React.StrictMode in React is a tool that is used to provide potential problems in applications like Fragment. if there is any visible UI(User interface) then React.StrictMode does not render.
React.StrictMode provides additional checks and warning for its descendants. when we start our first project of React then in App.js we can see that React.StrictMode is in use.
import React from 'react';
import ReactDOM from 'react-dom';
import './index.css';
import App from './App';
import reportWebVitals from './reportWebVitals';
ReactDOM.render(
<React.StrictMode>
<App />
</React.StrictMode>,
document.getElementById('root')
);
// If you want to start measuring performance in your app, pass a function
// to log results (for example: reportWebVitals(console.log))
// or send to an analytics endpoint. Learn more: https://bit.ly/CRA-vitals
reportWebVitals();
For more detail go to the official documentation of React