gistfile1.js

function sum(n){
  arguments.callee.toString = function(){
    return sum;
  }
  if(!arguments.callee.sum){
    arguments.callee.sum=0
  }
  arguments.callee.sum += n
  var sum = arguments.callee.sum
  return arguments.callee
}

alert(sum(1)(2)(3)(4)(5));