React.StrictMode in React is that tool which is used to provide potential problems in application 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 first project of React then in App.js we can see that React.StrictMode is in used.
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 official documentation of React