This instance has already started one or more requests. properties can only be modified before sending the first request.

Error Explanation:

The error message “this instance has already started one or more requests. properties can only be modified before sending the first request.” indicates that you are trying to modify certain properties of an instance (such as headers, data, or configuration settings) after sending the first request using that instance. In most cases, these properties can only be modified before the first request is sent.

Error Example:

Let’s consider an example using JavaScript and the Fetch API to make an HTTP request:

  const request = new Request('https://api.example.com/data', {
    method: 'POST',
    headers: {
      'Content-Type': 'application/json'
    },
    body: JSON.stringify({ name: 'John Doe' })
  });

  fetch(request)
    .then(response => response.json())
    .then(data => console.log(data))
    .catch(error => console.log(error));

  // Trying to modify the headers after the first request
  request.headers.append('Authorization', 'Bearer your-token');
  

In the above example, we create a new Request object with a POST method, some headers, and a request body containing a JSON payload. Then, we use the Fetch API to send the request to the server. However, if we try to modify the headers of the existing request (request.headers.append) after the fetch request is already sent, it will result in the mentioned error.

To fix this error, ensure that you modify the properties of the request object before sending the fetch request. In the above example, you can update the headers before creating the Request object or modify the headers before sending the fetch request.