auth.js

 module.exports = function auth () {
   const rl = require('readline-sync')
   const Mastodon = require('./node_modules/mastodon-api/lib/mastodon.js')
   const fs = require('fs')

   let clientId
   let clientSecret

   var baseUrl = rl.question('Please enter the web address of instance:')
   if (baseUrl.indexOf('https://') === -1) {
     baseUrl = 'https://' + baseUrl
   }

   Mastodon.createOAuthApp(baseUrl + '/api/v1/apps', 'TWITIMAGE2MASTODON', 'read write follow')
    .catch(err => console.error(err))
    .then((res) => {
      console.log('Please save \'id\', \'client_id\' and \'client_secret\' in your program and use it from now on!')
      console.log(res)

      clientId = res.client_id
      clientSecret = res.client_secret

      return Mastodon.getAuthorizationUrl(clientId, clientSecret, baseUrl)
    })
    .then(url => {
      console.log('This is the authorization URL. Open it in your browser and authorize with your account!')
      console.log(url)
      return rl.question('Please enter the code from the website: ')
    })
    .then(code => Mastodon.getAccessToken(clientId, clientSecret, code, baseUrl))
    .catch(err => console.error(err))
    .then(accessToken => {
      console.log(`This is the access token. \n${accessToken}\nSaved as access_token.\nPlease add Twitter consumer_key, consumer_secret, access_token, access_token_secret and desired userid to the created pref.json and\nexec "node index.js" again.`)
      fs.writeFileSync('pref.json', JSON.stringify(
        {
          baseUrl: baseUrl,
          access_token: accessToken,
          twitter: {
            consumer_key: '',
            consumer_secret: '',
            access_token: '',
            access_token_secret: '',
            userid: '',
            hashtag: []
          }
        }), 'utf-8')
    })
 }