⚛️ React StrictMode

? Quick Overview

React.StrictMode is a development-only helper that highlights potential problems in React applications. It performs additional checks, warnings, and validations without affecting production builds.

? Key Concepts

• Runs only in development mode
• Does not render visible UI
• Helps detect unsafe patterns early
• Prepares apps for future React features

⚛️ Introduction

React.StrictMode is a tool that helps developers identify potential problems in their React applications. It runs only in development mode and does not affect production builds.

Think of it as React’s “lint mode” — it performs additional checks and warnings to help write cleaner, future-proof code.

? How to Use StrictMode

// index.js - enabling StrictMode at root level
import React from "react";
import ReactDOM from "react-dom/client";
import App from "./App";

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

You can also wrap specific components to isolate checks in smaller parts of the component tree.

? What StrictMode Checks

• Detects usage of unsafe or deprecated lifecycle methods.
• Warns about legacy string refs and improper ref usage.
• Identifies unexpected side effects during rendering.
• Ensures predictable behavior in concurrent rendering.
• Double-invokes lifecycle methods to verify idempotence.

⚙️ Example: Detecting Side Effects

// Component demonstrating double execution in StrictMode
import React, { useEffect } from "react";

function Example() {
useEffect(() => {
console.log("Effect executed");
}, []);

console.log("Component rendered");
return <p>Check your console!</p>;
}

export default Example;

When wrapped in StrictMode, the console will log:

// StrictMode intentional double invocation
Component rendered
Effect executed
Component rendered
Effect executed

This helps detect unwanted side effects such as API calls during rendering.

? StrictMode in React 18+

• Simulates concurrent rendering behavior
• Runs effects multiple times
• Validates transitions and Suspense boundaries

⚠️ Common Misunderstandings

• “StrictMode causes bugs.” — It only exposes existing ones.
• “It slows my app.” — Only in development.
• “I should disable it.” — Fix warnings instead.