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function LodashFactory($window) {  
  if(!$window._){
    // If lodash is not available you can now provide a
    // mock service, try to load it from somewhere else,
    // redirect the user to a dedicated error page, ...
  }
  return $window._;
}

// Define dependencies
LodashFactory.$inject = ['$window'];

// Register factory
yourModule.factory('_', LodashFactory);