A zero read through a variable renders nothing: `width: var(--v)` over `--v: 0px` sets no width

Start with parseUnparsed in src/compiler/declarations.ts and trace how reduceParseUnparsed represents usable values versus no result. Run the reproducer test src/__tests__/native/falsy-runtime-values.test.tsx from the issue. Done means all four zero-valued cases produce the expected styles while the 0% case still passes.

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

A custom property whose value is a falsy number renders nothing on the runtime path: width: var(--v) over --v: 0px sets no width, and so do margin-top, flex-grow and z-index at 0. 0% works, because it reaches React Native as the truthy string "0%". A variable takes the runtime path when the compiler keeps it — declared more than once, as a theme with a dark override declares it, listed in inlineVariables.exclude, or compiled with inlineVariables: false.

The first four fail, the element receiving no style where each expects its zero; 0% passes.

parseUnparsed (src/compiler/declarations.ts) guards its array branch with if (!args) return; on the value reduceParseUnparsed returns, which signals nothing usable with undefined alone, so 0, "" and false are dropped with no descriptor and no warning.

Library version

main at 2421ea7 (0.0.0-nightly.2421ea7)

Environment info

System:
  OS: Windows 11 10.0.26300
  CPU: "(32) x64 AMD Ryzen 9 9950X3D 16-Core Processor          "
  Memory: 155.26 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:
    API Levels:
      - "33"
      - "35"
      - "35"
      - "35"
      - "36"
      - "37"
    Build Tools:
      - 35.0.0
      - 36.0.0
      - 36.1.0
      - 37.0.0
    System Images:
      - android-26 | Google APIs Intel x86_64 Atom
      - android-28 | Google APIs Intel x86_64 Atom
      - android-29 | Google APIs Intel x86_64 Atom
      - android-30 | Google APIs Intel x86_64 Atom
      - android-31 | Google APIs Intel x86_64 Atom
      - android-32 | Google APIs Intel x86_64 Atom
      - android-33 | Google APIs Intel x86_64 Atom
      - android-34 | Google APIs Intel x86_64 Atom
      - android-35 | Google APIs Intel x86_64 Atom
      - android-36.1 | Google Play Intel x86_64 Atom
      - android-36.1 | Pre-Release 16 KB Page Size Google Play Intel x86_64 Atom
      - android-37.0 | Google APIs Intel x86_64 Atom
    Android NDK: Not Found
  Windows SDK: Not Found
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 2421ea7 with one file added, src/__tests__/native/falsy-runtime-values.test.tsx, and install:

   git clone --branch repro/534-falsy-runtime-values https://github.com/YevheniiKotyrlo/react-native-css
   cd react-native-css
   yarn install
   

2. Run yarn jest src/__tests__/native/falsy-runtime-values.test.tsx.

src/__tests__/native/falsy-runtime-values.test.tsx:

import { render, screen } from "@testing-library/react-native";
import { View } from "react-native-css/components/View";
import { registerCSS, testID } from "react-native-css/jest";

test.each([
  ["width", "0px", { width: 0 }],
  ["margin-top", "0px", { marginTop: 0 }],
  ["flex-grow", "0", { flexGrow: 0 }],
  ["z-index", "0", { zIndex: 0 }],
  ["width", "0%", { width: "0%" }],
])("%s: var(--v) over --v: %s renders the value", (property, value, expected) => {
  registerCSS(`.a { --v: ${value}; ${property}: var(--v); }`, { inlineVariables: false });
  render(<View testID={testID} className="a" />);
  expect(screen.getByTestId(testID).props.style).toStrictEqual(expected);
});

Expected: width: var(--v) over --v: 0px sets width: 0, and margin-top, flex-grow and z-index at 0 set 0.

Actual: each sets nothing.

FAIL src/__tests__/native/falsy-runtime-values.test.tsx
  × width: var(--v) over --v: 0px renders the value (23 ms)
  × margin-top: var(--v) over --v: 0px renders the value (5 ms)
  × flex-grow: var(--v) over --v: 0 renders the value (1 ms)
  × z-index: var(--v) over --v: 0 renders the value (1 ms)
  √ width: var(--v) over --v: 0% renders the value (1 ms)
  ● width: var(--v) over --v: 0px renders the value
    expect(received).toStrictEqual(expected) // deep equality
    - Expected  - 3
    + Received  + 1
    - Object {
    -   "width": 0,
    - }
    + Object {}
      12 |   registerCSS(`.a { --v: ${value}; ${property}: var(--v); }`, { inlineVariables: false });
      13 |   render(<View testID={testID} className="a" />);
    > 14 |   expect(screen.getByTestId(testID).props.style).toStrictEqual(expected);
         |                                                  ^
      15 | });
      16 |
      at toStrictEqual (src/__tests__/native/falsy-runtime-values.test.tsx:14:50)
  ● margin-top: var(--v) over --v: 0px renders the value
    expect(received).toStrictEqual(expected) // deep equality
    - Expected  - 3
    + Received  + 1
    - Object {
    -   "marginTop": 0,
    - }
    + Object {}
      12 |   registerCSS(`.a { --v: ${value}; ${property}: var(--v); }`, { inlineVariables: false });
      13 |   render(<View testID={testID} className="a" />);
    > 14 |   expect(screen.getByTestId(testID).props.style).toStrictEqual(expected);
         |                                                  ^
      15 | });
      16 |
      at toStrictEqual (src/__tests__/native/falsy-runtime-values.test.tsx:14:50)
  ● flex-grow: var(--v) over --v: 0 renders the value
    expect(received).toStrictEqual(expected) // deep equality
    - Expected  - 3
    + Received  + 1
    - Object {
    -   "flexGrow": 0,
    - }
    + Object {}
      12 |   registerCSS(`.a { --v: ${value}; ${property}: var(--v); }`, { inlineVariables: false });
      13 |   render(<View testID={testID} className="a" />);
    > 14 |   expect(screen.getByTestId(testID).props.style).toStrictEqual(expected);
         |                                                  ^
      15 | });
      16 |
      at toStrictEqual (src/__tests__/native/falsy-runtime-values.test.tsx:14:50)
  ● z-index: var(--v) over --v: 0 renders the value
    expect(received).toStrictEqual(expected) // deep equality
    - Expected  - 3
    + Received  + 1
    - Object {
    -   "zIndex": 0,
    - }
    + Object {}
      12 |   registerCSS(`.a { --v: ${value}; ${property}: var(--v); }`, { inlineVariables: false });
      13 |   render(<View testID={testID} className="a" />);
    > 14 |   expect(screen.getByTestId(testID).props.style).toStrictEqual(expected);
         |                                                  ^
      15 | });
      16 |
      at toStrictEqual (src/__tests__/native/falsy-runtime-values.test.tsx:14:50)
Test Suites: 1 failed, 1 total
Tests:       4 failed, 1 passed, 5 total
Snapshots:   0 total
Time:        1.126 s, estimated 2 s
Ran all test suites matching /src\\__tests__\\native\\falsy-runtime-values.test.tsx/i.

Reproducible example repository

https://github.com/YevheniiKotyrlo/react-native-css/tree/repro/534-falsy-runtime-values