[TypeScript] react-native-strict-api transform can be `undefined`

[MatiPl01]

Description

While working on the react-native-strict-api support in Reanimated I noticed that the type of the transforms array element changed. Before it could be an object with a single transform property and value. In new types it can be also undefined which doesn't seem to be valid because the transforms array should not contain undefined values.

Steps to reproduce

1. Enable react-native-strict-api in tsconfig.json
2. Copy the following code to the app:

const transforms: ViewStyle['transform'] = [];

transforms.forEach(transform => {
  console.log(Object.keys(transform));
});

3. Observe that TS is complaining about the Object.keys(transform) call and says that transform can be undefined

React Native Version

0.81.4

Affected Platforms

Build - Linux, Build - Windows, Build - MacOS, Runtime - Desktop, Runtime - Web, Runtime - iOS, Runtime - Android

Output of npx @react-native-community/cli info

System:
  OS: macOS 15.6
  CPU: (11) arm64 Apple M3 Pro
  Memory: 152.89 MB / 18.00 GB
  Shell:
    version: "5.9"
    path: /bin/zsh
Binaries:
  Node:
    version: 22.14.0
    path: ~/.nvm/versions/node/v22.14.0/bin/node
  Yarn:
    version: 1.22.22
    path: ~/.nvm/versions/node/v22.14.0/bin/yarn
  npm:
    version: 10.9.2
    path: ~/.nvm/versions/node/v22.14.0/bin/npm
  Watchman:
    version: 2025.06.30.00
    path: /opt/homebrew/bin/watchman
Managers:
  CocoaPods:
    version: 1.14.3
    path: /Users/mateu/.rbenv/shims/pod
SDKs:
  iOS SDK:
    Platforms:
      - DriverKit 25.0
      - iOS 26.0
      - macOS 26.0
      - tvOS 26.0
      - visionOS 26.0
      - watchOS 26.0
  Android SDK:
    API Levels:
      - "30"
      - "33"
      - "34"
      - "35"
      - "36"
    Build Tools:
      - 30.0.3
      - 33.0.1
      - 34.0.0
      - 35.0.0
      - 36.0.0
    System Images:
      - android-34 | Android TV ARM 64 v8a
      - android-35 | Google Play ARM 64 v8a
    Android NDK: Not Found
IDEs:
  Android Studio: 2025.1 AI-251.26094.121.2513.14007798
  Xcode:
    version: 26.0/17A324
    path: /usr/bin/xcodebuild
Languages:
  Java:
    version: 17.0.15
    path: /opt/homebrew/opt/openjdk@17/bin/javac
  Ruby:
    version: 3.2.2
    path: /Users/mateu/.rbenv/shims/ruby
npmPackages:
  "@react-native-community/cli":
    installed: 20.0.0
    wanted: 20.0.0
  react:
    installed: 19.1.0
    wanted: 19.1.0
  react-native:
    installed: 0.81.4
    wanted: 0.81.4
  react-native-macos: Not Found
npmGlobalPackages:
  "*react-native*": Not Found
Android:
  hermesEnabled: true
  newArchEnabled: true
iOS:
  hermesEnabled: Not found
  newArchEnabled: false

Stacktrace or Logs

No overload matches this call.
  Overload 1 of 2, '(o: {}): string[]', gave the following error.
    Argument of type 'Readonly<MaximumOneOf<MergeUnion<{ readonly perspective: number | AnimatedNode; } | { readonly rotate: string | AnimatedNode; } | { readonly rotateX: string | AnimatedNode; } | ... 10 more ... | { ...; }>>>' is not assignable to parameter of type '{}'.
      Type 'undefined' is not assignable to type '{}'.
  Overload 2 of 2, '(o: object): string[]', gave the following error.
    Argument of type 'Readonly<MaximumOneOf<MergeUnion<{ readonly perspective: number | AnimatedNode; } | { readonly rotate: string | AnimatedNode; } | { readonly rotateX: string | AnimatedNode; } | ... 10 more ... | { ...; }>>>' is not assignable to parameter of type 'object'.
      Type 'undefined' is not assignable to type 'object'.

MANDATORY Reproducer

https://github.com/MatiPl01/react-native-strict-api-reproducer/tree/undefined-transforms

Screenshots and Videos

[MatiPl01]

The issue seems to be caused by this internally used type:

type MergeUnion<T> = { [K in KeysOfUnion<T>]?: ValueOfUnion<T, K> };

It makes all fields optional, hence transforms like { readonly translateX: number | string | AnimatedNode; } become { readonly translateX?: number | string | AnimatedNode; }

Removing the ? from the MergeUnion type fixes the issue.