StyleSheet.flatten with a falsy value returns undefined

Description

On iOS and Android (not web), calling StyleSheet.flatten with null/undefined/false returns undefined.

The TypeScript signature of flatten is flatten<T>(style?: StyleProp<T>): T. null/undefined/false are valid StyleProps, so passing them to flatten should be valid, but flatten is supposed to return T (some style object), and undefined is usually not a valid T.

Widening the return type of flatten to T | undefined is an option, but would be a public API change. I think changing the function to return {} for falsy values is more correct, since you probably want to get a final computed, flattened style object, and don't care what the input was or if it was falsy.

Steps to reproduce

On iOS or Android:

console.log(typeof StyleSheet.flatten(false)) // => 'undefined'

React Native Version

0.75.2

Affected Platforms

Runtime - Android, Runtime - iOS

Output of npx react-native info

System:
  OS: macOS 14.2
  CPU: (8) arm64 Apple M2
  Memory: 339.17 MB / 8.00 GB
  Shell:
    version: "5.9"
    path: /bin/zsh
Binaries:
  Node:
    version: 22.3.0
    path: ~/.volta/tools/image/node/22.3.0/bin/node
  Yarn:
    version: 3.6.4
    path: ~/.volta/tools/image/yarn/4.3.1/bin/yarn
  npm:
    version: 10.8.1
    path: ~/.volta/tools/image/node/22.3.0/bin/npm
  Watchman:
    version: 2024.07.01.00
    path: /opt/homebrew/bin/watchman
Managers:
  CocoaPods:
    version: 1.15.2
    path: /Users/hazel/.rbenv/shims/pod
SDKs:
  iOS SDK:
    Platforms:
      - DriverKit 23.5
      - iOS 17.5
      - macOS 14.5
      - tvOS 17.5
      - visionOS 1.2
      - watchOS 10.5
  Android SDK: Not Found
IDEs:
  Android Studio: Not Found
  Xcode:
    version: 15.4/15F31d
    path: /usr/bin/xcodebuild
Languages:
  Java: Not Found
  Ruby:
    version: 2.7.4
    path: /Users/hazel/.rbenv/shims/ruby
npmPackages:
  "@react-native-community/cli": Not Found
  react:
    installed: 18.3.1
    wanted: 18.3.1
  react-native:
    installed: 0.75.2
    wanted: 0.75.2
  react-native-macos: Not Found
npmGlobalPackages:
  "*react-native*": Not Found
Android:
  hermesEnabled: true
  newArchEnabled: false
iOS:
  hermesEnabled: true
  newArchEnabled: fals

Stacktrace or Logs

N/A

Reproducer

https://snack.expo.dev/@catboyfan/stylesheet-flatten-undefined

Screenshots and Videos

No response

まず StyleSheet.flatten の実装とその TypeScript 宣言を見つけ、次にリンクされた Snack を使って iOS または Android で動作を再現します。null、undefined、false の入力を確認します。falsy 値が公開されている戻り値の型に従う空のスタイルオブジェクトを一貫して生成すれば完了です。