test-2.jsx

// app/javascript/components/App.js

import React from 'react'

class App extends React.Component {
  render() {
    return (
      <div>
        Hello world!
      </div>
    )
  }
}

export default App

test.jsx

// app/javascript/components/App.js

...
class App extends React.Component {
  render() {
    return (
      <div>
        <Switch>
          <Route exact path="/" component={Home} />
          <Route exact path="/posts" component={Posts} />
          <Route exact path="/new_post" component={NewPost} />
        </Switch>
      </div>
    )
  }
}

export default App