how to create custom function and use them in mongodb

mongoCustomFunction.js

// use admin database
use admin

// create function
db.system.js.save(
   {
     _id: "testEchoFunc",
     value : function(x) { return x; }
   }
)

// load functions
db.loadServerScripts();

// run function
db.runCommand( {
    eval: function(){return testEchoFunc(65)}
})