handle_response.js

class ErrorResponseServiceNotAvailable extends Response {}
class ErrorResponseBackendFailed extends Response {}
class ErrorResponseValidation extends Response {}
class SuccessResponse extends Response {}

// the function

function handleResponse(response) {
  if (response.status === 200) {
    return new SuccessResponse(response);
  }
  if (response.status === 400) {
    return new ErrorResponseValidation(response);
  }
  if (response.status > 400) {
    return new ErrorResponseBackendFailed(response);
  }
  if (response.status >= 500) {
    throw new ErrorResponseServiceNotAvailable(response);
  }
  return response;
}

// run!

fetch('...').then(handleResponse).then(response => {
  if (response instanceof SuccessResponse) {
    // everithing is ok
  }
  if (response instanceof ErrorResponseValidation) {
    // show validation errors
  }
  if (response instanceof ErrorResponseValidation) {
    // something PREDICTABLE goes wrong
  }
  if (response instanceof ErrorResponseServiceNotAvailable) {
    // something UNPREDICTABLE goes wrong
  }
});