What is StrictMode, and what checks does it perform in development?

Short answer

StrictMode helps developers detect deprecated practices and hidden problems; it renders components and reruns Effects twice in development.

Explanation

StrictMode adds nothing visible to the DOM. In development only, it runs render twice to detect impure functions, runs the setup and cleanup of useEffect twice to check for subscriptions and memory leaks, and warns about deprecated lifecycle methods or findDOMNode.

Code example

import { StrictMode } from 'react';
import { createRoot } from 'react-dom/client';

const root = createRoot(document.getElementById('root')!);
root.render(
  <StrictMode>
    <App />
  </StrictMode>
);

Common mistakes

• Treating a double useEffect call as a React bug and disabling StrictMode instead of writing proper cleanup logic.

Follow-up questions

• What are Pure Components and purity in React functions (Pure Functions)?

Sources

• React docs — StrictMode