Adding custom middleware - node.js

Question

I have a route with a req.param of :groupIndex in it. I would like to process this index as middleware in order to get a specific id.
Node and express are new to me, so I'm possibly missing something simple but reading docs and looking at other implementations hasn't seemed to work.
Any idea where I might be going wrong?
// routes.js
const express = require("express");
const router = express.Router();
const customMiddleware = ('./customMiddleware.js');
const db = require('./mysqlCon.js');
router.get('/person/:groupIndex(\\d+)', customMiddleware(), function (req, res) {
let id = req.params.id;
let query = `
SELECT
*
FROM
data
WHERE
id = ?
`;
let query_params = [id];
db.query(
query,
query_params,
function(error, result, fields) {
if ( result.length == 1 ) {
res.status(200).json( result[0] );
} else {
res.status(401).json({});
}
}
);
});
// customMiddleware.js
const db = require('./mysqlCon.js');
module.exports = (req, res, next) => {
let groupIndex = parseInt(req.params.groupIndex);
let query = `
SELECT
id
FROM
listofids
LIMIT 1
OFFSET ?
`;
let query_params = [groupIndex];
db.query(
query,
query_params,
function(error, result, fields) {
if ( result.length == 1 ) {
req.params.id = result[0].id;
} else {
res.status(401).json({});
}
}
);
next();
}

Answer 1

I would review the middleware guide, but generally all middleware functions have the following signature:
function myMiddleware(req, res, next)
Where req, res, next are passed in from Express itself. You do not invoke the function, you simply pass it in as an argument (higher-order functions) to your route definition:
router.get('/person/:groupIndex(\\d+)', myMiddleware, ...)