Mastering useState with useEffect in React: A Comprehensive Guide to Avoiding State Overwrites

Learn how to properly use `useState` in combination with `useEffect` in React to avoid state overwrites and ensure efficient rendering of data from promises.
---

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Append To useState Array inside useEffect inside a Promise

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Mastering useState with useEffect in React: A Comprehensive Guide to Avoiding State Overwrites

In today's guide, we'll address a common challenge in React development: how to effectively update an array using useState inside useEffect without accidentally overwriting its content. If you've faced a situation where your array render only the last added value, you’re not alone. Let's explore the problem and dissect a solution that ensures your application handles state updates as expected.

The Problem

When working with asynchronous data retrieval in React, you may find yourself wanting to populate an array with multiple entries fetched from a promise. However, a common pitfall is the array being overwritten with each new update, which results in rendering issues. In our example, we want to fetch several data points but end up only displaying the last value.

Example Scenario

Consider the following simplified React component that simulates fetching data asynchronously:

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

Expected vs Actual Output

Expected Output:

Data 0

Data 1

Data 2

Data 3

Data 4

Data 5

Actual Output:

Data 0

Data 5

As you can see, instead of accumulating data, our array only retains the last fetched item. So, what's the solution?

The Solution

To address this issue, we can leverage React's functional setState() approach. This allows us to explicitly reference the current state value while updating it. Furthermore, we need to avoid redundant calls to the fetching logic using an additional state variable to track ongoing data fetching.

Step-by-Step Correction

Utilize Functional Updates in setState: Instead of overwriting the current state with a new array directly, we will pass a function to setArr that provides access to the previous state value.

Introduce an Additional State Variable: To control when we should fetch more data, introduce a state variable called isFetchingMoreData.

Here’s the updated component with the necessary changes:

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

Key Improvements

Functional Updates: By using (prev) => [...prev, resolve], we ensure that the previous state is properly merged with the new data rather than being overwritten.

Data Fetch Control: With isFetchingMoreData, we ensure that the fetching logic isn't redundantly executed, which prevents unnecessary state updates and possible infinite loops of effect triggering.

Conclusion

By following the outlined strategy, you can effectively manage your state in React and prevent issues related to state overwriting. This approach will help ensure that when fetching data asynchronously, each entry is correctly added to the state and rendered accordingly. Always remember to plan for functional updates and control the flow of data fetching to keep your components efficient and predictable.

With this understanding, you’re now better equipped to harness the power of useState and useEffect in your React applications! Happy coding!