checkbox全选JS方法

 function selectall1()
{
    var a = document.getElementsByTagName("input");
    if(a[0].checked==true){
    for (var i=0; i<a.length; i++)
       if (a[i].type == "checkbox") a[i].checked = false;
    }
    else
    {
    for (var i=0; i<a.length; i++)
       if (a[i].type == "checkbox") a[i].checked = true;
    }
}
//第二种方法
function selectall2() {
var tform = document.forms['form1'];
for (var i=0;i<tform.length;i++)
{
   var e = tform.elements[i];
   if (e.type == "checkbox")
    e.checked = !e.checked;
}
}
//第三种方法，结合上述两种方法
function selectall3()
{
    var a = document.getElementsByTagName("input");
    for (var i=0; i<a.length; i++)
       if (a[i].type == "checkbox") a[i].checked =!a[i].checked;
   }

 

 

 

<form id="form1" name="form1" method="post" action="">
   <input type="checkbox" name="sid" value="1" />
<input type="checkbox" name="sid" value="2" />
<input type="checkbox" name="sid" value="3" />
<input type="checkbox" name="sid" value="4" />
   <input name="thes" type="button" onclick="javascript:selectall1()" value="第一种" />
   <input name="thes" type="button" onclick="javascript:selectall2()" value="第二种" />
   <input name="thes" type="button" onclick="javascript:selectall3()" value="第三种" />
</form>