Edit in JSFiddle

var ________________ = [] + [];
var _ = +[];
_++;
var _____ = _ + _;
var ___ = _____ + _____;
var __ = ___ + ___;
var ____ = __ + __;
var ______ = ____ + ____;
var _______ = ______ + _;
var ___________ = ______ + ______ + __;
var ______________ = ___________ + ____ - _;
var ____________ = _ + _____;
var ________ = _______ * ____________ + _;
var _________ = ________ + _;
var _____________ = ______________ + ______ - ___ - _;
var __________ = _____________ -
  ____________;
var _______________ = __________ - ____________;
document.write(________________ +
  String.fromCharCode(___________, _________, _______________, _______________, __________,
    ______, ______________, __________, _____________, _______________, ________, _______));