Snippet to get access tokens from Google Identity API, given a refresh token.

google.js

const buildAccessTokenUri = (refreshToken, clientId, clientSecret) =>
  `https://www.googleapis.com/oauth2/v4/token?client_id=${clientId}&client_secret=${clientSecret}&refresh_token=${refreshToken}&grant_type=refresh_token`;
 
const getAccessToken = async (refreshToken, clientId, clientSecret) => {
  accessTokenUri = buildAccessTokenUri(refreshToken, clientId, clientSecret);
 
  const rawResponse = await fetch(accessTokenUri, { method: 'POST' });
  const jsonResponse = await rawResponse.json();
 
  return jsonResponse.access_token;
};
 
/*
 * Main Function
 */
(async() => {
  const refreshToken = 'REFRESH TOKEN';
  const clientId = 'CLIENT ID';
  const clientSecret = 'CLIENT SECRET';
  const accessToken = await getAccessToken(refreshToken, clientId, clientSecret);
  console.log(`Access token: ${accessToken}`);
})();