Call Express router manually - node.js

Question

Нello! I am looking to call a function which has been passed to an expressRouter.post(...) call.
This expressRouter.post(...) call is occurring in a file which I am unable to modify. The code has already been distributed to many clients and there is no procedure for me to modify their versions of the file. While I have no ability to update this file for remote clients, other developers are able to. I therefore face the issue of this POST endpoint's behaviour changing in the future.
I am also dealing with performance concerns. This POST endpoint expects req.body to be a parsed JSON object, and that JSON object can be excessively large.
My goal is to write a GET endpoint which internally activates this POST endpoint. The GET endpoint will need to call the POST endpoint with a very large JSON value, which has had URL query params inserted into it. The GET's functionality should always mirror the POST's functionality, including if the POST's functionality is updated in the future. For this reason I cannot copy/paste the POST's logic. Note also that the JSON format will never change.
I understand that the issue of calling an expressjs endpoint internally has conventionally been solved by either 1) extracting the router function into an accessible scope, or 2) generating an HTTP request to localhost.
Unfortunately in my case neither of these options are viable:
I can't move the function into an accessible scope as I can't modify the source, nor can I copy-paste the function as the original version may change
Avoiding the HTTP request is a high priority due to performance considerations. The HTTP request will require serializing+deserializing an excessively large JSON body, re-visiting a number of authentication middlewares (which require waiting for further HTTP requests + database queries to complete), etc
Here is my (contrived) POST endpoint:
expressRouter.post('/my/post/endpoint', (req, res) => {
if (!req.body.hasOwnProperty('val'))
return res.status(400).send('Missing "val"');
return res.status(200).send(`Your val: ${req.body.val}`);
});
If I make a POST request to localhost:<port>/my/post/endpoint I get the expected error or response based on whether I included "val" in the JSON body.
Now, I want to have exactly the same functionality available, but via GET, and with "val" supplied in the URL instead of in any JSON body. I have attempted the following:
expressRouter.get('/my/get/endpoint/:val', (req, res) => {
// Make it seem as if "val" occurred inside the JSON body
let fakeReq = {
body: {
val: req.params.val
}
};
// Now call the POST endpoint
// Pass the fake request, and the real response
// This should enable the POST endpoint to write data to the
// response, and it will seem like THIS endpoint wrote to the
// response.
manuallyCallExpressEndpoint(expressRouter, 'POST', '/my/post/endpoint', fakeReq, res);
});
Unfortunately I don't know how to implement manuallyCallExpressEndpoint.
Is there a solution to this problem which excludes both extracting the function into an accessible scope, and generating an HTTP request?

Answer 1

This seems possible, but it may make more sense to modify req and pass it, rather than create a whole new fakeReq object. The thing which enables this looks to be the router.handle(req, res, next) function. I'm not sure this is the smartest way to go about this, but it will certainly avoid the large overhead of a separate http request!
app.get('/my/get/endpoint/:val', (req, res) => {
// Modify `req`, don't create a whole new `fakeReq`
req.body = {
val: req.params.val
};
manuallyCallExpressEndpoint(app, 'POST', '/my/post/endpoint', req, res);
});
let manuallyCallExpressEndpoint = (router, method, url, req, res) => {
req.method = method;
req.url = url;
router.handle(req, res, () => {});
};

Answer 2

How about a simple middleware?
function checkVal(req, res, next) {
const val = req.params.val || req.body.val
if (!val) {
return res.status(400).send('Missing "val"');
}
return res.status(200).send(`Your val: ${val}`);
}
app.get('/my/get/endpoint/:val', checkVal)
app.post('/my/post/endpoint', checkVal)
This code isn't tested but gives you rough idea on how you can have the same code run in both places.
The checkVal function serves as a Express handler, with request, response and next. It checks for params first then the body.