HerokuGithubAppGetsInstallationRepos.js

const jwt = require('jsonwebtoken')
const fetch = require('node-fetch')

const appId = process.env.GITHUB_APP_ID
const privateKey = process.env.GITHUB_APP_PRIVATE_KEY

const getRepositories = (token) => {
  return fetch('https://api.github.com/installation/repositories', {
    headers: {
      Authorization: `token ${token}`,
      Accept: 'application/vnd.github.machine-man-preview+json'
    }
  })
  .then(res => res.json())
}

const getInstallationRepos = (installationId) => {
  const tokenUrl = `https://api.github.com/installations/${installationId}/access_tokens`
  const token = jwt.sign({}, privateKey, {
    algorithm: 'RS256',
    expiresIn: (60 * 10), // 10min
    issuer: appId
  })
  
  return fetch(tokenUrl, {
    method: 'POST',
    headers: {Authorization:`Bearer ${token}`, Accept:'application/vnd.github.machine-man-preview+json'}
  })
  .then(res => res.json())
  .then(json => getRepositories(json.token))
  .catch(err => console.error(err))
}

getInstallationRepos('123456789')
.then(repos => {
  console.log(repos)
})