Скачать или смотреть Mastering React Hooks: How to Prevent Child Component Re-Renders in a Menu Structure

Discover effective strategies for preventing unnecessary re-renders of child components in React using Hooks, `React.memo()`, and `useCallback()`.
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This video is based on the question https://stackoverflow.com/q/71174560/ asked by the user 'FJTAIL' ( https://stackoverflow.com/u/8002971/ ) and on the answer https://stackoverflow.com/a/71174833/ provided by the user 'JS Chewy' ( https://stackoverflow.com/u/17958141/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Introduction

Are you a newbie in React and find yourself puzzled by the concept of component re-rendering? You're not alone! As developers, we often strive to optimize our applications, ensuring that they run efficiently. One common challenge is preventing child components from rendering unnecessarily, especially when using hooks and maintaining a clean architecture in a component tree. In this guide, we’ll explore how to tackle this problem using a simple menu application as an example.

Problem Statement

Imagine an app structured with the following hierarchy:

The App component acts as the parent of the Menu component.

The Menu component maps through an array of items to create individual MenuItem components.

Each time a MenuItem is clicked, it updates the state with the selected item's value. However, the challenge arises when we notice that every time the state updates, all components re-render — a behavior we want to avoid.

Below is a simplified version of our architecture:

Current Code Structure

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Despite using React.memo, which should theoretically reduce unnecessary re-renders, all components still update every time the state changes. Let's dig into why this happens.

Understanding React's Rendering Behavior

Reference Equality in JavaScript

One important aspect to grasp is how JavaScript handles equality. For primitive values like strings and numbers, equality checks are straightforward ('abc' === 'abc' returns true). However, for objects, functions, and arrays, a new reference is created each time an array or object is defined, resulting in a false equality ([] !== []).

Key Points on Render Behavior

Hooks and State: Each time your state updates in React, the entire component tree under that state gets checked for updates.

Functions & Arrays: When new instances of functions or arrays are created, they are treated as different, leading to unnecessary re-renders of child components.

With this in mind, let's delve into the solution for our problem.

Solution Overview

By utilizing React Hooks effectively, particularly useCallback, we can control when and how our components re-render. Here’s how we can modify our existing code to prevent child components from re-rendering unnecessarily.

Step 1: Update the App.js Component

We need to make two key changes in App.js:

Move the declaration of the data array outside the function to avoid creating a new instance on each render.

Wrap the handleMenuItem function with useCallback, passing an empty dependency array so it retains the same reference unless components are mounted or unmounted.

Here’s the updated code:

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Step 2: Refactor the Menu.js Component

The Menu.js component doesn’t require drastic changes; just ensure quality with syntax conventions. Here’s the updated version for better practice:

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Step 3: Optimize the MenuItem.js Component

To further cache our click event in MenuItem.js, we similarly use useCallback on the click handler:

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

By applying useCallback and ensuring we define our data outside of the component function, we effectively prevent unnecessary re-renders of child components in React. This not only enhances performance by avoiding excessive rendering but also creates a cleaner, more maintainable code structure.

In summary, focus on reference equality, leverage React Hooks wisely with caching mechanisms, and always verify that you're minimizing re-rendering whenever possible. With thi