JavaScriptでPad

pad.js

String.prototype.pad = function() {
  String.PAD_LEFT = 'STRING_PAD_LEFT';
  String.PAD_RIGHT = 'STRING_PAD_RIGHT';
  var default_pad_string = ' ';
  var default_pad_type = String.PAD_RIGHT;
  return function (length, pad_string, pad_type) {
    var result = this.toString();
    pad_string = pad_string == undefined ? default_pad_string : pad_string.toString();
    pad_type = pad_type || default_pad_type;
    if (pad_type == String.PAD_LEFT) {
      while (result.length < length) {
        result = pad_string + result;
      }
      return result.substr(result.length - length, length);
    } else {
      while (result.length < length) {
        result += pad_string; 
      }
      return result.substr(0, length);
    }
  };
}();

Number.prototype.pad = function(length) {
  return this.toString().pad(length, 0, String.PAD_LEFT);
};

// 'おっぱい'.pad(10, 'おっぱい') => 'おっぱいおっぱいおっ'

// (7).pad(3) => '007'