这个form表单return check(this.form)的this指向哪里？

Question

<form action="login.do?act=login" method="post">用户帐号<input type=text name="userId" size="18" value="" >登录密码 <input type="password" name="password" size="19" value=""/> <input type=submit name="submit1" value="登陆" onclick="return check(this.form)"></form>