How to login with Google in Microsoft Word Add-In

In the link below it is suggested to use Google Auth Lib, but the github link provided does not refer to any javascript library:

• Authorize external services in your Office Add-in

As far as I know I cannot use the Code FLow as it needs a redirect URL. How should I get Access Token from Google in Microsoft Word Add-In?

In order to Login using Google, Facebook, etc you can use office-js-helpers:

Install it using npm:

npm install --save @microsoft/office-js-helpers

And in your code, inside of Office.initialize:

Office.initialize = function (reason) {
    //...
    // This to inform the Authenticator to automatically close the authentication dialog once the authentication is complete.
    if (OfficeHelpers.Authenticator.isAuthDialog()) return;

    // register Google endpoint using
    authenticator.endpoints.registerGoogleAuth('GOOGLE-CLIENT-ID');
    authenticator
         .authenticate(OfficeHelpers.DefaultEndpoints.Google)
         .then(function (token) { console.log('_GOOGLE_TOKEN: ', token); })
         .catch(OfficeHelpers.Utilities.log);
}

That's it!