Dynamically load script as a Promise

script.js

/**
 * @param {string} url
 * @param {boolean} [useCredentials=false]
 * @param {string} [integrity]
 * @return {Promise<void>}
 */
export function loadScript(url, useCredentials, integrity) {
  return new Promise((resolve, reject) => {
    const scripts = document.head.getElementsByTagName('script');
    for (let i = 0; i < scripts.length; i += 1) {
      if (scripts.item(i).src.toLowerCase() === url) {
        // already mounted
        resolve();
        return;
      }
    }
    const newScript = document.createElement('script');
    newScript.async = true;
    newScript.src = url;
    newScript.crossOrigin = useCredentials ? 'use-credentials' : 'anonymous';
    if (integrity) {
      newScript.integrity = integrity;
    }
    newScript.addEventListener('load', function fn() {
      resolve();
      newScript.removeEventListener('load', fn);
    });
    newScript.addEventListener('error', function fn(event) {
      reject(new Error(event.message));
      newScript.removeEventListener('error', fn);
    });
    document.head.appendChild(newScript);
  });
}