title: react: why static propTypes
body: <p>I am looking the <a href="https://github.com/reactjs/redux/blob/85e2368ea9ff9b308fc873921ddf41929638f130/examples/todomvc/src/components/Header.js#L5" rel="nofollow noreferrer">redux todomvc codes</a>. What is the <code>static</code> keyword in <code>static propTypes</code>?</p>