How to apply styles to a child class in JSS - reactjs

Question

I'm using React, Material UI with JSS and React Router.
I'm hooking in to <NavLink> to apply an active class like:
<NavLink to={'/dashboard'} activeClassName={classes.active}
<button className={classes.btn}>Link</button>
/>
The class is being added fine to the parent, but I'm having an issue applying the style to the child button if it's a class. When targeting the element it works, just not the class.
I've looked into using nested JSS, but this still does not work. Any ideas?
active: {
'& .btn': { // This doesn't work
backgroundColor: '#2A354F'
},
'& button': { // This works
backgroundColor: '#2A354F'
}
}

Answer 1

If btn is another class defined via JSS, then you need to refer to it using $btn.
See this part of the JSS documentation.
Here's some sample code that works:
import React from "react";
import ReactDOM from "react-dom";
import { NavLink, BrowserRouter } from "react-router-dom";
import { withStyles } from "#material-ui/core/styles";
const styles = {
btn: {},
active: {
"& $btn": {
backgroundColor: "#2A354F",
color: "#fff"
}
}
};
function App(props) {
return (
<BrowserRouter>
<div className="App">
<NavLink to="/" activeClassName={props.classes.active}>
<button className={props.classes.btn}>Link</button>
</NavLink>
</div>
</BrowserRouter>
);
}
const StyledApp = withStyles(styles)(App);
const rootElement = document.getElementById("root");
ReactDOM.render(<StyledApp />, rootElement);

Answer 2

It does not work due to class name ".btn" because the root class "active" after rendering of React will not have the same class name and it cannot found out it.
just set {classes.btn}
<button className={classes.btn}>Link</button>