How to return a value in a JS asynchronous callback function - gapi

Wrap the load in a Promise so that you can await it like the rest of your code.

try {
  await new Promise((resolve,reject) => {
    gapi.load('client:auth2', resolve);
  });
  await authenticate();
  await loadYoutubeApi();
} catch (error) {
  throw Error(`Error initializing gapi client: ${error}`);
}
//is Initialized

You can wrap gapi.load part in a promise like this:

const initialize = async () => {
  const { gapi } = window;
  await new Promise((resolve, reject) => {
    gapi.load('client:auth2', async () => {
      try {
        await authenticate();
        await loadYoutubeApi();
        resolve();
      } catch (error) {
        throw Error(`Error initializing gapi client: ${error}`);
      }
    });
  });
  return true;
};

initialize(); // returns 'true' when done.