on our res now it will not be reflective of the res state after the router has done its thing.

We need to add an event hook to be invoked after the finish event, more info on this event can be found in the docs: https://nodejs.org/api/http.html#http_event_finish

Crucially however we need to be aware of the purpose of the event

Emitted when the response has been sent. More specifically, this event is emitted when the last segment of the response headers and body have been handed off to the operating system for transmission over the network. It does not imply that the client has received anything yet.

 const requestLoggerMiddleware = ({ logger }) => (req, res, next) => {
   ...
   res.on("finish", () => {
     logger("SEND >>>", res.statusCode);
   });
   next();
 };

This gives us the basic ability to observe information about the response after handoff, good for observability but bad for interception, and the res is also missing the crucial content sent to res.send

Intercepting res.send

For this last piece we need to monkey patch the res.send method. This means when we call it in our router, for example:

 app.get(["/", "/api/health"], (req, res) => {
   res.send({ message: "OK", uptime: process.uptime() });
 });

We are actually going to be calling our patched res.send, doing something, then calling the original res.send. We call this the interceptor because it's intercepting the call , performing some actions and handing back off.

This is going to give us a critical point to interject and do whatever we need. For observability , we will simply bind the content passed to res.send to the res under res.contentBody giving us access later, but you could just as easily perform some mutation and intercept the content for redacting, modifying, adapting etc.

Let's add the interceptor and explain it's signature.

const requestLoggerMiddleware = ({ logger }) => (req, res, next) => {
   ...  
   res.send = resDotSendInterceptor(res, res.send);
   ...
   next();
};

We can see how the interceptor replaces the default res.send, as this middleware runs BEFORE our router. We know that this patch is now in place and any subsequent calls to res.send will go via our interceptor.

Designing the interceptor

Our interceptor needs two things:

1. To be a function that can take the content sent to res.send normally
2. To be a function which ultimately calls the original res.send so the request/response chain can complete naturally.

Therefore, we write our interceptor as a Higher Order Function taking the res, and the original unmodified send. Finally, that returns a function which takes the content passed to res.send in the router.

const resDotSendInterceptor = (res, send) => (content) => {
 res.contentBody = content;
 res.send = send;
 res.send(content);
};

By the time the rouer has called res.send and the request finish event is broadcast, our interceptor has put the content passed to res.send in the router onto our res under the field contentBody making this now accessible in the finish callback.

const requestLoggerMiddleware = ({ logger }) => (req, res, next) => {
 ...
 res.on("finish", () => {
   logger("SEND >>>", res.contentBody);
 });
 next();
};

Conclusion

While the topic at first can seem quite confusing I highly recommend taking a look at the middleware and app discussed above.

Checkout the project on Github to see things running.