JavaScript获取元素偏移量offset的原理与实现

//例如
    document.parentNode//浏览器天生自带的一个属性：父亲节点的属性 null (因为一个页面中的document已经是最顶级元素了，它没有父亲)
    document.parentnode//undefined (因为没有parentnode这个属性)

var outer = document.getElementById('outer');
    var inner = document.getElementById('inner');
    var center = document.getElementById('center');

    center.parentNode //inner

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Document</title>
  <style>
    *{
      margin:0;
      padding:0;
    }
    #outer{
      width:180px;
      height:180px;
      margin:50px auto;
      border:10px solid #000;
      background:orange;
      padding:50px;
    }
    #inner{
      width:80px;
      height:80px;
      padding:50px;
      border:10px solid #000;
      background:green;
    }
    #center{
      width:50px;
      height:50px;
      border:10px solid #000;
      background:red;
    }
  </style>
</head>
<body>
  <div id="outer">
    <div id="inner">
      <div id="center"></div>
    </div>
  </div>

  <script>
    var outer = document.getElementById('outer');
    var inner = document.getElementById('inner');
    var center = document.getElementById('center');

    outer.style.position = "relative";//这样inner和center的参照物都是outer
    center.offsetParent//outer
    inner.offsetParent//outer
    outer.offsetParent//body
    outer.style.position = "relative";//
    inner.style.position = "relative";
    center.offsetParent//inner
    inner.offsetParent//outer
    outer.offsetParent//body
  </script>
</body>
</html>

function offset(curEle){
      var totalLeft = null,totalTop = null,par = curEle.offsetParent;
      //首先加自己本身的左偏移和上偏移
      totalLeft+=curEle.offsetLeft;
      totalTop+=curEle.offsetTop
      //只要没有找到body，我们就把父级参照物的边框和偏移也进行累加
      while(par){
        if(navigator.userAgent.indexOf("MSIE 8.0")===-1){
          //累加父级参照物的边框
          totalLeft+=par.clientLeft;
          totalTop+=par.clientTop
        }
        
        //累加父级参照物本身的偏移
        totalLeft+=par.offsetLeft;
        totalTop+=par.offsetTop
        par = par.offsetParent;
      }

      return{
        left:totalLeft,
        top:totalTop
      }
    }
    console.log(offset(center).left)