req.body is undefined when use router.post() but visible when use app.post()

Question

I have the next app.js:

const express = require('express');
const bodyParser = require('body-parser');
const authRoutes = require('./routes/auth');
const app = express();

app.use(`$/api/auth`, authRoutes)

app.use(bodyParser.json())
app.use(bodyParser.urlencoded({extended: true}))

module.exports = app;

And routes/auth.js:

const express = require("express");
const router = express.Router();

router.post("/register",  (req, res) =>{
    res.status(200).json(req.body);
})

So, if I try to take req.body in auth.js, I get undefined, and if i pass any json instead of req.body, it will work fine.

Also if I'll move post to app.js, and it will look like this:

const express = require('express');
    const bodyParser = require('body-parser');
    const authRoutes = require('./routes/auth');
    const app = express();
    
    // app.use(`$/api/auth`, authRoutes)
    
    app.use(bodyParser.json())
    app.use(bodyParser.urlencoded({extended: true}))

    app.post("/api/auth/register", (req, res) =>{
        res.status(200).json(req.body)
    })
     
    module.exports = app;

it also will work fine. What don't I see?

Answer 1

Slebetman in comments gave the answer.

So as contents of app.use() are middleware they depends on an order in which it disposed. Therefore I should have put lines with bodyParser above routing middleware.