// create OAuth2 client
const oauth2Client = new OAuth2(
    OAUTH_CLIENT_ID,
    OAUTH_CLIENT_SECRET,
    'https://developers.google.com/oauthplayground'
);

// set refresh token
oauth2Client.setCredentials({
    refresh_token: OAUTH_REFRESH_TOKEN
});

// get access token using promise
const accessToken = oauth2Client.getAccessToken()