freeCodeCamp Intermediate Algorithm Scripting: Arguments Optional

addTogether.js

// Create a function that sums two arguments together. If only 
// one argument is provided, then return a function that expects 
// one argument and returns the sum.
// For example, addTogether(2, 3) should return 5, and addTogether(2) 
// should return a function.
// Calling this returned function with a single argument will then
// return the sum:
// var sumTwoAnd = addTogether(2);
// sumTwoAnd(3) returns 5.
// If either argument isn't a valid number, return undefined.


function addTogether() {
  let returnValue;
  if (typeof arguments[0] === "number") {
    if (typeof arguments[1] === "undefined") {
      let firstArg = arguments[0];
//      var sumTwoAnd = addTogether(arguments[0]);
      return function(secondVal) {
        let returnValue2;
        if (typeof secondVal === "number") {
          returnValue2=firstArg + secondVal;
        }
        return returnValue2;
      }
     }
     else if (typeof arguments[1] === "number") {
      returnValue = arguments[0] + arguments[1]; 
    }
  }
  return returnValue;
}

addTogether(2,3);
addTogether(2)(3); // 5.
addTogether("http://bit.ly/IqT6zt"); // undefined.
addTogether(2, "3"); // undefined.
addTogether(2)([3]); // undefined.