自己动手实现getElementsByClassName()行为

摘录一个Js实现getElementsByClassName()的程序：

function getElementsByClassName(className){  
    var elements = [];  

    if(!document.getElementsByClassName){  
        var domNodes = document.getElementsByTagName("*");  
        for(var i =0 ;i<domNodes.length;i++){  
            if(domNodes[i].className){  
                var classNames = domNodes[i].className.split(" "); /*这里其实还要考虑类名间隔大于一个空格的情况*/  
                for(var j = 0;j<classNames.length;j++){  
                    if(classNames[j]==className){  
                        elements.push(domNodes[i]);              
                    }  
                }  
            }  
        }  
    }else{  
　　　　 /*这里既然浏览器支持这个行为，为何不直接return document.getElementsByClassName(className) ？？？*/
        var domNodes = document.getElementsByClassName(className);  
        for(var i =0 ;i<domNodes.length;i++){  
            elements.push(domNodes[i]);      
        }  
    }  
    return elements;  
}