ReactJS. Webpack. Как проверить версию браузера до подключения библиотек к проекту?

/**
 * ReactElementValidator provides a wrapper around a element factory
 * which validates the props passed to the element. This is intended to be
 * used only in DEV and could be replaced by a static type checker for languages
 * that support it.
 */
 
{
  var currentlyValidatingElement = null;
 
  var propTypesMisspellWarningShown = false;
 
  var getDisplayName = function (element) {
    if (element == null) {
      return '#empty';
    } else if (typeof element === 'string' || typeof element === 'number') {
      return '#text';
    } else if (typeof element.type === 'string') {
      return element.type;
    } else if (element.type === REACT_FRAGMENT_TYPE) {
      return 'React.Fragment';
    } else {
      return element.type.displayName || element.type.name || 'Unknown';
    }
  };
 
  var getStackAddendum = function () {
    var stack = '';
    if (currentlyValidatingElement) {
      var name = getDisplayName(currentlyValidatingElement);
      var owner = currentlyValidatingElement._owner;
      stack += describeComponentFrame(name, currentlyValidatingElement._source, owner && getComponentName(owner));
    }
    stack += ReactDebugCurrentFrame.getStackAddendum() || '';
    return stack;
  };
 
 
  // ПРОБЛЕМА ТУТ!!!!!!!!!!!!!!!
  var VALID_FRAGMENT_PROPS = new Map([['children', true], ['key', true]]); // ПАДАЕТ ТУТ!
}

navigator.sayswho= (function(){
    var ua= navigator.userAgent, tem, 
    M= ua.match(/(opera|chrome|safari|firefox|msie|trident(?=\/))\/?\s*(\d+)/i) || [];
    if(/trident/i.test(M[1])){
        tem=  /\brv[ :]+(\d+)/g.exec(ua) || [];
        return 'IE '+(tem[1] || '');
    }
    if(M[1]=== 'Chrome'){
        tem= ua.match(/\b(OPR|Edge)\/(\d+)/);
        if(tem!= null) return tem.slice(1).join(' ').replace('OPR', 'Opera');
    }
    M= M[2]? [M[1], M[2]]: [navigator.appName, navigator.appVersion, '-?'];
    if((tem= ua.match(/version\/(\d+)/i))!= null) M.splice(1, 1, tem[1]);
    return M.join(' ');
})();
 
console.log(navigator.sayswho);