兼容IE的getElementsByClassName()方法

兼容IE8及以下的getElementsByClassName()方法

    function getElementsByClassName = function(className, tagName) {
        //如果浏览器支持getElementsByClassName()就直接返回
        if(document.getElementsByClassName){
            return document.getElementsByClassName(className)
        }
        //否则遍历document文档指定的标签名集合
        var children = document.getElementsByTagName(tagName);
        //此数组用于保存遍历后得到的class元素
        var elements = new Array();
     
        for (var i = 0; i < children.length; i++) {
            var child = children[i];
            var classNames = child.className.split(' ');            //分割多个class元素
            for (var j = 0; j < classNames.length; j++) {
                if (classNames[j] == className) {
                    elements.push(child);
                    break;
                }
            }
        }
        return elements;
    };