text: // This Javascript module contains methods for using Google Sheets.
  //
  // Use this module as follows:
  //
  //     const {google} = require('googleapis');
  //     const sheets = require('./googleSheets');
  //
  //     function checkIntoEvent() {
  //       // auth (google.auth.OAuth2): The authenticated Google OAuth client
  //       api(function(auth) {
  //         const sheets = google.sheets({version: 'v4', auth});
  //         sheets.spreadsheets.values.get({
  //           spreadsheetId: '16baCd9Qd9Q1kJSUGqGzhCuoMFKRxNwO6EJ2lertzDJo',
  //           range: 'Class Data!A2:E',
  //         }, (err, res) => {
  //           if (err) return console.log('The API returned an error: ' + err);
  //           // Access/modify data
  //         });
  //       });
  //     }
  //
  //     sheets.api(checkIntoEvent);
  
  const fs = require('fs');
  const readline = require('readline');
  const {google} = require('googleapis');
  const {GoogleAuth, JWT, OAuth2Client} = require('google-auth-library');
  
  // The file credentials.json stores the downloaded API credentials from
  // Google Cloud Platform.
  const CREDENTIALS_FILE = 'credentials.json';
  
  // Upon modification, delete the file token.json
  const SCOPES = ['https://www.googleapis.com/auth/spreadsheets'];
  
  // 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.
  const TOKEN_PATH = 'token.json';
  
  // LOCAL FUNCTIONS
  
  /**
   * 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 getNewToken(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 while trying to retrieve 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 hasEnvCredentials() {
    return process.env.GOOGLE_CLIENT_EMAIL && process.env.GOOGLE_PRIVATE_KEY;
  }
  
  /**
   * 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.
   */
  function authorizeLocalCredentials(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 getNewToken(oAuth2Client, callback);
      }
  
      oAuth2Client.setCredentials(JSON.parse(token));
      callback(oAuth2Client);
    });
  }
  
  function authorizeEnvCredentials() {
    return new Promise(resolve => {
        const auth = new GoogleAuth();
        const jwtClient = new JWT(
            process.env.GOOGLE_CLIENT_EMAIL,
            null,
            process.env.GOOGLE_PRIVATE_KEY.replace(/\\n/g, '\n'), 
            SCOPES
        );
  
        jwtClient.authorize(() => resolve(jwtClient));
    });
  }
  
  // EXPORTS
  
  // This function accesses the API, given any template function.
  // The provided function must take in one parameter (auth) and complete
  // all the required steps for a single operation.
  exports.api = function(func) {
    if (hasEnvCredentials()) {
      console.log('Using Service Account credentials');
      authorizeEnvCredentials()
        .then(func)
        .catch((err) => {
            console.log('Authentication Error:', err);
        });
    }
    else {
      console.log('Using local credentials');
      fs.readFile(CREDENTIALS_FILE, (err, content) => {
        if (err) {
          return console.log('Error loading client secret file:', err);
        }
        authorizeLocalCredentials(JSON.parse(content), func);
      });
    }
  }
  
  /**
   * This example function prints the names and majors of students in a
   * sample spreadsheet:
   * @see https://docs.google.com/spreadsheets/d/1BxiMVs0XRA5nFMdKvBdBZjgmUUqptlbs74OgvE2upms/edit
   */
  exports.listMajorsExample = function() {
    // auth (google.auth.OAuth2): The authenticated Google OAuth client
    exports.api(function(auth) {
      const sheets = google.sheets({version: 'v4', auth});
      sheets.spreadsheets.values.get({
        spreadsheetId: '1BxiMVs0XRA5nFMdKvBdBZjgmUUqptlbs74OgvE2upms',
        range: 'Class Data!A2:E',
      }, (err, res) => {
        if (err) {
          return console.log('The API returned an error: ' + err);
        }
    
        const rows = res.data.values;
        if (rows.length) {
          console.log('Name, Major:');
          // Print columns A and E, which correspond to indices 0 and 4.
          rows.map((row) => {
            console.log(`${row[0]}, ${row[4]}`);
          });
        }
        else {
          console.log('No data found.');
        }
      });
    });
  }