evil_fetch.js

// Make fetch evil - useful for testing retry logic
// MAKE FETCH EVIL - THIS SHOULD NOT BE IN THE FINAL COMMIT
const _fetch = window.fetch
const evilFetch = function () {
  const r = Math.random()

  if (r < 0.5) {
    return _fetch.apply(null, arguments)
  } else if (r < 0.75) {
    console.warn('mocking fetch with TypeError')
    return Promise.reject(new TypeError('blarg'))
  } else {
    console.warn('mocking fetch with 404')
    return _fetch('/this/will/404')
  }
}
window.fetch = evilFetch