input输入密码变黑点密文的实现方法

<form id="login-form" method="post" onsubmit="return checkForm()">
 输入密码<input type="password" id="input-password">
 <input type="hidden" id="md5-password" name="password">
 <button type="submit">Submit</button>
</form>

function checkForm() {
 var input_pwd = document.getElementById('input-password');
 var md5_pwd = document.getElementById('md5-password');
 // 把用户输入的明文变为MD5:
 md5_pwd.value = toMD5(input_pwd.value);
 // 继续下一步:
 return true;
}