Adding external javascript script to React failed!

I'd like to include & run some `js` file in the `React` using `Helmet` component. Here is the simple code: *index.js:* import React from "react"; import ReactDOM from "react-dom"; import { Helmet } from "react-helmet"; import "./styles.css"; function App() { console.log("op"); return ( ); } and ultra simple `js` script to include & run: *hello.js:* console.log("opl882...") document.body.style.backgroundColor = "red"; But the script seems NOT to work! - i have no console output and/or background color changed. What's odd when I use the `js` code as an inline code like: it works! Why doesn't the external `js` file work?