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>