test(flags): cover experiment bucketing and add changeset

Author: Just-Insane

.changeset/feature-flag-env-vars.md

@@ -0,0 +1,5 @@
+---
+default: minor
+---
+
+Add build-time client config overrides via environment variables, with typed deterministic experiment bucketing helpers for progressive feature rollout and A/B testing.

src/app/hooks/useClientConfig.test.ts

@@ -0,0 +1,101 @@
+import { describe, it, expect } from 'vitest';
+import { selectExperimentVariant, type ExperimentConfig } from './useClientConfig';
+
+const baseExperiment: ExperimentConfig = {
+  enabled: true,
+  rolloutPercentage: 100,
+  controlVariant: 'control',
+  variants: ['alpha', 'beta'],
+};
+
+describe('selectExperimentVariant', () => {
+  it('returns control when experiment is disabled', () => {
+    const result = selectExperimentVariant(
+      'threadUI',
+      { ...baseExperiment, enabled: false },
+      '@alice:example.org'
+    );
+
+    expect(result.inExperiment).toBe(false);
+    expect(result.variant).toBe('control');
+  });
+
+  it('returns control when subject id is missing', () => {
+    const result = selectExperimentVariant('threadUI', baseExperiment, undefined);
+
+    expect(result.inExperiment).toBe(false);
+    expect(result.variant).toBe('control');
+  });
+
+  it('returns control when rollout is 0', () => {
+    const result = selectExperimentVariant(
+      'threadUI',
+      { ...baseExperiment, rolloutPercentage: 0 },
+      '@alice:example.org'
+    );
+
+    expect(result.inExperiment).toBe(false);
+    expect(result.variant).toBe('control');
+    expect(result.rolloutPercentage).toBe(0);
+  });
+
+  it('normalizes rollout less than 0 to 0', () => {
+    const result = selectExperimentVariant(
+      'threadUI',
+      { ...baseExperiment, rolloutPercentage: -10 },
+      '@alice:example.org'
+    );
+
+    expect(result.inExperiment).toBe(false);
+    expect(result.variant).toBe('control');
+    expect(result.rolloutPercentage).toBe(0);
+  });
+
+  it('normalizes rollout greater than 100 to 100', () => {
+    const result = selectExperimentVariant(
+      'threadUI',
+      { ...baseExperiment, rolloutPercentage: 999 },
+      '@alice:example.org'
+    );
+
+    expect(result.inExperiment).toBe(true);
+    expect(result.rolloutPercentage).toBe(100);
+    expect(['alpha', 'beta']).toContain(result.variant);
+  });
+
+  it('falls back to control when variants are missing after filtering', () => {
+    const result = selectExperimentVariant(
+      'threadUI',
+      {
+        ...baseExperiment,
+        variants: ['', 'control'],
+      },
+      '@alice:example.org'
+    );
+
+    expect(result.inExperiment).toBe(false);
+    expect(result.variant).toBe('control');
+  });
+
+  it('is deterministic for the same key and subject', () => {
+    const first = selectExperimentVariant('threadUI', baseExperiment, '@alice:example.org');
+    const second = selectExperimentVariant('threadUI', baseExperiment, '@alice:example.org');
+
+    expect(second).toEqual(first);
+  });
+
+  it('uses default control variant when none is provided', () => {
+    const result = selectExperimentVariant(
+      'threadUI',
+      {
+        enabled: true,
+        rolloutPercentage: 100,
+        variants: ['alpha'],
+      },
+      '@alice:example.org'
+    );
+
+    expect(result.inExperiment).toBe(true);
+    expect(result.variant).toBe('alpha');
+  });
+});