how to capitalize a letter based on an index in javascript

function capitalize(s,arr){
  s = s.split("");
  for(var i = 0; i < arr.length; i++){
    if (s[arr[i]] == null) { continue }
    s[arr[i]]=s[arr[i]].toUpperCase();  
  }
  s = s.join('');
  return s;
};