Скачать или смотреть Understanding useEffect() in React: The Dangers of Not Using a Cleanup Function

Discover why you should use a cleanup function with `useEffect()` in React to prevent issues like multiple event listeners affecting your application’s performance.
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Understanding useEffect() in React: The Dangers of Not Using a Cleanup Function

When developing in React, one of the key hooks you'll encounter is useEffect(). It's a powerful function that allows you to run side effects in your components. However, if used carelessly—especially without a cleanup function—useEffect() can lead to unexpected behavior and performance issues. In this post, we'll delve into a common problem related to useEffect() and provide a clear solution.

The Problem Explained

Let's say you're building a simple counter application using React. The idea is to increment a counter every time the user clicks anywhere on the document. The code might look something like this:

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

As you interact with the application, you might notice that the count increments by more than one. For example, it jumps from 1 to 3, then to 7, and so on. This happens because each time the component re-renders (for example, when the state changes), a new event listener is added—resulting in multiple listeners being fired for a single click!

Why Does This Happen?

Without an array of dependencies, the useEffect() function runs after every render. Therefore, each change in state—like updating the click count—causes useEffect() to execute again, adding yet another mousedown event listener every time.

Breakdown of useEffect() Usage

To understand why this is problematic, let's review three typical ways to use useEffect():

Invoking on Every Re-render:

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

Invoking on Component Mount Only:

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

Invoking on Component Mount and When a Specific State Changes:

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

The Solution: Using a Cleanup Function

To avoid piling up event listeners, it's crucial to clean up after your effects when the component unmounts or before re-running the effect. Here’s how you can implement the cleanup function:

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

By doing this, you ensure that there's only one event listener at a time, which will eliminate the issue of the counter incrementing by more than one on each click.

Optional Optimization with useCallback

To further optimize your component, consider wrapping the increment function with useCallback. This ensures that the function reference doesn't change on each re-render, preventing unnecessary additions of event listeners:

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

Key Takeaways

Always provide a cleanup function inside useEffect() when adding event listeners.

Use dependencies wisely to control when your useEffect() function runs.

Consider using useCallback for functions that will be passed as dependencies to avoid performance issues.

By keeping these principles in mind, you can create efficient and error-free components in React using useEffect(). Always remember: cleanup is crucial!