js淡出效果 javascript怎么实现不停淡入淡出

<h1 id="text">床前明月光，疑是地上霜。</h1>
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var duration = 3000;
var startTime = new Date();
var p = 0;
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  var el = document.querySelector("#text");

  var time = ( new Date - startTime ) / duration;
  el.style.opacity = Math.sin(2 * Math.PI * time);
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});