`styled()` with a mapping makes the class props it adds read-only

まず src/runtime.types.ts にある参照先の StyledComponent と StyledConfiguration の定義を確認し、次に再現テスト src/tests/native/styled-mapped-class-props.test.ts を調べます。対象を絞ったテストを実行し、マップされた class props とマップされていない class props の割り当てを比較します。両方のケースが診断なしで型チェックを通り、テストが成功すれば完了です。

Before submitting a new issue

• I tested using the latest version of the library, as the bug might be already fixed.
• I tested using a supported version of react native.
• I checked for possible duplicate issues, with possible answers.

Bug summary

styled(component, mapping) takes its mapping as const M extends StyledConfiguration<C>, so a literal such as { className: "style" } is inferred with read-only members, and StyledComponent adds each class prop by mapping over keyof M, which keeps them read-only. Code that sets className on a mapped component's props fails to type-check, while the component's own props and the className of styled() without a mapping are writable.

Library version

main at df32ce3

Environment info

System:
  OS: Windows 11 10.0.26300
  CPU: "(32) x64 AMD Ryzen 9 9950X3D 16-Core Processor          "
  Memory: 160.57 GB / 255.67 GB
Binaries:
  Node:
    version: 26.12.1
    path: C:\Program Files\nodejs\node.EXE
  Yarn:
    version: 4.9.2
    path: C:\Users\<user>\AppData\Roaming\npm\yarn.CMD
  npm:
    version: 12.2.1
    path: C:\Program Files\nodejs\npm.CMD
  Watchman: Not Found
SDKs:
  Android SDK: Not Found
  Windows SDK:
    AllowDevelopmentWithoutDevLicense: Enabled
    Versions:
      - 10.0.19041.0
      - 10.0.26100.0
IDEs:
  Android Studio: AI-262.9437.185.2621.16467767
  Visual Studio: Not Found
Languages:
  Java:
    version: 17.0.20.1
    path: C:\Program Files\Microsoft\jdk-17.0.20.101-hotspot\bin\javac.EXE
  Ruby: Not Found
npmPackages:
  "@react-native-community/cli": Not Found
  react:
    installed: 19.2.3
    wanted: 19.2.3
  react-native:
    installed: 0.86.3
    wanted: 0.86.3
  react-native-windows: 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. Clone the reproducer branch, which is main at df32ce3 with two files added, src/__tests__/native/styled-mapped-class-props.test.ts and reproduce.sh:

   git clone --branch repro/styled-mapped-class-props-readonly https://github.com/YevheniiKotyrlo/react-native-css
   cd react-native-css
   

2. Run ./reproduce.sh. It runs yarn install --immutable and the test, and ends with one line: DEFECT: … and exit status 1 while the mapped className is read-only, FIXED: … and 0 once it is writable, ERROR: … and 2 when the install fails or a case fails another way, the unmapped control among them.

src/__tests__/native/styled-mapped-class-props.test.ts:

import { join } from "node:path";

import ts from "typescript";

const root = join(__dirname, "../../..").replaceAll("\\", "/");
const fixtureName = `${root}/src/__tests__/native/styled-mapped-class-props.fixture.ts`;

function diagnostics(text: string): readonly string[] {
  const { config } = ts.readConfigFile(`${root}/tsconfig.json`, (path) =>
    ts.sys.readFile(path),
  );
  const { options } = ts.parseJsonConfigFileContent(config, ts.sys, root);
  const host = ts.createCompilerHost(options, true);
  const program = ts.createProgram([fixtureName], options, {
    ...host,
    getCurrentDirectory: () => root,
    fileExists: (name) => name === fixtureName || host.fileExists(name),
    readFile: (name) => (name === fixtureName ? text : host.readFile(name)),
    getSourceFile: (name, languageVersion, ...rest) =>
      name === fixtureName
        ? ts.createSourceFile(name, text, languageVersion, true)
        : host.getSourceFile(name, languageVersion, ...rest),
  });
  return ts
    .getPreEmitDiagnostics(program, program.getSourceFile(fixtureName))
    .map(({ messageText }) =>
      ts.flattenDiagnosticMessageText(messageText, "\n"),
    );
}

test("a class prop a styled() mapping adds is as writable as the component's own props", () => {
  expect(
    diagnostics(
      [
        `import type { ComponentProps } from "react";`,
        `import { Image } from "react-native";`,
        `import { styled } from "react-native-css";`,
        `const StyledImage = styled(Image, { className: "style" });`,
        `type Props = ComponentProps<typeof StyledImage>;`,
        `export function withRoundedCorners(props: Props): Props {`,
        `  const next: Props = { ...props };`,
        '  next.className = `${props.className ?? ""} rounded`;',
        `  next.style = props.style;`,
        `  return next;`,
        `}`,
      ].join("\n"),
    ),
  ).toStrictEqual([]);
});

test("the className styled() adds without a mapping is writable", () => {
  expect(
    diagnostics(
      [
        `import type { ComponentProps } from "react";`,
        `import { View } from "react-native";`,
        `import { styled } from "react-native-css";`,
        `const StyledView = styled(View);`,
        `type Props = ComponentProps<typeof StyledView>;`,
        `export function withPadding(props: Props): Props {`,
        `  const next: Props = { ...props };`,
        '  next.className = `${props.className ?? ""} p-4`;',
        `  return next;`,
        `}`,
      ].join("\n"),
    ),
  ).toStrictEqual([]);
});

Expected: both cases pass: the className a mapping adds is as writable as the component's own style, and the className of styled() without a mapping stays writable. The script ends with FIXED: 2 of 2 cases pass and exits 0.

Actual: the mapped case reports Cannot assign to 'className' because it is a read-only property., and the unmapped one passes; the script ends with this line and exits 1:

DEFECT: the className a styled() mapping adds is read-only: 1 of 1 cases show it: a class prop a styled() mapping adds is as writable as the component's own props

System: MINGW64_NT-10.0-26300 3.6.10-5a1665c8.x86_64, Node v26.12.1, Yarn 4.9.2, react-native-css e851452
yarn install --immutable: done
FAIL src/__tests__/native/styled-mapped-class-props.test.ts (73.514 s)
  × a class prop a styled() mapping adds is as writable as the component's own props (11460 ms)
  √ the className styled() adds without a mapping is writable (5049 ms)
  ● a class prop a styled() mapping adds is as writable as the component's own props
    expect(received).toStrictEqual(expected) // deep equality
    - Expected  - 1
    + Received  + 3
    - Array []
    + Array [
    +   "Cannot assign to 'className' because it is a read-only property.",
    + ]
Test Suites: 1 failed, 1 total
Tests:       1 failed, 1 passed, 2 total
jest: ran src/__tests__/native/styled-mapped-class-props.test.ts
DEFECT: the className a styled() mapping adds is read-only: 1 of 1 cases show it: a class prop a styled() mapping adds is as writable as the component's own props

Reproducible example repository

https://github.com/YevheniiKotyrlo/react-native-css/tree/repro/styled-mapped-class-props-readonly