Handle succcess and error responses for fetch calls, including scenarios where network error won't return a response body. Attempt to squeeze out all necessary info from success as well as failures.

fetch.js

fetch('/api/sendMessage', {
    method: 'POST',
    body: JSON.stringify(bodyObj),
    headers: new Headers({
      'Content-Type': 'application/json',
    }),
  })
  .then((res) => this.handlePushNotificationResponse(res))
  .catch((res) => this.handlePushNotificationResponse(res));  

handlePushNotificationResponse(res) {
    if (!res) {
      console.error('Error occurred while trying to read the fetch response');
      this.showError(res);
    } else if(!res.ok) {
      this.showError(res);
    } else {
      this.showSuccess(res);
    }
  }

  showSuccess(res) {
    this.setState({hasResults: true, isLoading: false, results: 'success'});
    this.logSuccessResponse(res);
  }

  logSuccessResponse(res) {
    const statusText = `Push notification call succeeded:: Fetch statusText: ${res && res.statusText}`;
    if (res && res.json) {
      res.json().then(body => console.info(`${statusText}, Response body:`, body))
      .catch(() => console.info(`${statusText}, No further info available as api error response body could not be parsed.`));
    } else {
      console.info(statusText);
    }
  }

  showError(res) {
    this.setState({hasErrors: true, isLoading: false, errors: 'error'});
    this.logErrorResponse(res);
  }

  logErrorResponse(res) {
    const statusText = `Push notification call failed:: Fetch statusText: ${res && res.statusText}`;
    if (res && res.json) {
      res.json().then(err => console.error(`${statusText}, `, err))
      .catch(() => console.error(`${statusText}, No further info available as api error response body could not be parsed.`));
    } else {
      console.error(statusText);
    }
  }