for(var i=0;i<li.length;i++){         li[i].onclick = function(){             for(var j=0; j<li.length; j++) {                 if (this == li[j]) {                     li[j].className = "on";                     div[j].className = "box active";                 }                 else {                     li[j].className = "";                     div[j].className = "box";                 }             }         }     }

this == li[j] 这怎么理解啊?