HTML60 秒倒计时

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>60秒倒计时</title>
</head>
<body>
<input type="button" id="btn" value="获取验证码" onclick="sendemail()" />
</body>
</html>
<script src="https://code.jquery.com/jquery-3.2.1.min.js"></script>
<script>
var countdown = 5
function sendemail() {
var obj = $("#btn")
settime(obj)
console.log('发送邮件')
}
function settime(obj) {
if (countdown == 0) {
obj.attr('disabled', false)
obj.val("获取验证码")
countdown = 5
return
} else {
obj.attr('disabled', true)
obj.val("重新发送(" + countdown + ")")
countdown--
}
setTimeout(function() {
settime(obj)
}, 1000)
}
</script>