Array#toString を少し見やすく。ただしコードは醜い。

Array#toString.js

Object.defineProperty(Array.prototype, 'toString', {
  value: (function () {
    var start = '[';
    var end = ']';
    var delimiter = ', '

    function toString() {
      var index = 0;
      var len = this.length;
      var result = '';

      result += start;

      for (; index < len; index++) {
        result += this[index];

        if (index < len - 1) {
          result += delimiter;
        }
      }

      result += end;

      return result;
    };

    return toString;
  })(),

  writable: true,
  enumerable: false,
  configurable: true
});