/*
* 根據(jù)元素clsssName得到元素集合
* @param fatherId 父元素的ID，默認為document
* @tagName 子元素的標簽名
* @className 用空格分開的className字符串
*/
function getElementsByClassName(fatherId,tagName,className){
node = fatherId&&document.getElementById(fatherId) || document;
tagName = tagName || "*";
className = className.split(" ");
var classNameLength = className.length;
for(var i=0,j=classNameLength;i<j;i++){
//創(chuàng)建匹配類名的正則
className[i]= new RegExp("(^|//s)" + className[i].replace(//-/g, "http://-") + "(//s|$)");
}
var elements = node.getElementsByTagName(tagName);
var result = [];
for(var i=0,j=elements.length,k=0;i<j;i++){//緩存length屬性
var element = elements[i];
while(className[k++].test(element.className)){//優(yōu)化循環(huán)
if(k === classNameLength){
result[result.length] = element;
break;
}
}
k = 0;
}
return result;
}

<div id="container">
<span class="aaa zzz ccc"></span>
<div class="aaa bbb ccc"></div>
</div>
<div class="aaa bbb ccc"></div>

window.onload = function(){
alert(getElementsByClassName(document,"div","aaa ccc").length);//2
alert(getElementsByClassName("container","div","aaa ccc").length);//1
alert(getElementsByClassName("container","span","aaa zzz").length);//1
}

//方式一
var arr = [];
var start = new Date();
for(var i=0;i<100000;i++){
arr.push(i);
}
//方式二
var arr = [];
var start = new Date();
for(var i=0;i<100000;i++){
arr[arr.length]=i;
}