basic-useswr.ts

import { useState, useEffect } from 'preact/hooks';

const cache = new Map<string, any>();
const inflight = new Map<string, Promise<any>>();

export interface SWRResponse<T> {
  data: T | undefined;
  error: Error | null;
  loading: boolean;
}

export function useSWR<T>(key: string | null, fetcher: (key: string) => Promise<T>): SWRResponse<T> {
  const [data, setData] = useState<T | undefined>(cache.get(key ?? ''));
  const [error, setError] = useState<Error | null>(null);
  const [loading, setLoading] = useState(!cache.has(key ?? ''));

  useEffect(() => {
    if (!key) return;
    
    (async () => {
      if (inflight.has(key)) {
        try {
          const result = await inflight.get(key);
          setData(result);
          setError(null);
        } catch (err) {
          setError(err as Error);
        } finally {
          setLoading(false);
        }
        return;
      }

      setLoading(true);
      const promise = fetcher(key);
      inflight.set(key, promise);

      try {
        const result = await promise;
        cache.set(key, result);
        setData(result);
        setError(null);
      } catch (err) {
        setError(err as Error);
      } finally {
        setLoading(false);
        inflight.delete(key);
      }
    })();
  }, [key]);

  return { data, error, loading };
}