http://www.freecodecamp.com/minsooshin 's solution for Bonfire: Arguments Optional

bonfire-arguments-optional.js

// Bonfire: Arguments Optional
// Author: @minsooshin
// Challenge: http://www.freecodecamp.com/challenges/bonfire-arguments-optional
// Learn to Code at Free Code Camp (www.freecodecamp.com)

function add() {
  var args = Array.prototype.slice.call(arguments),
      numberType = true;
  
  args.forEach(function(val) {
    if (typeof val !== 'number') {
      numberType = false;
      return;
    }
  });
  
  if (numberType) {
    if (args.length === 2) {
      return args[0] + args[1];
    } else {
      return function() {
        if (typeof arguments[0] !== 'number') return;
        else {
          return args[0] + arguments[0];
        }
      };
    }
  } else {
    return;
  }
}

add("http://bit.ly/IqT6zt"); //=> undefined
add(2, 3);                   //=> 5
add(2)(3);                   //=> 5
add(2, "3");                 //=> undefined