Generate base64-encoded bearer token in Node.js

auth.js

// import https module for making API calls
const https = require('https')

// import querystringify to construct a query from an object
const qs = require('querystringify')

const consumerApiKey = YOUR_CONSUMER_API_KEY
const apiSecretKey = YOUR_API_SECRET_KEY
const plainCredential = `${consumerApiKey}:${apiSecretKey}`
const bearerToken = Buffer.from(plainCredential).toString('base64')

// Example API request
const options = {
  hostname: 'api.twitter.com',
  port: 443,
  method: 'POST',
  path: '/oauth2/token?grant_type=client_credentials',
  headers: {
    'Authorization': 'Basic ' + bearerToken,
  },
}

const promise = new Promise((resolve, reject) => 
  https.request(options, res => {
    let data = ''
    res.on('data', chunk => data += chunk)
    res.on('end', () => {
      const auth = JSON.parse(data)
      if(auth.token_type !== 'bearer')
        return reject(new Error('Twitter auth failed.'))
      const accessToken = auth.access_token
      return resolve(accessToken)
    })
  }).end()
)