var sum = add(1,2); // 3
console.log(sum);
function add(x, y) {
    x = +x;
  y = +y;
    if(isNaN(x) || isNaN(y)) {
      return;
  }
  return x+y;
}

var sum = add(1,2); //Uncaught TypeError: add is not a function
console.log(sum);
var add = function (x, y) {
    x = +x;
  y = +y;
    if(isNaN(x) || isNaN(y)) {
      return;
  }
  return x+y;
}