router.js

import React from "react";
import pathToRegexp from "path-to-regexp";

function Home() {
  return <h1>Home</h1>;
}

function About() {
  return <h1>About</h1>;
}

function Contact() {
  return <h1>Contact</h1>;
}

function Inbox({ match, params }) {
  return (
    <div>
      <h1>Inbox</h1>
      <Message key={params.id}/>
      <div>
        {params.id ? <p>Now showing message {params.id}</p> : null}
      </div>
    </div>
  );
}

const RoutingContext = React.createContext();

class Router extends React.Component {
  state = {
    location: window.location
  };

  push = url => {
    window.history.pushState(null, null, url);
    this.setState({ location: window.location });
  };

  handlePopState = () => {
    this.setState({ location: window.location });
  };

  componentDidMount() {
    window.addEventListener("popstate", this.handlePopState);
  }

  componentWillUnmount() {
    window.removeEventListener("popstate", this.handlePopState);
  }

  render() {
    const { location } = this.state;

    return (
      <RoutingContext.Provider
        {...this.props}
        value={{ location, push: this.push }}
      />
    );
  }
}

function Route({ path, component: Component }) {
  const keys = [];
  const regexp = pathToRegexp(path, keys);

  return (
    <RoutingContext.Consumer>
      {({ location }) => {
        const match = regexp.exec(location.pathname);

        if (match) {
          const params = keys.reduce((memo, key, index) => {
            memo[key.name] = match[index + 1];
            return memo;
          }, {});

          return <Component match={match} params={params} />;
        }

        return null;
      }}
    </RoutingContext.Consumer>
  );
}

function Link(props) {
  const { to, ...anchorProps } = props;

  return (
    <RoutingContext.Consumer>
      {({ push }) => (
        <a
          {...anchorProps}
          href={to}
          onClick={event => {
            event.preventDefault();
            push(to);
          }}
        />
      )}
    </RoutingContext.Consumer>
  );
}

class App extends React.Component {
  render() {
    return (
      <Router>
        <div>
          <h1>Welcome to the app!</h1>

          <nav>
            <ul>
              <li>
                <Link to="/about">About Us</Link>
              </li>
              <li>
                <Link to="/contact">Contact Us</Link>
              </li>
              <li>
                <Link to="/">Home</Link>
              </li>
            </ul>
          </nav>

          <Redirect to="..."/>

          <Route path="/about" component={About} />
          <Route path="/contact" component={Contact} />
          <ParentRoute path="/" component={Home} />
        </div>
      </Router>
    );
  }
}

export default App;

Thanks for sharing! @mjackson