arguments-optional.txt

function addTogether(...args) {
  
  let length = args.length;

  let isValid = args.every(ele => typeof ele === 'number');

  if(length > 1 && isValid) {
    let sum = args.reduce((sum, next) => sum+next);
    return sum;
  }

  else if(length===1 && isValid) {
      return function sumUp(num) {
        if(typeof num === 'number') {
          return parseInt(args) + parseInt(num);
        } else {
          return undefined;
        }    
    }
  }

}



//addTogether("http://bit.ly/IqT6zt") //should return undefined.
//addTogether(2)([3])
//addTogether(2, "3");
console.log(addTogether(2)(3));