[Vlasenko Fedor]

document.onclick = function (e) {
	var info = document.getElementById("buttom-purchase-info");
	var but = document.getElementById("buttom-purchase");
	var el = e ? e.target : window.event.srcElement;
	switch (el) {
		case but:
			!info.className & (info.className = "active");
			break;
		case info:
			break;
		default:
			info.className = "";
	}
};

http://learn.javascript.ru/play/SPOkgb