Creating new `Blob` from `Uint8Array` produces garbage data

Description

I'm trying to manually convert base64 data to a Blob and vice versa. To do this I create a Uint8Array from the base64 data (with atob - polyfilled with base-64).

For example for a very simple PNG the resulting blob is about 3-4x as large as expected and the data is unrecognizable.

const base64Uri = "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAgAAAAIAQMAAAD+wSzIAAAABlBMVEX///+/v7+jQ3Y5AAAADklEQVQI12P4AIX8EAgALgAD/aNpbtEAAAAASUVORK5CYII=";

const base64ToBlob = (b64Data, contentType, sliceSize = 512) => {
  const byteCharacters = atob(b64Data);
  const byteArray = new Uint8Array(byteCharacters.length);

  for (let n = 0; n < byteCharacters.length; n++) {
    byteArray[n] = byteCharacters.charCodeAt(n);
  }

  return new Blob([byteArray], { type: contentType });
};
const blobToBase64Uri = (blob) => {
  return new Promise((resolve, reject) => {
    const reader = new FileReader();
    reader.onload = () => {
      resolve(reader.result);
    };

    reader.readAsDataURL(blob);
  });
};

const [_, type, base64] = base64Uri.match(/^data:(.*);base64,(.*)/) ?? [];

console.log(base64Uri)
// result: data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAgAAAAIAQMAAAD+wSzIAAAABlBMVEX///+/v7+jQ3Y5AAAADklEQVQI12P4AIX8EAgALgAD/aNpbtEAAAAASUVORK5CYII=
blobToBase64Uri(base64ToBlob(base64, type)).then(console.log)
// result: data:image/png;base64,MTM3LDgwLDc4LDcxLDEzLDEwLDI2LDEwLDAsMCwwLDEzLDczLDcyLDY4LDgyLDAsMCwwLDgsMCwwLDAsOCwxLDMsMCwwLDAsMjU0LDE5Myw0NCwyMDAsMCwwLDAsNiw4MCw3Niw4NCw2OSwyNTUsMjU1LDI1NSwxOTEsMTkxLDE5MSwxNjMsNjcsMTE4LDU3LDAsMCwwLDE0LDczLDY4LDY1LDg0LDgsMjE1LDk5LDI0OCwwLDEzMywyNTIsMTYsOCwwLDQ2LDAsMywyNTMsMTYzLDEwNSwxMTAsMjA5LDAsMCwwLDAsNzMsNjksNzgsNjgsMTc0LDY2LDk2LDEzMA==

The same code works fine on the web and outputs the same result for both log statements.

React Native Version

0.72.6

Output of npx react-native info

System:
OS: macOS 14.0
CPU: (10) arm64 Apple M1 Pro
Memory: 551.34 MB / 32.00 GB
Shell:
version: "5.9"
path: /bin/zsh
Binaries:
Node:
version: 20.8.0
path: /opt/homebrew/bin/node
Yarn:
version: 1.22.19
path: /usr/local/bin/yarn
npm:
version: 10.2.0
path: /opt/homebrew/bin/npm
Watchman:
version: 2023.10.09.00
path: /opt/homebrew/bin/watchman
Managers:
CocoaPods:
version: 1.13.0
path: /opt/homebrew/bin/pod
SDKs:
iOS SDK:
Platforms:
- DriverKit 23.0
- iOS 17.0
- macOS 14.0
- tvOS 17.0
- watchOS 10.0
Android SDK: Not Found
IDEs:
Android Studio: 2021.2 AI-212.5712.43.2112.8609683
Xcode:
version: 15.0/15A240d
path: /usr/bin/xcodebuild
Languages:
Java:
version: 11.0.20.1
path: /usr/bin/javac
Ruby:
version: 2.6.10
path: /usr/bin/ruby
npmPackages:
"@react-native-community/cli": Not Found
react:
installed: 18.2.0
wanted: 18.2.0
react-native:
installed: 0.72.5
wanted: 0.72.5
react-native-macos: Not Found
npmGlobalPackages:
"react-native": Not Found
Android:
hermesEnabled: Not found
newArchEnabled: Not found
iOS:
hermesEnabled: Not found
newArchEnabled: Not found

Steps to reproduce

1. Install base-64
2. run the code given in the instructions somewhere in your app

Snack, screenshot, or link to a repository

https://snack.expo.dev/@stefan-5gpay/base64blobbug

On the web version both are the same:

On device (same for iOS and Android) data is different:

Start with the provided Snack reproduction and run the base64-to-Blob example on web, iOS, and Android to compare the encoded results. Trace the React Native Blob and Uint8Array handling involved in the repro; done means the round trip preserves the original PNG data on devices as it does on the web.