school_compare/nextjs-app/__tests__/lib/flags.test.ts

import { getFlags, FLAGS_REVALIDATE } from '@/lib/flags';

// jsdom provides no global fetch, so there is nothing for jest.spyOn to attach
// to — assign it and restore the original afterwards. This is the first test
// here to mock fetch; later ones should follow this shape.
const realFetch = global.fetch;

function mockFetch(impl: () => Promise<unknown>) {
  global.fetch = jest.fn(impl) as unknown as typeof fetch;
}

describe('getFlags', () => {
  afterEach(() => { global.fetch = realFetch; });

  it('returns the flags the API reports', async () => {
    mockFetch(async () => ({
      ok: true,
      json: async () => ({ admission_distance: true }),
    }));
    await expect(getFlags()).resolves.toEqual({ admission_distance: true });
  });

  it('returns no flags rather than throwing when the API is down', async () => {
    // A page that cannot read flags must render everything dark, not 500.
    // Fail-closed is the same direction as the backend's default.
    mockFetch(async () => { throw new Error('ECONNREFUSED'); });
    await expect(getFlags()).resolves.toEqual({});
  });

  it('returns no flags rather than throwing on a non-200', async () => {
    mockFetch(async () => ({ ok: false, status: 503 }));
    await expect(getFlags()).resolves.toEqual({});
  });

  /*
   * Reading a flag pins the calling route's ISR floor: Next uses the LOWEST
   * revalidate among a route's fetches for the whole route. That is why the
   * revalidate is an argument rather than the constant.
   *
   * Every SEO route here declares `revalidate = 604800`. A gate that read
   * flags at the 300s default would drop the whole school and place corpus
   * from a weekly cache to a 5-minute one, which is a large origin-load
   * regression to pay for a feature flag.
   */
  it('reads at the 300s floor by default', async () => {
    mockFetch(async () => ({ ok: true, json: async () => ({}) }));
    await getFlags();
    expect((global.fetch as jest.Mock).mock.calls[0][1])
      .toEqual({ next: { revalidate: FLAGS_REVALIDATE } });
  });

  it('lets a caller pass its own route floor instead', async () => {
    mockFetch(async () => ({ ok: true, json: async () => ({}) }));
    await getFlags(604800);
    expect((global.fetch as jest.Mock).mock.calls[0][1])
      .toEqual({ next: { revalidate: 604800 } });
  });
});