Traversing the Mesh Object3D fails

I won’t bother the github people – I want to traverse the Mesh Object3D object. The routine I have uses Object.getPrototypeOf to follow the prototype chain. It fails with “Uncaught TypeError”. The following is a simplified routine to go up the chain. It fails. It probably involves “toJSON” somehow. TIA.

<!doctype html>
    <title> D-C </title>
    <script src=""></script>
  "use strict";
  var logdiv, level=0;
window.onload = function() {
  logdiv = document.getElementById("logdiv");
  var geometry = new THREE.PlaneGeometry(2,3);
  var material = new THREE.MeshBasicMaterial();
  var mesh = new THREE.Mesh(geometry, material);
  doit(1, mesh); return;
function doit(level, obj) {
  z_log("level "+level+", obj type is " + typeof obj); // print level and obj type
  z_log("level "+level, JSON.stringify(obj));          // print JSON.., but error
  if (obj == null) return;                             // recurse until obj == null
  doit(level+1, Object.getPrototypeOf(obj));           // recurse
function z_log(args) { // like console.log
  var s = "";
  for (var i=0; i<arguments.length; i+=1) {
    s += ((i>0) ? ", " : "") + arguments[i];
  logdiv.innerHTML += s + "<br /><br />\n";
<div id="logdiv"></div>