Скачать или смотреть How to Fix useEffect Not Triggering on State Change in React

Learn how to resolve the issue of `useEffect` not being triggered when removing items from the state array in React. This post offers a detailed explanation and solution for proper state management.
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This video is based on the question https://stackoverflow.com/q/64001934/ asked by the user 'AsianTemptation' ( https://stackoverflow.com/u/11723715/ ) and on the answer https://stackoverflow.com/a/64001963/ provided by the user 'CertainPerformance' ( https://stackoverflow.com/u/9515207/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding the Problem with useEffect in React

If you're new to React, you may encounter certain challenges while managing state in your components. A common issue arises when using the useEffect hook in conjunction with state changes. For example, you may experience a situation where useEffect only runs when a value is added to a state array but does not trigger when a value is removed. This can be frustrating, especially when you're trying to build dynamic applications, such as a scheduling app.

In this guide, we’ll explore a specific scenario where useEffect does not work as expected. We’ll provide an example of the code you're working with and guide you through the solution step by step.

The Code Example

Consider the following snippets of code from a basic scheduling application:

App.js

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AllEvents.js

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NavBar.js

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Identifying the Issue

In your NavBar component, you have a loop designed to check if a filter value exists in your eventFilters array. If it does, you remove it by using splice(). However, this approach has a crucial flaw: it’s mutating the state. In React, mutations directly affect the state, which can inhibit updates and prevent the component from re-rendering as expected.

The Problematic Code

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The Solution: Use Non-Mutating State Updates

To resolve this issue, you must avoid mutating the state directly in React. Instead, you should create a copy of the current state and update that copy. Here's how you can do it effectively using the filter method:

Updated Code

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Explanation of the Solution

Check presence: First, we check if passedFilter is already included in eventFilters.

Update state: We use the filter method to create a new array that contains all elements of eventFilters except the one that matches passedFilter. This new array is passed to setEventFilters, which correctly triggers a re-render of the component.

Conclusion

By ensuring that you never mutate the state in React and always provide a fresh copy of the state when updating, you can avoid issues where useEffect fails to respond as expected. Understanding and following these practices will help you build more robust and dynamic components in your React applications.

If you have any questions or need further clarification, feel free to leave a comment below! Happy coding!