why a component renders twice in strict mode in react

In React, when a component renders twice in strict mode, it is by design and serves a specific purpose. This behavior helps identify potential issues and ensure that your components are resilient and free from certain types of bugs. Here’s a detailed explanation of why this happens:

React Strict Mode

React’s Strict Mode is a tool for highlighting potential problems in an application. It activates additional checks and warnings for its descendants. One of its features is intentionally double-invoking certain lifecycle methods and functions, including the render function of components.

Reasons for Double Rendering

1. Identifying Side Effects: React aims to ensure that components are pure and free of side effects. By rendering a component twice, React can help detect side effects that might occur during the rendering process. For example, if your component performs an operation that should only happen once but happens twice, this behavior can help catch such issues.

2. Detecting Unsafe Lifecycle Methods: The double rendering can help identify problems with unsafe lifecycle methods (e.g., componentWillMount in class components). React can highlight issues where these methods might cause unexpected behavior.

3. Ensuring Clean Code: React encourages clean code practices. By double rendering, React helps developers adhere to best practices, such as avoiding side effects in the render phase and ensuring that components are idempotent (i.e., rendering them multiple times with the same props and state should produce the same result).

Example of Double Rendering in Strict Mode

Consider the following example:

import React, { useState } from 'react';
import ReactDOM from 'react-dom';

const App = () => {
  const [count, setCount] = useState(0);

  console.log('Rendering App');

  return (
    <div>
      <p>Count: {count}</p>
      <button onClick={() => setCount(count + 1)}>Increment</button>
    </div>
  );
};

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

Explanation

• Initial Render: When the App component is rendered for the first time, console.log('Rendering App') is executed, logging "Rendering App" to the console.
• Double Rendering: Due to Strict Mode, the App component’s render function is called again immediately. This logs "Rendering App" to the console a second time.

When Does This Happen?

• Development Mode: Double rendering occurs only in development mode. In production builds, Strict Mode checks (including double rendering) are disabled to avoid performance impacts.
• Initial Mounting: The double rendering typically happens during the initial mounting phase to help catch potential issues early in the component lifecycle.

Addressing Double Renders

While developing, it’s essential to understand that double renders in Strict Mode are a feature, not a bug. To handle this:

1. Avoid Side Effects in Render: Ensure that your components do not have side effects in the render phase. Side effects should be handled in hooks like useEffect that are designed for such purposes.
2. Use Pure Functions: Make sure that your components are pure functions of their props and state.
3. Optimize Performance: Avoid expensive operations in the render phase. If necessary, use memoization techniques (useMemo, useCallback) to prevent unnecessary computations.