getElementsByTagName() 兼容性

window.onload = function () {
    var div = getClassNames('lg' , 'div');
    function getClassNames(classStr,tagName){
        if (document.getElementsByClassName) {
            return document.getElementsByClassName(classStr)
        }else {
            var nodes = document.getElementsByTagName(tagName),ret = [];
            for(i = 0; i < nodes.length; i++) {
                if(hasClass(nodes[i],classStr)){
                    ret.push(nodes[i])
                }
            }
            return ret;
        }
    }
    function hasClass(tagStr,classStr){
        var arr=tagStr.className.split(/\s+/ ); //這個正則表達式是由於class能夠有多個,判斷是否包含
        for (var i=0;i<arr.length;i++){
            if (arr[i]==classStr){
                return true ;
            }
        }
        return false ;
    };
//下邊寫你須要的代碼

var div = getClassNames('lg' , 'div');

</script>