javascript语句问题：this['className']?this['className']='nav-swit

javascript语句问题：this['className']?this['className']='nav_switch' :this['class']='nav_switch';
this['className']?this['className']='nav_switch' :this['class']='nav_switch';这句是什么意思啊?this['className']='nav_switch' 这一句就可以实现功能,为什么要加上各判断,这句 this['class']='nav_switch'到底有什么用?