Array flatter using recursion.

app.js

function flatter(arr , res){
  if(!Array.isArray(arr)){
    return;
  }
  for(var elem in arr){
    if(Array.isArray(arr[elem])){
      flatter(arr[elem], res);
    }else {
      res.push(arr[elem]);
    }
  }
  return res;
}

var res = flatter([1,[2,3,[4,5,6,[7,8]],9],10], []);
console.log(res);