Express Middleware acting on every router after being added - node.js

Question

I'm having a problem when adding middleware in express.Router().
let router = express.Router();
let mid = function(req, res, next) {
console.log("mid");
next();
}
router.get("/", function(req, res) {
res.send("public");
})
let userRouter = express.Router();
userRouter.use(mid);
userRouter.get("/users", function(req,res) {
res.send("users");
})
router.use(userRouter);
app.use("/", router);
router = express.Router();
router.get("/", function(req, res) {
res.send("api");
})
app.use("/api", router);
and here is the result:
accessing "/" don't triggers the middleware.
accessing "/users" triggers the middleware.
accessing "/api" triggers the middleware.
I don't get why "/api" is triggering the middleware.

Answer 1

Check the Express documentation for router middleware.
The Express API documentation might be useful too.
I organized your code a little bit, and now it is working as you wanted.
'use strict';
let express = require('express');
let expressApp = express();
// --- middleware function ---
let usersMiddleware = function(req, res, next) {
console.log("users router middleware");
next();
}
// --- middleware function ---
let appMiddleware = function(req, res, next) {
console.log("app middleware");
next();
}
// --- public router ---
let publicRouter = express.Router();
publicRouter.get("/", function(req, res) {
res.send("public");
});
// --- user router ---
let userRouter = express.Router();
// --- this is a router based middleware,
// --- to be used only for this router ---
userRouter.use(usersMiddleware);
userRouter.get("/", function(req,res) {
res.send("users");
});
// --- api router ---
let apiRouter = express.Router();
apiRouter.get("/", function(req, res) {
res.send("api");
});
// --- this is a app based middleware,
// --- it will be used for every route of the app after this line
expressApp.use(appMiddleware);
// --- adding the routers to the app ---
expressApp.use("/", publicRouter);
expressApp.use("/users", userRouter);
expressApp.use("/api", apiRouter);
expressApp.listen(8080);
Note that there are two kinds of middleware used here, the one attached to an specific router (that's the one you wanted), and one attached to the app, used for all the further routes (I added this one to show the difference).
/api was triggering the middleware because you added it to the app and not to the router instance.