JS实现简单计数器

<div class="body">
   <div class="text">
    <font>Counter</font>
   </div>
   <div class="count" >
    <span id="demo" class="ft">
     2
    </span>
   </div>
   <div class="btn">
    <button type="button" οnclick="add()" class="btn1">＋</button>
    <button type="button" οnclick="zero()" class="btn2">零</button>
    <button type="button" οnclick="sub()" class="btn1">－</button>

   </div>
</div>

 .body {
    width: 300px;
    height: 500px;
    background-color: #211d5a;
    border-radius: 20px;
    display: flex;
    flex-direction: column;
    align-items: center;
   }
   
   .text {
    font-size: 24px;
    color: white;
    margin-top: 60px;
    text-shadow: 2px 2px 2px #fff;
   }
   
   .count {
    position: relative;
    width: 200px;
    height: 200px;
    margin-top: 60px;
    border: 10px solid;
    border-color: rgb(79, 59, 156);
    border-radius: 50%;
    display: flex;
   }
   
   .ft {
    font-size: 100px;
    color: #fff;
    /*left: 50%;
                margin-left: -102px;
                margin-top: 40px;*/
    margin: auto;
   }
   
   .btn {
    width: 220px;
    display: flex;
    /*flex-direction: row;*/
    justify-content: space-between;
    margin-top: 60px;
   }
   
   .btn1 {
    width: 50px;
    height: 30px;
    border: 0px;
    border-radius: 4px;
    background-color: rgb(79, 59, 156);
    font-size: 20px;
    color: #efdaff;
   }
   
   .btn2 {
    width: 50px;
    height: 30px;
    border: 0px;
    border-radius: 4px;
    background-color: rgb(79, 59, 156);
    font-size: 22px;
    color: #efdaff;
   }

<script>
   var counter = document.getElementById("demo").innerHTML;
   function add() {
     counter++;
     document.getElementById("demo").innerHTML = counter;
   }

   function sub() {
    if(counter == 0) {
       counter = 0;
    } else {
     counter--;
        document.getElementById("demo").innerHTML = counter;
    }
   }

   function zero() {
    counter = 0;
       document.getElementById("demo").innerHTML = counter;
   }
</script>