Why does undefined come up when sending a PUT request?

Question

Node.js

Good afternoon! I send a fetch request from the client with this body: body: JSON.stringify({"id": "2"}). I process this put request from the server side, but undefined comes to the request body, what is the problem?

Серверная часть:
router.put('/', async (req, res) => {
    const {id} = req.body;
    res.send(console.log(id))
});
Клиентская часть:
fetch('http://localhost:5000/api', {
            method: 'PUT',
            body: JSON.stringify({
                "id": "2"
            })
        })

Answer 1

The problem seems to be that the middleware is asynchronous, and there is no notification about the end of its execution (it is not called next()).
Option 1. Either remove asyncit - judging by the synchronous operation of the content, it is not needed here:

router.put('/', (req, res) => {
    const {id} = req.body;
    console.log(id);
    res.send(id);
});

Option 2. If you still plan some asynchronous work inside the middleware, then use next() to announce the end of the execution of the asynchronous middleware

router.put('/', async (req, res, next) => {
    const {id} = req.body;
    console.log(id);
    res.send(id);
    next();
});

UPD1: In the client side, when accessing /api, the "content-type" header corresponding to the passed body is not passed.
It should be like this:

fetch('http://localhost:5000/api', {
            method: 'PUT',
            headers: {
              'Content-Type': 'application/json',
            },
            body: JSON.stringify({
                "id": "2"
            })
        })

UPD2: as Igor Makhov rightly noted - you hardly need console.log in res.send.

Answer 2

Are you kidding? Why do you res.sendneed console.log?