What is the purpose of the React.StrictMode component?

MediumIntermediateReact

By FrontendAtlas Team · Updated Jan 30, 2026

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Quick Answer

React.StrictMode runs extra checks in development to surface unsafe lifecycles, deprecated APIs, and side-effect bugs. It does not affect production output. StrictMode catches unsafe patterns by double-invoking in dev; ensure effects are idempotent and tested.

Answer

Overview

React.StrictMode is a wrapper component provided by React to help developers identify unsafe or deprecated code patterns. It activates additional development checks and warnings to ensure components adhere to modern React practices and will behave correctly in concurrent rendering scenarios.

JSX

import React from 'react';
import ReactDOM from 'react-dom/client';
import App from './App';

ReactDOM.createRoot(document.getElementById('root')).render(
  <React.StrictMode>
    <App />
  </React.StrictMode>
);

Feature	Behavior
Detects Unsafe Lifecycles	Warns when using outdated lifecycle methods like `componentWillMount` or `componentWillReceiveProps`.
Detects Side-Effect Issues	Runs certain functions twice (like useEffect callbacks) to ensure side effects are idempotent.
Highlights Deprecated APIs	Warns about legacy string refs, findDOMNode, and legacy context usage.
Identifies Unexpected Behavior	Detects potential issues in asynchronous rendering with React concurrent features.

Key development checks enabled by React.StrictMode

Why It Matters

Encourages developers to write cleaner, future-proof React code.
Ensures that effects and updates behave consistently in upcoming React features.
Improves debugging and helps migrate older codebases to modern React hooks and concurrent rendering.

Important Notes

Only active in development mode; it has no effect in production builds.
May cause duplicate console logs or API calls in development due to intentional double-invocation of certain functions.

<li>

JSX

// Example of StrictMode highlighting a side-effect
function Example() {
  useEffect(() => {
    console.log('Effect triggered');
    return () => console.log('Cleanup');
  }, []);

  return <p>Hello World</p>;
}

// The effect above runs twice in development to test cleanup behavior.

Think of React.StrictMode as React’s 'training mode' — it helps you find potential bugs early, making your app stronger and more resilient in production.

Practical scenario
In development, you notice effects running twice; StrictMode is intentionally exposing unsafe side effects.

Common pitfalls

Assuming double-invocation happens in production (it does not).
Writing non-idempotent effects that break when re-run.
Doing side effects directly in render.

Trade-off or test tip
StrictMode makes dev noisier but catches bugs early. Test by making effects idempotent and verifying cleanup works.

Summary

StrictMode helps detect side effects, deprecated APIs, and unsafe lifecycles.
It runs additional checks in development without affecting production.
Useful for ensuring your React app is ready for future updates and concurrent rendering.