JS---案例：滚动条

DOCTYPE html>
html>

head lang="en">
  meta charset="UTF-8">
  title>title>
  style>
    * {
      margin: 0;
      padding: 0;
    }

    .box {
      width: 300px;
      height: 500px;
      border: 1px solid red;
      margin: 100px;
      position: relative;
      overflow: hidden;
    }

    .content {
      padding: 5px 18px 5px 5px;
      position: absolute;
      top: 0;
      left: 0;

    }

    .scroll {
      width: 18px;
      height: 100%;
      position: absolute;
      top: 0;
      right: 0;
      background-color: #eee;
    }

    .bar {
      height: 100px;
      width: 100%;
      position: absolute;
      top: 0;
      left: 0;
      background-color: red;
      border-radius: 10px;
      cursor: pointer;
    }
  style>
head>

body>
  div class="box" id="box">
    div class="content" id="content">
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    div>
    
    div id="scroll" class="scroll">
      
      div class="bar" id="bar">div>
      
    div>
  div>
  script src="common.js">script>
  script>

    //获取需要的元素

    //最外面的div
    var box = my$("box");
    //文字div
    var content = my$("content");
    //装滚动条的div---容器
    var scroll = my$("scroll");
    //滚动条
    var bar = my$("bar");

    //设置滚动条的高度
    //滚动条的高/装滚动条div的高=box的高/文字div的高
    //滚动条的高=装滚动条div的高*box的高/文字div的高
    var height = scroll.offsetHeight * box.offsetHeight / content.offsetHeight;
    bar.style.height = height + "px";

    //移动滚动条
    bar.onmousedown = function (e) {
      var spaceY = e.clientY - bar.offsetTop;
      document.onmousemove = function (e) {
        var y = e.clientY - spaceY;
        y = y  0 ? 0 : y;
        y = y > scroll.offsetHeight - bar.offsetHeight ? scroll.offsetHeight - bar.offsetHeight : y;
        bar.style.top = y + "px";

        //设置鼠标移动的似乎和，文字不被选中
        window.getSelection ? window.getSelection().removeAllRanges() : document.selection.empty();
        //滚动条的移动距离/文字的div移动距离=滚动条最大的移动距离/文字div的最大移动距离
        var moveY = y * (content.offsetHeight - box.offsetHeight) / (scroll.offsetHeight - bar.offsetHeight);
        //设置位子div的移动距离
        content.style.marginTop = -moveY + "px";
      };
    };


  script>

body>

html>