21 Bonfire Arguments Optional.js

/* Bonfire: Arguments Optional
Difficulty: 2/5

Create a function that sums two arguments together. If only one argument is provided, return a
function that expects one additional argument and will return the sum.

For example, add(2, 3) should return 5, and add(2) should return a function that is waiting for an
argument so that var sum2And = add(2); return sum2And(3); // 5

If either argument isn't a valid number, return undefined.

Remember to use RSAP if you get stuck. Try to pair program. Write your own code.

Here are some helpful links:

Global Function Object
Arguments object

Code by Rafael Rodriguez & https://github.com/mildused & https://github.com/ozydesign
rafase282@gmail.com
http://www.freecodecamp.com/rafase282
*/

function add() {
  // Function to check if a number is actually a number
  // and return undefined otherwise.
  var checkNum = function(num) {
    if (typeof num !== 'number') {
      return undefined;
    } else
      return num;
  };

  // Check if we have two parameters, check if they are numbers
  // handle the case where one is not
  // returns the addition.
  if (arguments.length > 1) {
    var a = checkNum(arguments[0]);
    var b = checkNum(arguments[1]);
    if (a === undefined || b === undefined) {
      return undefined;
    } else {
      return a + b;
    }
  } else {
    // If only one parameter was found, returns a new function that expects two
    // Store first argument before entering the new function scope
    var c = arguments[0];

    // Check the number again, must be outside the function to about returning an object
    // instead of undefined.
    if (checkNum(c)) {
      // Return function that expect a second argument.
      return function(arg2) {
        // Check for non-numbers
        if (c === undefined || checkNum(arg2) === undefined) {
          return undefined;
        } else {
          // if numbers then add them.
          return c + arg2;
        }
      };
    }
  }
}

add(2, 3);