feature-flags

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());

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

Adding a New Flag Add to ReactFeatureFlags.js with default value Add to each fork file (.www.js, .native-fb.js, etc.) If it should vary in www/RN, set to VARIANT in the fork file 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 # VARIANT = true /test www variant false # VARIANT = false

Debugging Channel-Specific Failures Run /flags --diff to compare values Check @gate conditions - test may be gated to specific channels Run /test to isolate the failure 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')