checkbox实现全选的多种方法

<script language=javascript>    
//第一种方法    
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;    
}    
//第四种方法    
function selectall4(id){ //用id区分    
var tform=document.forms['form1'];    
for(var i=0;i<tform.length;i++){    
var e=tform.elements[i];    
if(e.type=="checkbox" && e.name==id) e.checked=!e.checked;    
}    
}    
//第五种方法    
function selectall(theform,thename){ //theform指定的form,thename是checkbox的name属性    
var tform=document.forms[theform];    
document.getElementById("thewen").value='反选';    
for(var i=0;i<tform.length;i++){    
var e=tform.elements[i];    
if(e.type=='checkbox' && e.name==thename)e.checked=!e.checked;    
}    
}    
</script>    
<form id="form1" name="form1" method="post" action="">    
<input type="checkbox" name="sid" value="1" />    
<input name="thes" type="button" onclick="javascript:selectall3()" value="全选" />    
</form>