Version

3.0.0 + 4.0.0

For 3.0.0, I used this:

   componentDidMount(){
    this.props.router.setRouteLeaveHook(
      this.props.route,
      this.routerWillLeave
    )

    routerWillLeave(nextLocation){
      return 'Are you sure you want to leave?';
    }

For 4.0.0, I simply tested with the example in https://react-router.now.sh/

I tested on iOS 10, with Chrome, Firefox, UC Browser, Opera Mini all working well.

The reason is most likely that Safari doesn't support beforeunload and pagehide is used instead.
https://developer.apple.com/library/content/documentation/AppleApplications/Reference/SafariWebContent/HandlingEvents/HandlingEvents.html

However, it's worst when I used it anyway on my 3.0.0 react SPA with browserHistory or hashHistory, as trying to navigate away by clicking links or back button not only doesn't trigger a leave prompt, the back button and any links on the page becomes malfunctioned as clicking on it has no effect at all, just like being trapped in those spam sites that forces you to stay.

I wonder what could be the solution? Would it be better to have something as follows implemented?

if ('onpagehide' in window) {
//if(window.onpagehide || window.onpagehide === null){
   window.addEventListener('pagehide', exitFunction, false);
} else {
   window.addEventListener('unload', exitFunction, false);
}

Source:
http://stackoverflow.com/questions/6906146/how-to-detect-browser-support-for-pageshow-and-pagehide