Recoil.js selector family that allows for equality checking

equals-selector-family.tsx

function equalSelectorFamily<T, P extends SerializableParam>(
  options: EqualSelectorFamilyOptions<T, P>
) {
  const inner = selectorFamily<T, P>({
    key: `${options.key}_inner`,
    get: options.get,
  });

  const priorValues: Map<P, T | undefined> = new Map();

  return selectorFamily<T, P>({
    ...options,
    key: options.key,
    get:
      (param: P) =>
      ({ get }) => {
        const latest = get(inner(param));
        const prior = priorValues.get(param);
        if (prior != null && options.equals(latest, prior)) {
          return prior;
        }
        priorValues.set(param, latest);
        return latest;
      },
  });
}