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];
  let extra = function(test){
     if(typeof test == 'Number')
     return test+a;
  return extra;


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:

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