A special system collection named system.js, can store JavaScript functions for reuse. Made use of this feature to implement the auto-increment feature found in traditional rdbms.

mongo_js_function_loader.js

# To store a function use the db.collection.save() method

db.system.js.save(
   {
     _id: "foo_bar",
     value : function(x, y) { return x * y; }
   }
)

db.system.js.save(
   {
     _id : "bar_buzz" ,
     value : function (x, y){ return x * y; }
   }
);

# Fire up the mongo shell
db.loadServerScripts();

foo_bar(9, 8);