title: reactjs fetch - body: json.stringify
body: <p>My Json Object looks like this:</p>
  
  <pre><code>[
  {
      "keys": {
          "_1": ".",
          "_2": ".",
          "_3": "."
      },
      "credits": -200,
      "_id": "5c1e3e7531dccf0017cf70b9",
      "firstName": "Martin",
      "lastName": "Seubert",
      "username": "admin",
      "createdDate": "2018-12-22T13:39:01.244Z",
      "__v": 0,
      "id": "5c1e3e7531dccf0017cf70b9"
  }
  ]
  </code></pre>
  
  <p>An I want following function to set the keys to a value, I am fetching from another API:</p>
  
  <pre><code>updateKeys() {
    let user = JSON.parse(localStorage.getItem('user'));
    var requestUrl = 'https://mighty-atoll-75521.herokuapp.com/users/';
    var id = user._id
    fetch(requestUrl + id, {
        method: 'put',
        headers: {
          ...authHeader(),
          'Accept': 'application/json, text/plain, */*',
          'Content-Type': 'application/json'
        },
        body: JSON.stringify({keys._1:  this.props.code})
      })
  }
  </code></pre>
  
  <p>But i get this Error Message in the browser:</p>
  
  <p><a href="https://i.stack.imgur.com/vihF2.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/vihF2.png" alt="enter image description here"></a></p>
  
  <p>Have you any suggestions for me?</p>
  
  <p>Best regards</p>
  
  <p>Martin</p>
  
  <p><strong>EDIT:</strong></p>
  
  <p>Here you can find the full code:
  <a href="https://github.com/kamami/prototype/blob/RealDatabase/src/components/ProgressMobileStepper.jsx" rel="nofollow noreferrer">https://github.com/kamami/prototype/blob/RealDatabase/src/components/ProgressMobileStepper.jsx</a></p>