from where we get the access token and refresh token and how to set the access token and refresh token oauth2Client.setCredentials in google calendar via nodejs

Access tokens and refresh tokens are typically obtained through the OAuth 2.0 authentication process. In order to obtain these tokens for the Google Calendar API, you will need to follow these steps:

1. Create a new project in the Google Cloud Console and enable the Google Calendar API.
2. Set up OAuth 2.0 credentials for your project, including a client ID and client secret. These credentials will be used to authenticate your application and obtain access to the user's Google Calendar data.
3. Use the Google API client library for Node.js to obtain an access token and refresh token. This is typically done by calling the authenticate() method on an OAuth2Client instance.
4. Once you have obtained the access token and refresh token, you can set them on your OAuth2Client instance using the setCredentials() method.

Here's an example of how to obtain and set the access token and refresh token in a Node.js application:

const {google} = require('googleapis');
const readline = require('readline');
const {OAuth2Client} = require('google-auth-library');

const SCOPES = ['https://www.googleapis.com/auth/calendar.readonly'];
const TOKEN_PATH = 'token.json';

// Load client secrets from a local file.
fs.readFile('credentials.json', (err, content) => {
  if (err) {
    console.log('Error loading client secret file:', err);
    return;
  }

  // Authorize a client with the loaded credentials.
  authorize(JSON.parse(content), listEvents);
});

function authorize(credentials, callback) {
  const {client_secret, client_id, redirect_uris} = credentials.installed;
  const oAuth2Client = new google.auth.OAuth2(
      client_id, client_secret, redirect_uris[0]);

  // Check if we have previously stored a token.
  fs.readFile(TOKEN_PATH, (err, token) => {
    if (err) return getAccessToken(oAuth2Client, callback);
    oAuth2Client.setCredentials(JSON.parse(token));
    callback(oAuth2Client);
  });
}

function getAccessToken(oAuth2Client, callback) {
  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) return console.error('Error retrieving access token', err);
      oAuth2Client.setCredentials(token);
      // Store the token to disk for later program executions
      fs.writeFile(TOKEN_PATH, JSON.stringify(token), (err) => {
        if (err) return console.error(err);
        console.log('Token stored to', TOKEN_PATH);
      });
      callback(oAuth2Client);
    });
  });
}

function listEvents(auth) {
  const calendar = google.calendar({version: 'v3', auth});
  calendar.events.list({
    calendarId: 'primary',
    timeMin: (new Date()).toISOString(),
    maxResults: 10,
    singleEvents: true,
    orderBy: 'startTime',
  }, (err, res) => {
    if (err) return console.log('The API returned an error: ' + err);
    const events = res.data.items;
    if (events.length) {
      console.log('Upcoming 10 events:');
      events.map((event, i) => {
        const start = event.start.dateTime || event.start.date;
        console.log(`${start} - ${event.summary}`);
      });
    } else {
      console.log('No upcoming events found.');
    }
  });
}

In this example, the authorize() function is used to obtain an OAuth2Client instance with the appropriate credentials. If a token is already stored on disk, it is loaded and set on the OAuth2Client instance using the setCredentials() method. If a token is not found, the getAccessToken() function is called to initiate the OAuth 2.0 flow and obtain a new token. Once a token is obtained, it is stored to disk and set on the OAuth2Client instance using the setCredentials() method.

You can adapt this example to suit your own needs, such as by modifying the SCOPES array to include the permissions your application requires, or by calling the setCredentials() method directly with your own access token and refresh token.