javascript阻止表单提交

<form action="*" method="post" onSubmit="return check()">
</form>
<script>
function check(){
if(条件)
{return:false}
else{return:true}
}
</script> 

或者：

<form action="*" method="post" onSubmit=" check()">
</form>
<script>
function check(){
window.event.returnValue = false;
}
</script