googleSheets.js

injectScript = (src) => {
  return new Promise((resolve) => {
    var script = document.createElement('script');
    script.type = 'text/javascript';
    script.async = true;
    script.src = src;
    script.onload = () => resolve(window.gapi);
    document.body.appendChild(script);
  });
};
googleClientScriptSrc = 'https://apis.google.com/js/client.js';
promise = injectScript(googleClientScriptSrc);

promise.then((gapi) => {
  // Do stuff with `gapi`.
});