devdating.net solution

devdating.js

return map.map(function(column_str) {
  var col = column_str.split(""),
      y = col.length;
      
  while(y >= 0) {
    if (col[y] == 'o' && col[y + 1] === '.') {
      col[y + 1] = col[y];
      col[y] = '.';
      y++;
    } else y--;
  }
  
  return col.join(""); 
});

i did something similar:

(function() {
    function dropStones(row) {
        for (i = row.length-2; i >= 0; i--) {
            if (row[i] === 'o') {
                var tmpI = i;
                while(row[tmpI + 1] === '.') {
                    row[tmpI] = '.';
                    row[++tmpI] = 'o';
                }
            }
        }
        return row.join("");
    }

    map.forEach(function(row, y) {
        map[y] = dropStones(row.split(''));
    });

    return map;
})();