Use js to get SMS verification code countdown

html code part

	<form  action=" " method="post">
    	<input id="phone" name="phone" placeholder="手机号码" autofocus/>
    	<input id="code" placeholder="短信验证码"/>
    	<button id="btn>获取验证码</button>
        <input type="submit"  value="立即登录/注册"/><br>			       
    </form>

The js code is as follows

 var userphone = document.getElementById("phone");//获取手机号码标签
 var usercode = document.getElementById("code");获取验证码标签
 var btn = document.getElementById("btn");//获取验证码点击事件
 
   **//给btn添加一个点击事件**
 btn.onclick = function(){
     var timer = null;
    var num = 60;//60秒时间长度
    btn.disabled = true //防止在倒计时时第二次点击
   timer = setInterval(function(){  
      if(num<=0){
       clearInterval(timer);
       btn.innerHTML= "获取验证码";
       btn.disabled = " false";
      }else{
      num--;
      btn.innerHTML= num+"秒后重新发送";
  }
   },1000)//每隔一秒
**效果图**
![在这里插入图片描述](https://img-blog.csdnimg.cn/20201025130835589.png#pic_center)
![在这里插入图片描述](https://img-blog.csdnimg.cn/2020102513094737.png#pic_center)