gapi-tools.js

function gapiLoad(s) {
  return new Promise(r => {
    gapi.load(s, () => r(gapi[s]));
  });
}

function getClient(s, v) {
  if (!v) v = 'v1';
  return gapiLoad('client').then(client => {
    client.load(s, v).then(function() {
      return gapi.client[s];
    });
  });
}