code: function tryApplyUpdates(onHotUpdateSuccess) {
    if (!module.hot) {
      // HotModuleReplacementPlugin is not in Webpack configuration.
      window.location.reload();
      return;
    }
  
    if (!isUpdateAvailable() || !canApplyUpdates()) {
      return;
    }
  
    function handleApplyUpdates(err, updatedModules) {
      if (err || !updatedModules) {
        window.location.reload();
        return;
      }
  
      if (typeof onHotUpdateSuccess === 'function') {
        // Maybe we want to do something.
        onHotUpdateSuccess();
      }
  
      if (isUpdateAvailable()) {
        // While we were updating, there was a new update! Do it again.
        tryApplyUpdates();
      }
    }
  
    // https://webpack.github.io/docs/hot-module-replacement.html#check
    var result = module.hot.check(/* autoApply */true, handleApplyUpdates);
  
    // // Webpack 2 returns a Promise instead of invoking a callback
    if (result && result.then) {
      result.then(
        function(updatedModules) {
          handleApplyUpdates(null, updatedModules);
        },
        function(err) {
          handleApplyUpdates(err, null);
        }
      );
    }
  }
docs: Attempt to update code on the fly, fall back to a hard reload.