Polyfill with a Promise

polyfill.js

function polyfill (feature, url) {
  return new Promise(function (resolve, reject) {
    if (!!feature) return resolve(feature)
    var el = document.createElement('script')
    el.async = el.defer = true
    el.src = url
    el.onload = function () { resolve(url) }
    el.onerror = function (e) { reject(e) }
    document.head.appendChild(el)
  })
}