dynamic-polyfilling.js

dynamic-polyfilling.js

polyfillIfNeeded().then(() => {
    // we (ab)use synchronous `require`, which is not standard JS (but is provided by webpack)
    require('./app.js').start()
})

function polyfillIfNeeded(callback) {
    if (browserSupportsAllFeatures()) {
        return Promise.resolve()
    } else {
        return new Promise((resolve) => {
            const polyfill = document.createElement('script')
            polyfill.src = '/path/to/polyfill.js'
            polyfill.onload = resolve
            document.head.appendChild(polyfill)
        })
    }
}

function browserSupportsAllFeatures() {
    return (
      Promise.prototype.finally &&
      NodeList.prototype.forEach &&
      // ...
    )
}