? var picImg = document.getElementById('pic').getElementsByTagName('img');? //圖片? var tabLi = document.getElementById('tab').getElementsByTagName('li'); ?? //按鈕? var index = 0; ?? picImg[0].style.display = 'block';? tabLi[0].className = 'on'; ? ??? for(var i=0;i<tabLi.length;i++){? ? tabLi[i].index = i; ?? //自定義屬性index??? tabLi[i].onclick = function(){????? picImg[index].style.display = 'none'; ??//每個(gè)li都有index自定義屬性? ?? ? ? tabLi[index].className = '';????? index = this.index;????? picImg[indexr].style.display = 'block';????? tabLi[index].className = 'on';??? }? }網(wǎng)上說(shuō)每個(gè)li都有index自定義屬性，我無(wú)法理解picImg[index].style.display = 'none';? tabLi[index].className = '';中index的含義，求兩行代碼具體解釋。