static-cms/packages/netlify-cms-lib-util/src/loadScript.js

/**
 * Simple script loader that returns a promise.
 */
export default function loadScript(url) {
  return new Promise((resolve, reject) => {
    let done = false;
    const head = document.getElementsByTagName('head')[0];
    const script = document.createElement('script');
    script.src = url;
    script.onload = script.onreadystatechange = function() {
      if (
        !done &&
        (!this.readyState || this.readyState === 'loaded' || this.readyState === 'complete')
      ) {
        done = true;
        resolve();
      } else {
        reject();
      }
    };
    script.onerror = error => reject(error);
    head.appendChild(script);
  });
}