Fetch with polyfill and error wrapper

Fetch

import 'whatwg-fetch'; //polyfill

 window.fetch(Routing.generate('booking_data'), { //window.fetch -> polyfill
      method: 'post',
      headers: {
          'Content-Type': 'application/json',
          'Accept': 'application/json'
      },
      body: JSON.stringify({
          checkin: event.start.toDateString(),
          checkout: addDays(event.start, 7).toDateString()
      })
  })
      .then(checkStatus)
      .then(response => response.json())
      .catch(error => console.log(error));
      
function checkStatus(response) {
    if (response.status >= 200 && response.status < 300) {
        return response
    } else {
        let error = new Error(response.statusText);
        error.response = response;
        throw error;
    }
}