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Learn how to correctly run multiple asynchronous queries in a functional component using React's `useEffect` and render the results in your application.
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How to Run Multiple Async Queries on Component Load with React's useEffect

In React, it's common to need to fetch data asynchronously when a component loads. However, running multiple queries and accurately handling their results can sometimes be tricky, especially when using the useEffect hook. In this guide, we will explore how to effectively run multiple async queries on the load of a functional component and render the results in the component.

The Problem: Queries Returning Empty Arrays

Imagine you have a functional component where, upon loading, you need to loop through an array of objects and run several asynchronous queries to gather data. Even after writing your code and seeing that it correctly fetches data (verified by console logging intermediate results), you notice that the state you are trying to update remains empty.

The structure of your code might look something like this:

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

In this case, the reason for the empty relatedTableItems array lies in how asynchronous functions execute. The tempArray is updated immediately before any API calls complete, leading to the component rendering without any data.

The Solution: Modify Your useEffect

To effectively gather your data, you'll want to ensure that you are waiting for all asynchronous operations to complete before setting the state. Here’s how you can accomplish this:

Step 1: Rewrite the Async Fetch Function

Instead of directly mutating tempArray in the map, return the data for each query and resolve it within Promise.all():

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

Step 2: Understand the Changes Made

Here are the key changes and their impacts:

Awaiting Async Calls: By returning an array of promises that resolve once the async calls have finished, Promise.all() will ensure all requests complete before the results are handled.

Setting State After Resolving: Only after waiting for Promise.all to resolve do we call setRelatedTableItems(res), ensuring that relatedTableItems is populated with fetched data before it is rendered.

Benefits of This Approach

Efficient State Management: Your component state is updated only when all data has been fetched, thus avoiding unnecessary renders with empty data.

Clearer Code Structure: By returning values in the map, your code is cleaner and easier to understand.

Better Error Handling: Your fetch function can be easily expanded to include error handling if needed.

Conclusion

By restructuring your async data fetching approach within the useEffect hook, you can effectively manage the state and ensure it correctly reflects the data retrieved from your queries. This not only leads to improved functionality but also enhances the readability and maintainability of your React components.

Now that you've learned how to properly handle asynchronous operations in functional components with React, you're better equipped to tackle more complex data-fetching scenarios in your applications!