http://ja.wikibooks.org/wiki/JavaScript_If%E6%96%87%E3%81%AE%E5%BE%A9%E7%BF%92%E3%81%A8%E3%80%81%E4%B8%89%E9%A0%85%E6%BC%94%E7%AE%97%E5%AD%90