Issue with React Native Fetch API: Handling Arrays of Objects on iOS 15.7.7 and Simulated iPhone 7 (iOS 14.5)

Question

5teve OP

Created Jul ’23

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Hey fellow developers!

I'm encountering a perplexing issue with my React Native app, specifically related to handling arrays of objects when using the Fetch API on iOS 15.7.7 and even on a simulated iPhone 7 running iOS 14.5. I'm reaching out for some collective wisdom to help shed some light on this situation.

Here's a brief overview of the problem:

Affected Platforms: The issue is observed on physical devices running iOS 15.7.7, as well as simulated iPhone 7 with iOS 14.5. However, it's important to note that the app works flawlessly on iPad devices with iOS 15.7.7 and various other simulators.
The Fetch Request: In my React Native app, I'm making a simple fetch request to retrieve an array of objects from my backend API. The server returns a valid JSON response with the expected data.

async function fetchSomething(id: string): Promise<SomeResponse> {
 const response = await fetch(`https://www.website.com/api/${id}`);
 return await response.json();
}

Inconsistent Behavior: The strange part is that while the fetch request works perfectly fine on iPad devices and other simulators, it fails to handle the array of objects correctly on the affected platforms.
Direct JSON File Fetch: To eliminate any issues with the backend API, I experimented by fetching a JSON file directly from the server with the same data that the API should provide. Surprisingly, the direct JSON file fetch works perfectly on all platforms, including the affected iOS devices.
Accessing the Desired Array of Objects: Upon further investigation, I noticed that when attempting to access the desired array of objects in the response data:

Working Example (iPad and Simulators):
```
someArrayWithObjects [{"bodytext": "...", "ctype": "...", "header": "...", "headerLayout": "...", "pid": 0, "uid": 0}]
```
Non-Working Example (iOS 15.7.7 and Simulated iPhone 7):
```
someArrayWithObjects [{"ctype": null, "headerLayout": null}]
```
The someArrayWithObjects in the non-working example lacks essential data compared to the working example, which might be causing the issue. By the way, the error occurs with any other array of objects of the same data response.
Differences in Request Header Content-Length: Additionally, as mentioned in the original post, the "content-length" in the request header for the non-working example (iOS 15.7.7 and simulated iPhone 7) differs from the working example (iPad and other simulators).

I'm puzzled as to why the "content-length" differs and how it might be impacting the processing of the response data on these specific iOS platforms. Perhaps the length differs because of the missing data in the response.

Has anyone encountered a similar issue with React Native fetch requests on iOS 15.7.7 or older versions? Any ideas or insights into why this discrepancy in "content-length" might be causing the problem? Or any thoughts on why the "contentArray" is lacking data in the non-working example?

I appreciate any help or suggestions to resolve this issue, as it's been quite challenging to pinpoint the root cause.

Thank you in advance for your support and expertise. 🙏

Happy coding! 5steve

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Answer 1

5teve OP

Jul ’23

SOLVED 🚀

As mentioned earlier, we encountered a peculiar behavior with our fetch requests on certain iOS devices. However, upon extensive investigation and collaboration, we realized that it was not a bug in the conventional sense. Instead, it was an implementation detail that we had overlooked during development.

The Lingering "Accept-Language" Header

Our epiphany came when we revisited the fetch request headers and noticed the "Accept-Language" parameter. This header tells the server the language preferences of the client. By default, the fetch request included the system language as the preferred language, which explained why the response from the backend contained mixed content in different languages (this is the current behavior of the backend: if content is requested in any other available language than the default, the fallback mechanism makes sure that the missing pieces will be filled with content in the default language).

The App's Single-Language Support

The crucial detail we had missed was that our app, in its current state, only supported a single language (the set default language of the backend). Thus, when the fetch request brought back a response with mixed content, it led to unexpected behavior during data processing. The fetch request itself was technically correct, but the app was not prepared to handle multi-language content.

Embracing Lessons Learned

While the implementation detail was initially perplexing, it provided us with valuable insights. We recognized the need for clarity in our application's language support. To ensure consistent behavior, we made changes to the fetch request to explicitly set the preferred language to the one our app supported.

Future-Proofing for Multi-Language Support

This experience also opened up new possibilities. We realized that by leveraging the "Accept-Language" header in the future, we could develop a multi-language version of our app with ease.

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