<script type="text/javascript">
var getElementsByClassName = function (searchClass, node,tag) 
{ 
    if(document.getElementsByClassName){ 
        return  document.getElementsByClassName(searchClass) 
    }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; 
    } 
}
function test()
{
	alert(getElementsByClassName('test')[0].value);
}
</script>

<input class='test' id='test_id' type='text' name='test' value='test'>
<input type="button" value=" get " onclick="test()">