Polyfill loader for for React 16

polyfill.js

const features = [
  'Map',
  'Set',
  'requestAnimationFrame'
]

function browserSupportsAllFeatures() {
  return features.every(f => window[f])
}

function missingFeatures() {
  return features.filter(f => !window[f])
}

function loadScript(done) {
  // eslint-disable-next-line prefer-template
  const cdn = 'https://cdn.polyfill.io/v2/polyfill.min.js?features=' + missingFeatures().join()
  const js = document.createElement('script')
  
  js.src = cdn
  js.onload = function onLoad() {
    done()
  }
  
  js.onerror = function onError() {
    // eslint-disable-next-line prefer-template
    done(new Error('Failed to load script ' + cdn))
  }
  
  document.head.appendChild(js)
}

module.exports = (done) => {
  if (browserSupportsAllFeatures()) {
    return done()
  }
  
  loadScript(done)
}