Use updated req variable in middleware after route execution - express

Question

I have a middleware function I wrote to perform some logging for each HTTP request that comes in
export const winstonMiddlewareLogger = (req: express.Request, res: express.Response, next: express.NextFunction) => {
let _startTime = new Date();
res.on("finish", function() {
let responseTime = new Date().getTime() - _startTime.getTime() + "ms";
let method = req.method;
let url = req.originalUrl;
let body = req.body ? req.body : {};
let query = req.query;
let params = req.params;
let status = res.statusCode;
let msg = `HTTP ${req.method} ${req.url} ${responseTime}`;
let logDocument = {
msg,
method,
url,
params,
query,
body,
status,
responseTime,
};
logger.info(undefined, logDocument)
});
next();
};
Next thing I have a route
app.post("/slow", (req, res) => {
req.testParam = "test";
res.send("hello");
});
I want to be able to access the new property of the req parameter that is initiated in the /slow (testParam), in the res.on(...){...} event listener in my middleware function the next way:
if (req.hasOwnProperty('testParam')) {
console.log(req.testParam)
}
But the current state is that the req parameter is not updated no matter what I do in the route itself, because the req parameter it knows is only the one I get at the beginning of each request.
Changing the req variable in the route doesn't change it in the middleware which has already got a req variable of its own.
Any idea how to do it?

Answer 1

Ok so I found winston-express which actually does the things I wanted to.
What it does is to reassign the res.end function to a function he wrote himself.
This function can access the modified objects of req and res after the route has already executed.
So for the sake of showing how it works, I change my route to:
app.get("/", (req, res) => {
req._attributeFromRoute = "test";
res.send("hello");
});
And my middleware function looks something like this:
export const winstonMiddlewareLogger2 = (req: express.Request, res: express.Response, next: express.NextFunction) => {
let _startTime = new Date();
let end = res.end;
res.end = (chunk: any, encoding: string, cb?: () => void): void => {
res.end = end;
res.end(chunk, encoding, cb);
console.log(req._attributeFromRoute) // Notice that this property was not declared before the route execution, and now we can access it after it was defined
let responseTime = new Date().getTime() - _startTime.getTime() + "ms";
let method = req.method;
let url = req.originalUrl;
let body = req.body ? req.body : {};
let query = req.query;
let params = req.params;
let status = res.statusCode;
let msg = `HTTP ${req.method} ${req.url} ${responseTime}`;
let logDocument = {
msg,
method,
url,
params,
query,
body,
status,
responseTime,
};
logger.info(undefined, logDocument)
};
next();
};
And the way it will work is that the middleware function will wait for the route to execute and call the req.end function, which will enter the end function we created ourselfs.
We will then execute the original end function that had as a reference in the end variable.
Then we can do whatever we want with the updated objects of req and res.