/feature-flags

name: feature-flags
description: Use when feature flag tests fail, flags need updating, understanding @gate pragmas, debugging channel-specific test failures, or adding new flags to React.

React Feature Flags

Flag Files

| File | Purpose | |------|---------| | `packages/shared/ReactFeatureFlags.js` | Default flags (canary), `__EXPERIMENTAL__` overrides | | `packages/shared/forks/ReactFeatureFlags.www.js` | www channel, `__VARIANT__` overrides | | `packages/shared/forks/ReactFeatureFlags.native-fb.js` | React Native, `__VARIANT__` overrides | | `packages/shared/forks/ReactFeatureFlags.test-renderer.js` | Test renderer |

Gating Tests

`@gate` pragma (test-level)

Use when the feature is completely unavailable without the flag:

// @gate enableViewTransition
it('supports view transitions', () => {
  // This test only runs when enableViewTransition is true
  // and is SKIPPED (not failed) when false
});

`gate()` inline (assertion-level)

Use when the feature exists but behavior differs based on flag:

it('renders component', async () => {
  await act(() => root.render(<App />));

  if (gate(flags => flags.enableNewBehavior)) {
    expect(container.textContent).toBe('new output');
  } else {
    expect(container.textContent).toBe('legacy output');
  }
});

Adding a New Flag

1. Add to `ReactFeatureFlags.js` with default value 2. Add to each fork file (`*.www.js`, `*.native-fb.js`, etc.) 3. If it should vary in www/RN, set to `__VARIANT__` in the fork file 4. Gate tests with `@gate flagName` or inline `gate()`

Checking Flag States

Use `/flags` to view states across channels. See the `flags` skill for full command options.

`__VARIANT__` Flags (GKs)

Flags set to `__VARIANT__` simulate gatekeepers - tested twice (true and false):

/test www <pattern>              # __VARIANT__ = true
/test www variant false <pattern> # __VARIANT__ = false

Debugging Channel-Specific Failures

1. Run `/flags --diff <channel1> <channel2>` to compare values 2. Check `@gate` conditions - test may be gated to specific channels 3. Run `/test <channel> <pattern>` to isolate the failure 4. Verify flag exists in all fork files if newly added

Common Mistakes

• **Forgetting both variants** - Always test `www` AND `www variant false` for `__VARIANT__` flags
• **Using @gate for behavior differences** - Use inline `gate()` if both paths should run
• **Missing fork files** - New flags must be added to ALL fork files, not just the main one
• **Wrong gate syntax** - It's `gate(flags => flags.name)`, not `gate('name')`