JavaScript 实现网页头部加载进度条

<div id="barbg">
   <div id="bar">
   </div>
  </div>

 <style>
   *{margin:0;padding:0;}
   #barbg{height:5px; background:rgb(149,143,143)}
   #barbg div{width:0; height:5px; position:relative; background:rgb(230,10,10);}
  </style>

 <script>
   document.onreadystatechange = function(){
    var bar = document.getElementById('bar');
    var barbg = bar.parentNode;
    var wd = document.body.scrollWidth || document.documentElement.scrollWidth;
    var t = 10;
    var d = 0;
    var i = 0;
    var timer = setInterval(goto,10);
    function goto(){
     d = d + t ;
     bar.style.width = d + 'px';
     if(d >= wd){
      clearInterval(timer);
      bar.style.background = 'rgba(230,10,10,0)';
      none();
     }
    }
    function none(){
     var a = 10 - i;
     i++;
     if(a != 0 && a != 10){a = a * 0.1}
     if(a === 10){a = 1}
     if(a === 0){a = 0}
     barbg.style.background = 'rgba(230,10,10,' + a + ')';
     if(a === 0){barbg.style.display = 'none'}
     if(a != 0){setTimeout(none,50);}
    }
   }
  </script>