average-dom-depth.js

function getAverage (arr) {
  return arr.reduce(function (prev, cur) {
    return prev + cur;
  }) / arr.length;
}

function getNestedLevel (el) {
  var level = 0;
  while (el = el.parentNode) {
    level++;
  }
  return level;
}

function getAverageDomDepth () {
  var all = [].slice.call(document.querySelectorAll('*'));
  var levels = all.map(function (el) {
    return getNestedLevel(el);
  });
  return getAverage(levels);
}