Скачать или смотреть How to Handle undefined on Fetch Requests Inside a Map in React

Learn how to properly manage asynchronous fetch requests within a map function in React to avoid returning `undefined` values.
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This video is based on the question https://stackoverflow.com/q/71616954/ asked by the user 'michael' ( https://stackoverflow.com/u/18511508/ ) and on the answer https://stackoverflow.com/a/71617110/ provided by the user 'David' ( https://stackoverflow.com/u/328193/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Tackling the undefined Problem in Fetch Requests with React

When working with API calls in React, especially when retrieving data from a fetch request in an array (or map function), developers sometimes encounter an issue: having undefined values returned. If you've faced this problem, you're not alone! We will explore this situation and provide a clear solution.

The Problem: Fetching Data Inside a Map

Recently, a developer faced this issue when trying to retrieve and display weather data based on a list of favorite weather cards. The developer used the map function to create an array of fetch requests but always ended up with undefined. Let’s take a look at the code they used:

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In this case, the developer expected test to contain a list of weather data but received undefined instead.

Why undefined Happens

The root cause of this issue lies in the asynchronous nature of the getCurrentWeather function. When using map(), if the operations inside it are asynchronous and you don't wait for their results, the return value of map() will be undefined.

The Solution: Properly Handling Asynchronous Fetch Requests

Step 1: Use async Functions

To solve the problem, we can convert the fetch requests into an asynchronous operation. Here’s how we can restructure the code:

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Explanation of the Code

Asynchronous Function: We defined an async function named fetchWeather to handle asynchronous behavior.

Using Promise.all(): This method allows us to execute multiple promises concurrently and waits for all of them to resolve before proceeding.

Setting State: After we have all the results, we then update the state with the fetched data.

Step 2: For Ordered Fetching (If Necessary)

If the order of the fetched data matters, we can instead await each fetch operation individually:

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

Key Changes

For Loop: We used a for...of loop to iterate through each card, ensuring each fetch completes before moving on to the next.

Additional Notes: map() and React

It’s important to note two key takeaways:

Return Values: When using .map() with a block body (curly braces), be cautious. If you don't explicitly return a value, it defaults to undefined.

Async in useEffect: The function passed to useEffect cannot be async, but we can handle async operations internally by defining a separate async function.

Conclusion

Understanding how to effectively manage asynchronous calls in React is essential for tackling issues like returning undefined. By organizing your fetch requests using async functions and handling promises correctly, you can avoid potential pitfalls and ensure your components render the desired data seamlessly.

With the right strategies in place, you can optimize your React applications and enhance user experience. Happy coding!