js读复选框的值

< SCRIPT LANGUAGE ="JavaScript" >
<!--
function getValue(){
    var o = document.getElementById( ' checkBoxId ' ).getElementsByTagName( " input " );
    var value = "" ;
    for ( var i = 0 ; i < o.length ; i ++ ){
        if (o[i].checked == true )
            value += o[i].value;
    }
    alert(value);
}
// -->
</ SCRIPT >
< BODY >
    < span id ="checkBoxId" >
    < input type ="checkbox" value ="1" /> 1 < br >
    < input type ="checkbox" value ="2" /> 2 < br >
    </ span >
    < input type ="button" value ="getValue" onclick ="getValue()" />
</ BODY >