Feature Flags + Kill Switch

HOW IT IS WIRED

Real code from the GAS template

The code below is drawn from lib/feature-flags.ts in the gas-template repository. This is the code your generated app gets, not pseudocode, not a description of intent.

// lib/feature-flags.ts (gas-template) -- hook + evaluation core

const TTL_MS = 60_000; // re-poll once a minute

// Deterministic per-(userId, key) bucket: stable across re-renders and refreshes.
function bucket(userId: string, key: string): number {
  const cacheKey = `${userId}:${key}`;
  const cached = bucketCache.get(cacheKey);
  if (cached !== undefined) return cached;
  const value = assignBucket(userId, key, 100);
  bucketCache.set(cacheKey, value);
  return value;
}

// Evaluation order: kill_all -> enabled -> country -> user_segment -> rollout%
function resolveFlag(key: string, defaultValue: boolean): boolean {
  const killAll = snapshot['kill_all'];
  if (killAll?.enabled === true) return false; // kill switch: everything off
  const row = snapshot[key];
  if (!row) return defaultValue;
  const uid = userCtx?.userId ?? (deviceId ?? 'anonymous');
  return evaluate(row, uid, userCtx?.country, userCtx?.segment);
}

/** React hook -- subscribes to snapshot, re-renders on background polls. */
export function useFlag(key: string, defaultValue = false): boolean {
  const [value, setValue] = useState<boolean>(() => resolveFlag(key, defaultValue));
  useEffect(() => {
    let cancelled = false;
    ensureFresh().then(() => {
      if (!cancelled) setValue(prev => {
        const next = resolveFlag(key, defaultValue);
        return prev === next ? prev : next;
      });
    }).catch(() => { /* silent */ });
    const onUpdate = () => {
      if (!cancelled) setValue(prev => {
        const next = resolveFlag(key, defaultValue);
        return prev === next ? prev : next;
      });
    };
    listeners.add(onUpdate);
    return () => { cancelled = true; listeners.delete(onUpdate); };
  // eslint-disable-next-line react-hooks/exhaustive-deps
  }, [key]);
  return value;
}

Source: goodspeed-apps/gas-template → lib/feature-flags.ts