AJAX post request gives value undefined

I am using javascript, AJAX and Node js and I cannot get a value on server side when I do a post request. I have the following to test it and it says value undefined.

The app.js file

app.use(express.json());

app.post("/admin-usercategories/:id/edit", async function (req, res) {
    try {
        console.log('Title - ' + req.body.title);

        res.json({message: 'Insertion completed!'})

    } catch (error) {
        res.status(500).json(error);
    }
})

In my javascript file:

async function saveUsercategories(displayElement, userId) {

    const comment = { title: "HELLO", text: "How are you?"};

    await fetch(`/admin-usercategories/${userId}/edit`, {
        method: 'POST',
        body: JSON.stringify(comment)
    });            
}

See the "Uploading JSON data" example from MDN.

You haven't set a Content-Type request header, so fetch will default (based on you passing a string to body) to telling the server that you are sending text/plain which doesn't trigger the JSON body parsing middleware.

await fetch(`/admin-usercategories/${userId}/edit`, {
    method: 'POST',
    body: JSON.stringify(comment),
    headers: {
        "Content-Type": "application/json"
    }
});