Unable to return selected data angularjs

I created a service named ProductStorage to retrieve data from sqlite database using angularjs.It selecting data properly and get result in console when i call it like ProductStorage.product(1), but it return {} always.

.factory('ProductStorage', function() {
return {
   product:function(product_id){
      var product  = {};
      db.transaction(function (tx) {
            tx.executeSql("SELECT * FROM products WHERE product_id = "+product_id,[],function(tx, res){
              product = angular.fromJson(res.rows.item(0).productdata);   
              console.log(product);  // shows result in console   
            });
         });
      return product; // not returning product ,always return {}
   }
}
})