chebox全选， 反选， 是否全选中

是否全选中的代码
$("input[name='checkbox']").click(function(){
var de = $("input[name='checkbox']");
var num=0;
[color=red]for(i = 0; i < de.length; i++){
if(de[i].checked == true){
num++;
}
}[/color]
if(num==de.length){
$("#checkboxAll").attr("checked",true);
}else{
$("#checkboxAll").attr("checked",false);
}

/*复选框取值*/
if(this.checked){
add_jstc(this.value,"id_input_jstc");
}else{
reduce_jstc(this.value,"id_input_jstc");
}


});


function add_jstc(value,id){
var s_jstc=document.getElementById(id).value;
if(s_jstc.indexOf(value+",")>=0){
return;
}
else{
document.getElementById(id).value=s_jstc+value+","
}
}

function reduce_jstc(value,id){
var s_jstc=document.getElementById(id).value;

if(s_jstc.indexOf(value+",")>=0){
s_jstc=s_jstc.replace(value+",","");

document.getElementById(id).value=s_jstc
return;
}
else{
return;

}
}