Express: why middlewares doesn't work properly for independent routers?

Question

I have the following code with 3 independent routers

const Express = require("express")

const app = Express()

const usersRouter = Express.Router()
const productsRouter = Express.Router()
const storeRouter = Express.Router()

productsRouter.use((_, res, next) => {
  res.send("products fail")
  //next()
})

storeRouter.use((_, res, next) => {
  res.send("store fail")
  //next()
})

usersRouter.route("/users")
  .get((_, res) => res.send("users"))

productsRouter.route("/products")
  .get((_, res) => res.send("products"))

storeRouter.route("/store")
  .get((_, res) => res.send("store"))

app.use(usersRouter)
app.use(productsRouter)
app.use(storeRouter)

app.listen(80, () => console.log("running"))

But every time I request /store route it pass through productRouter middleware which is assigned only to it.

I can't understand this behavior.

Why is this? How can I manage independent middlewares for each one?

GET /store 200
products fail

Expected

GET /store 200
store fail

Answer 1

When you do this:

app.use(productsRouter)

that sends ALL requests to the productsRouter and thus its middleware runs for all requests. So, when you have this:

productsRouter.use((_, res, next) => {
  res.send("products fail")
  //next()
});

That will run on every single request.

If you want the router to only see certain requests, then register the router on a path instead so the router only gets requests destined for a certain path.

app.use("/products", productsRouter)

And, then remove the path itself from the router's routes since the path will have already been filtered.