Arguments Optional -- help needed

Tell us what’s happening:

Your code so far


function addTogether(a,...args) {
  if(typeof a != 'Number'){
  return ;}
  if(args.length == 1){
    if(typeof args[0] != 'Number')
    return ; 
    return a + args[0];
  }
else{
  let extra = function(test){
     if(typeof test == 'Number')
     return test+a;
  }
  return extra;
 }
}

console.log(addTogether(2)(3));

Your browser information:

User Agent is: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/71.0.3578.80 Safari/537.36.

Link to the challenge:
https://learn.freecodecamp.org/javascript-algorithms-and-data-structures/intermediate-algorithm-scripting/arguments-optional

Can anyone tell me what is wrong here ?
Thank You.