loadJS.js

function loadJS(url) {
  return new Promise((resolve, reject) => {
    const element = document.createElement('script');
    element.async = true;
    element.src = url;

    element.onload = () => {
      resolve(url);
    }

    element.onerror = () => {
      reject(url);
    }

    const script = document.getElementsByTagName('script')[0];
    script.parentNode.insertBefore(element, script);
  });
}

usage.md

loadJS('http://code.jquery.com/jquery-3.3.1.min.js')
        .then(() => {
          // Successfully
          // do something here
        })
        .catch(() => {
          // Fail
          // do something here
        });