code: function ArrayIndex (_length) {
    Object.defineProperty(this, 'length', {
      get: getLength,
      set: setLength,
      enumerable: false,
      configurable: true
    });
  
    this[length] = 0;
  
    if (arguments.length > 0) {
      setLength.call(this, _length);
    }
  }
docs: Subclass this.