html5 效果按下鼠标数值自动增长

<!doctype html>

<html>
<head>
<style>
* {
    margin:0;
    padding:0;
}
div {
    margin:10px auto;
    width:300px;
    text-align:center;
}
meter {
    border:1px solid red;
    width:300px;
}
</style>
</head>

<body>
<div>
  <meter id="song" min="0" max="100" high="70" value="10"></meter>
  <button id="add">增加</button>
  <button id="minus">减少</button>
  <div id="num">10</div>
</div>
<script>

function $(id)
{
    return document.getElementById(id);
}

$('add').onclick = function(){
    
    var val = $('song').value;
    val ++;
    $('song').value = val;
    
    $('num').innerHTML = val;
    
}

var timer = null;
var start_timer = null;

$('add').onmousedown = function(){
    
    //延迟两秒
    start_timer = setTimeout(function(){
        timer = setInterval(function(){
            
            var val = $('song').value;
            val += 1;
            $('song').value = val;
            
            $('num').innerHTML = val;
            
        },150)
        
    },500)
}

$('add').onmouseup = function(){
    //清除启动器
    clearTimeout(start_timer);
    
    //清除间隔启动器
    clearInterval(timer);
}

</script>
</body>
</html>
效果图：
　　 html5 效果按下鼠标数值自动增长