<!doctype html><html><head> <meta charset="UTF-8"><title>Document</title></head><body><ul id="list"><li>1</li><li>2</li><li>3</li><li>4</li></ul><p>class为on的p标签</p></body></html><script type="text/javascript"> window.onload=function(){ var len1= getByClass('on','list'); alert(len1.length); // 结果等于2为正确var len2= getByClass('on'); alert(len1.length); // 结果等于3为正确}function getByClass(clsName, parent){ //定义函数getByClass()实现获取document或指定父元素下所有class为on的元素 var arr = []; if(parent){ var oParent = document.getElementById(parent); var oElements = oParent.getElementsByTagName('*'); for(var i in oElements){ if(oElements[i].className==clsName){ arr.push(oElements[i]); } } }else{ var oElements = document.getElementsByTagName('*'); for(var j in oElements){ if(oElements[j].className==clsName){ arr.push(oElements[i]); } } } return arr;}</script>
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