LilyStilson / flutterjs_fetch_bug

Repository, demonstating a bug with flutter_js library with its fetch functionality.

flutterjs_fetch_bug

Repository, demonstrating a bug with flutter_js library with its fetch functionality.

The bug

Apparently, fetch in flutter_js library is not working properly. It does not escape automatically special characters in the response body, which leads to inability to parse the response as JSON. I made this repository and a Cloudflare worker to demonstrate the bug.

Faulty behavior

After I do this

let response = await fetch("https://flutterjs-test.nightskystudio.workers.dev/faulty")
let json = await response.json()

I expect json to be an object with parsed JSON object from the response. But instead, I get a string with the response body. This shows that something went wrong when parsing the response. After some tinkering, I found out that fetch in flutter_js does not escape special characters automatically and, in fact, does not support them at all. This poses a problem, because sometimes APIs DO return a sequence of \r\n\t and other special characters, which are not escaped, and thus, cannot be parsed by the JS engine.

Expected behavior

This will work as expected:

let response = await fetch("https://flutterjs-test.nightskystudio.workers.dev/normal")
let json = await response.json()

json will be an object with parsed JSON object from the response. Since return in /normal path does not contain any special characters.

Remarks

I was at least partially certain that this library will provide me with functionality that is very close to V8, but here we are...

Reproducing the bug

1. Clone this repository

2. cd flutterjs_fetch_bug

3. Run flutter pub get

4. Run flutter run

5. After application runs, select Normal/Faulty from the dropdown menu and press on the FAB button.

   Under Text output: you will see direct output that was received from the JavaScript runtime.

   Under JSON output: you will see the result of trying to re-parse JSON.stringify call by Dart language.

   The output of JsRuntime function callFetch(type) is an object with the source json parsing result and the type of that result. If it's an object, then fetch was successful and the response was parsed as JSON. If it's a string, then fetch was unsuccessful and the response was not parsed as JSON. (also, btw, response.json() should fail if the response is not a valid JSON, but it doesn't and fallbacks to response.text(), which is another bug)