auh_flow_step1.js

let redirectUrl = Expo.AuthSession.getRedirectUrl();
let response = await Expo.AuthSession.startAsync({
  authUrl:
  `https://accounts.google.com/o/oauth2/v2/auth?` +
  `&client_id=${GOOGLE_WEB_APP_ID}` +
  `&redirect_uri=${encodeURIComponent(redirectUrl)}` +
  `&response_type=code` +
  `&access_type=offline` +
  `&prompt=consent` +
 `&scope=${encodeURIComponent('https://www.googleapis.com/auth/plus.profile.emails.read https://www.googleapis.com/auth/calendar https://www.googleapis.com/auth/contacts')}`,
});

// response is like
// { type: "success",
//   errorCode: undefined,
//   url: "exp://localhost:19000/+expo-auth-session?code=4%2F…dVIg_D5axLSsDNjl9VIN_qTYcxH4ZQzEUGj_71OHxQo-9NLS8",
//   params: {
//     code: "4/AAAHF_zoV_I89Odu2qn5aX6ehHFZp1KKKMrTUhdVIg_D5axLSsDNjl9VIN_qTYcxH4ZQzEUGj_71OHxQo-9NLS8"
//   }
// }