Class-Based Header Assertion Spy for Next.js Middleware Testing

class HeaderSpy {
  #captured = new Map();
 
  record(request) {
    for (const [key, value] of request.headers.entries()) {
      this.#captured.set(key.toLowerCase(), value);
    }
  }
 
  assertHeaderEquals(key, expected) {
    const actual = this.#captured.get(key.toLowerCase());
    if (actual !== expected) {
      throw new Error(`Header mismatch: expected ${expected}, got ${actual}`);
    }
    return true;
  }
}
 
export function testMiddlewareHeaders(middlewareFn, mockReq) {
  const spy = new HeaderSpy();
  const res = middlewareFn(mockReq);
  spy.record(res);
  return spy.assertHeaderEquals('x-custom-tenant', 'acme-prod');
}