index.ts

let mswEnabled = false;
const customFetch = async (uri: string, options: RequestInit) => {
  if (typeof window !== 'undefined' && !mswEnabled) {
    const { worker } = await import('@/mocks/worker');
    await worker.start();
    mswEnabled = true;
  }

  return fetch(uri, options);
};

const httpLinkOption = {
  uri: URL_YOU_WANT_TO_MOCK,
  ...(process.env.NEXT_PUBLIC_USE_MOCK && { fetch: customFetch }),
};

const apiLink: ApolloLink = new HttpLink(httpLinkOption);

...
...
return new ApolloClient({
    link: apiLink,
    cache,
    ...
    ...
  });