How to correctly make a 404 error?

Question

JavaScript

there is react routing

<Route path="/" component={App}>
    <IndexRoute component={HomePage} />
    <Route path="about" component={AboutPage} />
    <Route path="admin" component={AdminPage} />
    <Route path="*" component={notFound} />
    </Route>

and there is express routing

app.get('*', function (req, res) {
  res.sendFile(path.join(__dirname, '/../dist/index.html'))
})

how to make it give a 404 error code? res.status(404).send().
With a direct transition from outside.
And in general, I just started to deal with React. Do search engines index react by default?