Uncaught TypeError: Cannot read property ‘toUpperCase’ of undefined in React

Ok, I gotta admit this is a really annoying issue because its root cause is pretty trivial.

You get this issue in React when

You are accidentally trying to render a component and that component itself is null or undefined. e.g you are trying to render <MyComponent /> when the value of MyComponent is itself null or undefined.

Ive encountered this issue several times, the most recent being when upgrading to react-router 1.0 release candidate which eliminated the the <RouteHandler /> component. As a result, at places in my tree where I was doing a require for RouteHandler, things worked because it simply returned undefiend, but the moment I tried to render it using <RouteHandler /> React gave me this pretty little error.

Hope this helps you save some productive time!