Helper function to inject a <script> tag

inject_script.js

// Helper function to inject a <script> tag.
function injectScript(url, onload, onerror) {
  var script = document.createElement("script");
  // onload fires even when script fails loads with an error.
  script.onload = onload;
  script.onerror = onerror || onload;
  script.src = url;
  document.head.appendChild(script);
}