auth.js

// Ref: https://developers.google.com/youtube/v3/quickstart/nodejs

const fs = require('fs');
const readline = require('readline');
var {google} = require('googleapis');
var OAuth2 = google.auth.OAuth2;

// If modifying these scopes, delete your previously saved credentials
// at ~/.credentials/youtube-nodejs-quickstart.json
const TOKEN_DIR = (process.env.HOME || process.env.HOMEPATH ||
                 process.env.USERPROFILE) + '/.credentials/';
const TOKEN_PATH = TOKEN_DIR + 'termtube.json';

exports.authorize = (callback, scopes, token_path) => {
    // Load client secrets from a local file.
    fs.readFile(__dirname + '/client_secret.json', function (err, content) {
        if (err) {
            console.log('Error loading client secret file: ' + err);
            return;
        }
        // Authorize a client with the loaded credentials, then call the YouTube API.
        authorize(JSON.parse(content), callback, scopes, token_path);
    });
}

/**
 * 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 authorize (credentials, callback, scopes, token_path) {
    var clientSecret = credentials.installed.client_secret;
    var clientId = credentials.installed.client_id;
    var redirectUrl = credentials.installed.redirect_uris[0];
    var oauth2Client = new OAuth2(clientId, clientSecret, redirectUrl);

    // Check if we have previously stored a token.
    fs.readFile(token_path.path, function(err, token) {
        if (err) {
            getNewToken(oauth2Client, callback, scopes, token_path);
        } else {
            oauth2Client.credentials = JSON.parse(token);
            callback(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 to call with the authorized
 *     client.
 */
function getNewToken(oauth2Client, callback, scopes, token_path) {
    var authUrl = oauth2Client.generateAuthUrl({
        access_type: 'offline',
        scope: scopes
    });
    console.log('Authorize this app by visiting this url: ', authUrl);
    var rl = readline.createInterface({
        input: process.stdin,
        output: process.stdout
    });
    rl.question('Enter the code from that page here: ', function(code) {
        rl.close();
        oauth2Client.getToken(code, function(err, token) {
            if (err) {
                console.log('Error while trying to retrieve access token', err);
                return;
            }
            oauth2Client.credentials = token;
            storeToken(token_path, token);
            callback(oauth2Client);
        });
    });
}

/**
 * Store token to disk be used in later program executions.
 *
 * @param {Object} token The token to store to disk.
 */
function storeToken(token_path, token) {
    try {
        fs.mkdirSync(token_path.dir);
    } catch (err) {
        if (err.code != 'EEXIST') {
            throw err;
        }
    }
    fs.writeFile(token_path.path, JSON.stringify(token), (err) => {
        if (err) throw err;
        console.log('Token stored to ' + token_path.path);
    });
}