function polyfillCustomEvent() {

   if (typeof window.CustomEvent === "function") return false;

   function CustomEvent(event, params) {

     params = params || { bubbles: false, cancelable: false, detail: undefined };

     var evt = document.createEvent("CustomEvent");
     evt.initCustomEvent(
       event,
       params.bubbles,
       params.cancelable,
       params.detail
     );

     return evt;
   }
 
   CustomEvent.prototype = window.Event.prototype;
 
   window.CustomEvent = CustomEvent;

 }

 function runOnceFor(object, flag, cb) {

   if (!object[flag]) {
     object[flag] = true;
     cb();
   }
 }
 
 let MISSING_HISTORY = Symbol("missing_history");
 export default function addMissingHistoryEvents(window, history) {

   runOnceFor(history, MISSING_HISTORY, () => {

     const pushState = history.pushState.bind(history);
     const replaceState = history.replaceState.bind(history);
 
     polyfillCustomEvent();
 

     history.pushState = function(state, title, url) {

       let result = pushState(...arguments);
 

       var pushstate = new CustomEvent("pushstate", {
         detail: { state, title, url }
       });

       window.dispatchEvent(pushstate);
       return result;
     };
 

     history.replaceState = function(state, title, url) {

       const result = replaceState(...arguments);
 

       var replacestate = new CustomEvent("replacestate", {
         detail: { state, title, url }
       });

       window.dispatchEvent(replacestate);
       return result;
     };
   });

 }