Overriding a JavaScript function while referencing the original

function a()
{
  new_code();
  original_a();
}

function a()
{
  original_a();
  other_new_code();
}

var a = (function() {
    var original_a = a;

    if (condition) {
        return function() {
            new_code();
            original_a();
        }
    }
    else {
        return function() {
            original_a();
            other_new_code();
        }
    }
})();

//Saving the original func

var org_foo = window.foo;

//Assigning proxy fucnc

window.foo = function(args){

   //Performing checks

   if(checkCondition(args)){

     //Calling original funcs

     org_foo(args);
   }
};

if (condition) { new_code(); original_a(); }
     else { original_a(); other_new_code(); }
});