<!DOCTYPE html>
<head>
  <script>
    function go() {
      var path = document.getElementById("myPath");
      var len = path.getTotalLength();
      console.log(len);
    }
  </script>
</head>
<body onload="go()">
  <svg>
    <path id="myPath" d="M45,-17592186044414A71,23 46,0,0 16382,98"/>
  </svg>
</body>