React context require contextType

I don't often use `context` when working with React, but today I came across a project which was using it. I was trying to add a new class but the context passed in was always empty, and I couldn't work out why - most of the code was identical to a class which was working.

The only difference was that the working class had defined contextTypes:

export default class Hello extends React.Component {
  static contextTypes = {
    store: React.PropTypes.object,
  };
}

When I added contextTypes to my class, suddenly the context was available! A bit more digging showed that this is a known quirk, and in fact is mentioned in the docs (but only in very small writing!).