spy.onCall(...args) for mocking based on arguments

[gkiely]

This is a suggestion to add a new spyOn method https://bun.sh/docs/test/mocks#spyon.

This would allow developers to mock functions based on specific arguments or paths.

This would be particularly useful for mocking fetch calls but could be useful elsewhere.

Currently this is done by using spy.mockImplementation() to overwrite the function and check the parameters or spy.mockResolvedValue(). The downside of mockImplementation is that it requires boilerplate. The downside of mockResolvedValue is that it will fire regardless of the arguments.

There is no related API in jest or vitest as far as I could tell. It is just something I am hoping for :)

Example:

const spy = spyOn(global, 'fetch');

// Mock specific URL for access token
spy.onCall('https://api.example.com/oauth/token')
.mockResolvedValueOnce(new Response(JSON.stringify({ access_token: 'mock_token' }), {
  status: 200,
  headers: { 'Content-Type': 'application/json' },
}));

// Mock another specific URL for user data
spy.onCall('https://api.example.com/users/123')
.mockResolvedValueOnce(new Response(JSON.stringify({ id: 123, name: 'Test User' }), {
  status: 200,
  headers: { 'Content-Type': 'application/json' },
}));

Example using mockImplementationOnce

spy.mockImplementationOnce(async (url: RequestInfo) => {
  if (url === 'https://api.example.com/oauth/token') {
    return new Response(JSON.stringify({ access_token: 'mock_token_for_path_1' }), {
      status: 200,
      headers: { 'Content-Type': 'application/json' },
    });
  }
  return new Response('Not Found', { status: 404 });
}

spy.mockImplementationOnce(async (url: RequestInfo) => {
  if (url === 'https://api.example.com/users/123') {
    return new Response(JSON.stringify({ access_token: 'mock_token_for_path_1' }), {
      status: 200,
      headers: { 'Content-Type': 'application/json' },
    });
  }
  return new Response('Not Found', { status: 404 });
}

Alternative name: withArgs

spy.withArgs('url').mockResolvedValue(...)

[gkiely]

Userland solution (fetch only):

const mockFetch = <T = unknown>(
  spy: Mock<typeof fetch>,
  url: string,
  data: T,
  options: ResponseInit = { status: 200 }
) => {
  const fn = (requestUrl: URL | RequestInfo) =>
    requestUrl === url
      ? Promise.resolve(new Response(JSON.stringify(data), options))
      : Promise.resolve(new Response('Not Found', { status: 404 }));
  fn.preconnect = () => {};
  return spy.mockImplementationOnce(fn);
};