https://medium.com/@tomat/a-simple-trick-to-make-googlebot-tell-you-about-javascript-errors-2fd5d9d2e064

show-errors.js

(function() {
  var errorBoxElement = false;
  var showError = function(error) {
    if (!errorBoxElement) {
      errorBoxElement = document.createElement('div');
      errorBoxElement.style.position = 'absolute';
      errorBoxElement.style.top = '0';
      errorBoxElement.style.bottom = '0';
      errorBoxElement.style.left = '0';
      errorBoxElement.style.right = '0';
      errorBoxElement.style.background = 'rgba(255, 0, 0, 0.7)';
      errorBoxElement.style.zIndex = 1000000;
      errorBoxElement.style.color = '#fff';
      errorBoxElement.style.padding = '10px';
      document.body.appendChild(errorBoxElement);
    }
    var errorLine = document.createElement('div');
    errorLine.innerHTML = error;
    errorBoxElement.appendChild(errorLine);
  };
  window.onerror = function(errorMsg, url, lineNumber) {
    showError('Error: ' + errorMsg + ' Script: ' + url + ' Line: ' + lineNumber);
  }
})();