Vanilla JavaScript method for loading a script into your document

getScript.js

function getScript(url, success = () => {}, async = true, defer = false, attributes) {
  var debugMode = debugMode || Boolean(window.console && console && (
    Boolean(window.location.href.indexOf('localhost')+1) || 
    Boolean(window.location.href.indexOf('dev.')+1)
  )),
  script = document.createElement('script'),
  deferLoadFunc = function() {
    script.src = url;
    script.type = 'text/javascript';
    if (async) script.async = true;
    if (defer) script.defer = true;
    var head = document.getElementsByTagName('head')[0], done = false;

    script.onload = script.onreadystatechange = function() { // Attach handlers for all browsers
      if (!done && (!this.readyState || this.readyState == 'loaded' || this.readyState == 'complete')) {
        done = true;
        if (typeof success === 'function') success();
        script.onload = script.onreadystatechange = null;
        // if (debugMode) {
        //   console.log('Script fully loaded... removing now:');
        //   console.log(script);
        // }
        // document.body.removeChild(script);
      }
    };

    if (attributes) {
      for (k in attributes) {
        if (attributes.hasOwnProperty(k)) {
          script.setAttribute(k, attributes[k]);
        }
      }
    }

    if (debugMode) {
      console.log('Inserting script tag with async=' + async + ' and defer=' + defer + ':');
      console.log(script);
    }
    document.body.appendChild(script);
  }; // deferLoadFunc()

  if (defer) { // Load a call AFTER all page resources are loaded:
    if (window.addEventListener) window.addEventListener('load', deferLoadFunc, false);
    else if (window.attachEvent) window.attachEvent('onload', deferLoadFunc);
    else window.onload = deferLoadFunc;
  } else if (async) setTimeout(deferLoadFunc, 57); // <-- don't defer, but keep it non-blocking
  else deferLoadFunc(); // <-- some calls actually WANT to be loaded sequentially
} // getScript(url, success)

// SAMPLE INVOCATION:
// getScript('//code.jquery.com/jquery-3.4.1.min.js', null, false, false, {
//   'data-foo': 'bar'
// });

This script is lacking any detection or protection to ensure that the same script isn't loaded multiple times.
I think I've got some code somewhere that I can add to this routine to make it do that properly.