code: function josu (num) {
    // 10-19, 110-119, 210-219...
    if (parseInt(num % 100 / 10 , 10) === 1) {
      return num + 'th';
    }
  
    switch(num % 10) {
    case 1:
      return num + 'st';
    case 2:
      return num + 'nd';
    case 3:
      return num + 'rd';
    default:
      return num + 'th';
    }
  }
docs: 序数
  @method josu
  @param  {Number} num
  @return {String} suji