JavaScript 实现点击复选框禁用提交按钮

<html>
<head>
<title>点击复选框按钮变为不可用</title>
</head>
<script>
var checkobj
function agreesubmit(el){
checkobj=el
if (document.all||document.getElementById){
for (i=0;i<checkobj.form.length;i++){ 
var tempobj=checkobj.form.elements[i]
if(tempobj.type.toLowerCase()=="submit")
tempobj.disabled=!checkobj.checked
}
}
}
function defaultagree(el){
if (!document.all && !document.getElementById){
if (window.checkobj && checkobj.checked)
return true
else{
alert("Please read/accept terms to submit form")
return false
}
}
}
</script>
<body>
<form name="agreeform" onSubmit="return defaultagree(this)">
<input name="agreecheck" type="checkbox" onClick="agreesubmit(this)"><b>你确认要提交？</b><br>
<input type="Submit" value="我一定要提交" disabled>
</form>
<script>
document.forms.agreeform.agreecheck.checked=false
</script>
</body>
</html>