Скачать или смотреть Understanding useEffect in React: How to Properly Update State with Async Data

Explore how to effectively update state within the `useEffect` hook in React. We'll walk you through common pitfalls, like forgetting the `await` keyword, and provide a clear example to help you manage async data.
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Mastering useEffect in React: Updating State with Async Data

When working with React, handling asynchronous data can sometimes be a challenge, especially for beginners. A common scenario is fetching data from an API and updating the component's state with that data using the useEffect hook. Today, we will answer a couple of questions regarding this process and clarify one of the most frequent pitfalls that developers encounter: forgetting to properly handle promises.

The Problem: State is Not Updating

In the example we will discuss, a component is fetching data from a Spring Boot application and attempts to store that data in a state variable called visitDetails. However, the developer notices that the state remains empty after attempting to set it with the fetched data. The questions posed are:

Can you directly set the state using the promise?

How can you use the map function to reference variables in the received JSON response?

Here's the starting point of their React component:

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

Today, let's break down the solution into clear steps to help our friend overcome these issues effectively.

Step 1: Adding the Missing await

The most critical step in resolving the issue revolves around ensuring that the response from the fetch request is properly parsed into JSON before setting it in the state. In their initial code, the developer made a minor error by missing the await keyword when calling res.json(). Here's the corrected code snippet:

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

Highlights of the Change

Await the Response: By adding await before res.json(), you ensure that the parsed JSON data gets returned before trying to use it.

Error Handling: It's a good practice to handle errors effectively so the user can understand what went wrong if the fetch fails.

Step 2: Mapping Through the Response Data

Once you retrieve the JSON data successfully, you may want to display certain properties. To access these properties, you can utilize the map function. Here’s an example of how you might do that using the visitDetails state:

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

What’s Happening Here?

Iterating with map: Each entry in the visitDetails state is rendered into a set of HTML elements, allowing you to display the data dynamically in your component.

Unique Key Prop: It’s essential to use a unique key prop with your mapped elements for optimal performance. Using scheduledVisitId from the data makes it easy.

Conclusion

By properly using the await keyword and understanding how to manipulate the fetched data, you can effectively update your component’s state with asynchronous data in React. Remember to always check for potential pitfalls like missing await before using promises. With these tips, you can strengthen your React skills and avoid common mistakes when using hooks!

Feel free to experiment further with mapping through various properties of your fetched data to customize your components even more.