This is an old, outdated answer for React Router v4
<Link> no longer has the activeClassName or activeStyle properties. In react-router v4 you have to use <NavLink> if you want to do conditional styling:
const Router = () => (
<BrowserRouter>
<div>
<Nav>
<NavLink exact activeClassName="is-active" to="https://stackoverflow.com/">Home</NavLink>
<NavLink activeClassName="is-active" to='/about'>About</NavLink>
</Nav>
<Match pattern="https://stackoverflow.com/" exactly component={Home} />
<Match pattern='/about' exactly component={About} />
<Miss component={NoMatch} />
</div>
</BrowserRouter>
)
I added an exact property to the home <NavLink>, I’m fairly sure that without it, the home link would always be active since / would match /about and any other pages you have.