document.getElementsByClassName的理想实现

function getElementsByClassName(searchClass, node,tag) { 
        if(document.getElementsByClassName){ 
          var nodes = (node || document).getElementsByClassName(searchClass),result = nodes;
if(tag!=undefined){
result = [];
for(var i=0 ;node = nodes[i++];){
if(tag !== "*" && node.tagName === tag.toUpperCase()){
result.push(node);
}else{
result.push(node);
}
}
}
return result;
        }else{ 
            node = node || document; 
            tag = tag || "*"; 
            var classes = searchClass.split(" "), 
            elements = (tag === "*" && node.all)? node.all : node.getElementsByTagName(tag), 
            patterns = [], 
            returnElements = [], 
            current, 
            match; 
            var i = classes.length; 
            while(--i >= 0){ 
                patterns.push(new RegExp("(^|\\s)" + classes[i] + "(\\s|$)")); 
            } 
            var j = elements.length; 
            while(--j >= 0){ 
                current = elements[j]; 
                match = false; 
                for(var k=0, kl=patterns.length; k<kl; k++){ 
                    match = patterns[k].test(current.className); 
                    if (!match)  break; 
                } 
                if (match)  returnElements.push(current);  
            } 
            return returnElements; 
        } 
    };