Detect when all requests initiated by scripts have completed. Requires Performance API

getCount.js

(function(P) {
  var timer = setInterval(function() {
    var requestCount = P.getEntriesByType("resources").filter(res => res.initiatorType === "script" && res.responseEnd === 0).length;

    if (requestCount === 0) {
      // all requests are loaded
      // begin next step here...
      clearInterval(timer);
    }
  }, 16 * 4); // interval set to approx. 100ms
})(window.performance);