googleAuth.js

const { AuthenticationService, JWTStrategy } = require('@feathersjs/authentication');
const { LocalStrategy } = require('@feathersjs/authentication-local');
const { expressOauth, OAuthStrategy } = require('@feathersjs/authentication-oauth');

class GoogleStrategy extends OAuthStrategy {

  // https://github.com/feathersjs/feathers/blob/be91206e3dba1e65a81412b7aa636bece3ab4aa2/packages/authentication-oauth/src/strategy.ts#L113-L119
  async createEntity(profile, params) {
    // check if the user exists and has access
    if (youShouldLetHimIn) {
      // access your services via this.app.service('service-name')
      return this.app.service('var').create(someData);
    } else {
      // decide what to do
      if (someDecision) {
        return super.createEntity(profile, params);
      } else {
        return {}; // not sure if this should be an error or just an empty object
      }
    }
  }

  // https://github.com/feathersjs/feathers/blob/be91206e3dba1e65a81412b7aa636bece3ab4aa2/packages/authentication-oauth/src/strategy.ts#L121-L128
  async updateEntity(entity, profile, params) {
    // check if the user exists and has access
    if (youShouldLetHimIn) {
      // access your services via this.app.service('service-name')
      return this.app.service('var').patch(someId, someData);
    } else {
      // decide what to do
      if (someDecision) {
        return super.createEntity(profile, params);
      } else {
        return {}; // not sure if this should be an error or just an empty object
      }
    }
  }

  async getEntityData(profile, entity, params) {
    const baseData = await super.getEntityData(profile);
    let { email, picture} = profile;
    let data = {
      ...baseData,
      profilePicture: picture,
      email
    };
    return data;
  }
}

module.exports = app => {
  const authentication = new AuthenticationService(app);

  authentication.register('jwt', new JWTStrategy());
  authentication.register('local', new LocalStrategy());
  authentication.register('google', new GoogleStrategy()); // added!

  app.use('/authentication', authentication);
  app.configure(expressOauth());
};