App.js

import React, { Component } from 'react';
...
class App extends Component {
  render() {

      const handleAuthentication = (nextState, replace) => {
          if (/access_token|id_token|error/.test(nextState.location.hash)) {
              const auth = new Auth();
              auth.handleAuthentication();
          }
      }
      
      // If user logged in, redirect to dashboard.
      if(localStorage.getItem('isLoggedIn')==='true'){
          history.replace('/dashboard');
      }
    
      return (
      <div className="container">
          <Router history={history}>
            <div>
               <Route exact path="/" render={(props) => <Login />} />
               <Route exact path="/dashboard" render={(props) => <Dashboard/>} />
                <Route path="/callback" render={(props) => {
                    handleAuthentication(props);
                      return <div>loading</div>;
                }}></Route>
            </div>
          </Router>
      </div>
    );
  }
}

export default App;