POST method sends clear string. REACT -> API

Question

I have component for SQL queries and when I want to send any query to API-server backend crash down because MySQL see empty string like a my request. My backend is working good because when I test it from postman.exe everything is fine.
My component with submit is on localhost:3000 and api on localhost:5000.
Here is my sending method:

submitData= () =>{
        const { queryFromEditor } = this.props;

        let queryFromEditorString = queryFromEditor.join(' ');
        console.log(queryFromEditorString);

       let xhr = new XMLHttpRequest();
        xhr.open("POST", 'http://localhost:5000/api/query', true);
        xhr.setRequestHeader('Content-Type', 'application/json');
        xhr.send(JSON.stringify({
            "value": queryFromEditorString,
        }));

    }

Here is my post method

app.post('/api/query', (req, res) => {
  console.log(`\n  Query input : ${JSON.stringify(req.body)}`);
  let queryInput = JSON.stringify(Object.values(req.body));

  queryInput = queryInput.slice(1,-1);// Opravit, aby to nevypadalo tak blbě
  queryInput = queryInput.slice(1,-1);

  let queryOutput;
  db.query(queryInput, (err, result) =>{
    if(err) throw err+' : '+queryInput;

    queryOutput = result;
    console.log("\nQuery output "+ JSON.stringify(queryOutput));
    res.json(queryOutput);
  });
});

Answer 1

Problem was on server.
Easy fix: add "app.use(express.json());"