client.js

const fs = require('fs');
const readline = require('readline');
const {google} = require('googleapis');

// If modifying these scopes, delete token.json.
const SCOPES = ['https://www.googleapis.com/auth/calendar.readonly'];
// The file token.json stores the user's access and refresh tokens, and is
// created automatically when the authorization flow completes for the first
// time.


// Authorize a client with credentials, then call the Google Calendar API.

module.exports = async function (credentialsPath, tokenPath, numberOfEvents) {
  const credentials = fs.readFileSync(credentialsPath);
  const client = await authorize(JSON.parse(credentials), tokenPath);
  const events = await listEvents(client, numberOfEvents);

  return events
}


/**
 * Create an OAuth2 client with the given credentials, and then execute the
 * given callback function.
 * @param {Object} credentials The authorization client credentials.
 * @param {function} callback The callback to call with the authorized client.
 */
async function authorize(credentials, token_path, callback) {
  const {client_secret, client_id, redirect_uris} = credentials.installed;
  const oAuth2Client = new google.auth.OAuth2(
      client_id, client_secret, redirect_uris[0]);

  try {
    const token = fs.readFileSync(token_path);
    oAuth2Client.setCredentials(JSON.parse(token));
    return oAuth2Client;
  } catch (e) {
    return getAccessToken(oAuth2Client);
  }

}

/**
 * Get and store new token after prompting for user authorization, and then
 * execute the given callback with the authorized OAuth2 client.
 * @param {google.auth.OAuth2} oAuth2Client The OAuth2 client to get token for.
 * @param {getEventsCallback} callback The callback for the authorized client.
 */
function getAccessToken(oAuth2Client) {
  return new Promise((resolve, reject) => {
    const authUrl = oAuth2Client.generateAuthUrl({
      access_type: 'offline',
      scope: SCOPES,
    });
    console.log('Authorize this app by visiting this url:', authUrl);
    const rl = readline.createInterface({
      input: process.stdin,
      output: process.stdout,
    });
    rl.question('Enter the code from that page here: ', (code) => {
      rl.close();
      oAuth2Client.getToken(code, (err, token) => {
        if (err) {
          reject(err);
        }

        oAuth2Client.setCredentials(token);
        // Store the token to disk for later program executions
        fs.writeFile(TOKEN_PATH, JSON.stringify(token), (err) => {
          if (err) {
            reject(err)
          }
          console.log('Token stored to', TOKEN_PATH);
        });
        resolve(oAuth2Client);
      });
    });
  });
}

/**
 * Lists the next n events on the user's primary calendar.
 * @param {google.auth.OAuth2} auth An authorized OAuth2 client.
 */
function listEvents(auth, numberOfEvents, calendarId = 'primary') {
  return new Promise((resolve, reject) => {
    const calendar = google.calendar({version: 'v3', auth});
    calendar.events.list({
      calendarId: 'primary',
      timeMin: (new Date()).toISOString(),
      maxResults: numberOfEvents,
      singleEvents: true,
      orderBy: 'startTime',
    }, (err, res) => {
      if (err) {
        reject(err)
      }
      const events = res.data.items;
      resolve(events);
    });
  });
}