sdad.js

/**
 * Your solution
 * @date JS Date object
 **/
function solution(date){
  date = new Date(Date.parse(date));
 
  return summate(date.getDate().toString() + (date.getMonth()+1).toString() + date.getFullYear().toString());
}
 
function summate(str_sum) {
  str_sum = str_sum.toString().split('');
  console.log(str_sum);
  
  var sum = 0;
  for(var i=0; i < str_sum.length; i++) {
    sum += str_sum[i];
    if(sum >= 10) {
     sum = summate(sum);
    }
  }
  return sum;
}