Example: Webpack

// webpack.config.js

module.exports = {
  entry: {
    'app': './index.tsx',
    'polyfills': [
      'core-js/stable',
      'regenerator-runtime/runtime',
      'whatwg-fetch',
      'fast-text-encoding'
    ]
  },
  // module property with your babel or typescript loaders
  // and any other plugins needed ...
  output: {
    filename: '[name].js'
  }
};

// top-level app file (aka entry point)

// Patch missing crypto library
if (!window.crypto && window.msCrypto) {
  window.crypto = window.msCrypto;
}
// Patch missing el.remove() method
if (!Element.prototype.remove) {
  Element.prototype.remove = function() {
    this.parentElement.removeChild(this);
  };
}

function hasSupportedFeatures() {
  return window.Promise &&
    window.URL &&
    window.fetch &&
    window.TextEncoder &&
    Object.assign;
}

function loadApp() {
  ReactDom.render(<Login />, document.getElementById('sdk-target'));
}

function loadPolyfills(done: () => void) {
  const js = document.createElement('script');
  js.src = '/js/polyfills.js';
  js.onload = () => done();
  js.onerror = () => console.error('Failed to load polyfills');
  document.head.appendChild(js);
}

if (hasSupportedFeatures()) {
  loadApp();
} else {
  loadPolyfills(loadApp);
}