How to refer external javascript and css files i react?

App.js
import React,{Component} from "react";
import ReactDOM from "react-dom";
import "./styles.css";
class App extends Component {
componentDidMount() {
const script = document.createElement("script");
script.async = true;
script.src = "https://some-scripturl.js";
this.div.appendChild(script);
}
render() {
return (
<div className="App" ref={el => (this.div = el)}>
<h1>Hello reacth1>
{/* Script is inserted here */}
div>
);
}
}
export default App;