hooks.js

render((
  <Router history={...}>
    <Route path="/" component={App}>
      <Route path="A" component={PageA} onEnter={/* E-A */} onLeave={/* L-A */} />
      <Route path="B" component={PageB} onEnter={/* E-B */} onLeave={/* L-B */} />
      <Route path="C" onEnter={/* E-C */} onLeave={/* L-C */} />
    </Route>
  </Router>
), document.getElementById('...'))