IndexRoute allows us to define a default child component to be rendered at a specific route when no other sub-route is available.
After your change /about path should not be existed right?
If you follow this video, and at the end click on the link created to go to about, rather than clicking the link for home, you will get a routing error. fyi, you need to add a Route for the about link to go to.
<IndexRoute /> is no longer supported in react-router v4, try using
<Switch /> instead