non-recursive sum of three numbers

  function sumIter() {
  var sum = 0;
  for (var i = 0; i < arguments.length; i++) {
    sum = sum + arguments[i]
  }
  return sum;
}

sumIter(3,4,5);   // 12

recursive:

  function sumRecur () {
  var args = [].slice.call(arguments);
  if (args.length <= 2) {
    return args[0] + args[1];
  }
  return (
    args[0] + sumRecur.apply(null,args.sice(1))
  );
}

sumRecur(3,4,5);    // 12

(in ES6 this can be express more pretty)

  function sumRecur(...args) {
  if (args.length <= 2) {
    return args[0] + args[1];
  }
  return (
    args[0] + sumRecur(...args.slice(1))
  );
}

sumRecur(3,4,5);    // 12

the magic is in ​...​ rest operator. Works in arguments as well (to call another function).Note: arguments are what you pass into a function, parameters are what the function receives.