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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')