[ОлегА]

function my_split(separator, str){
  var exp = {};
  arr = str.split(separator)

  if( arr.length > 1 ){
    first_val = arr[0];
    str = str.substr(first_val.length+separator.length);
    exp[ first_val ] = my_split(separator, str)
    return exp;
  } else {
    return str;
  }
}

str = 'a.b.c.d';

res = my_split('.', str);