google-signin.ts

import { enableProdMode } from '@angular/core';
import { platformBrowserDynamic } from '@angular/platform-browser-dynamic';

import { AppModule } from './app/app.module';
import { environment } from './environments/environment';

if (environment.production) {
  enableProdMode();
}
const gapi = window['gapi'];
const SCOPE = 'email profile https://www.googleapis.com/auth/drive.metadata.readonly';
if (gapi) {
  new Promise((resolve, reject) => {
    gapi.load('client:auth2', {
      callback: () => {
        console.log('client:auth2 loaded');
        gapi.client.init({
          clientId: '719627788647-ifgm8v5sghj4u6a1us2b10lplbq3pfe6.apps.googleusercontent.com',
          scope: SCOPE
        }).then(() => {
          if (gapi.auth2.getAuthInstance().isSignedIn.get()) {
            gapi.client.load('drive', 'v3', () => {
              gapi.client.drive.files.list({
                pageSize: 10,
                fields: 'nextPageToken, files(id, name)'
              }).then((r) => {
                console.log(r);
                resolve();
              })
            })
          } else {
            console.log('nop! not signed in at all');
            gapi.auth2.getAuthInstance().signIn({
              scope: SCOPE,
              ux_mode: 'redirect',
              redirect_uri: 'http://localhost:4200'
            }).then(() => console.log('Yay! now properly signed in'), () => console.log('Nop! not signed in yet'));
          }
        })
      }, onerror: () => {
        throw new Error('Error while loading client:auth2');
      }
    })
  }).then(() => {
    platformBrowserDynamic().bootstrapModule(AppModule)
      .catch(err => console.log(err));
  })
} else {
  throw new Error('gapi not ready yet');
}

Using Drive api just to ensure it is properly loading user data