Client-side js helper function for POSTing

post.js

      function post(url, data, successCallback, errorCallback, responseType) {
        fetch(url, {
          method: 'POST',
          headers: {'Content-Type': 'application/x-www-form-urlencoded'},
          body: new URLSearchParams(data),
        })
          .then(response => {
            if (!response.ok) { throw new Error(response.status); }
            const contentType = response.headers.get('content-type');
            const isJson = responseType === 'json' || (responseType === 'auto' && contentType && contentType.includes('application/json'))
            return isJson ? response.json() : response.text();
          })
          .then(result => { successCallback(result);})
          .catch(error => { errorCallback(parseInt(error.message, 10) || 0)})
      }