@tinkoff/mocker

A server and a middleware for API mocking.

Installation

Install @tinkoff/mocker:

npm install @tinkoff/mocker

Add your first mock to file mocks/first-api.js:

module.exports = {
  api: 'first-api',
  mocks: {
    'GET /endpoint': {
      status: 200,
      headers: {},
      payload: 'mocked response',
    },
  },
};

Set up mocker in your project:

import { Mocker, FileSystemMockRepository } from '@tinkoff/mocker';

// Repository will read mocks from directory `mocks` relative from current dir
const repository = new FileSystemMockRepository({ cwd: process.cwd(), root: 'mocks' });
// Mocker to all of the request on `/first-api/...` will response with mock found in fs or with proxying request to the source API
const options = {
  apis: {
    'first-api': {
      target: 'https://real-first-api.com/',
    },
  },
  passUnhandledRequests: true,
};

const mocker = new Mocker({ repositories: [repository], logger: console, ...options });

(async () => {
  // with this call mocker reads mocks from repository and creates according routes
  await mocker.init();

  mocker.start(4000, () => {
    console.log('Mocker is running at 4000 port');
  });
})();

Now we can make a GET request to mocker

(async () => {
  const response = await fetch('http://localhost:4000/first-api/endpoint');
  const data = await response.json();

  console.log(data); // "mocked response"
})();

Explanation

Library is based on express.

mocker can be used as standalone server or as a middleware for existing server through call mocker.use(req, res).

FileSystemMockRepository supports mock in js и json formats. js mocks are able to define custom express handler as a mock handler.

For choosing right mock for request next parameters of the request are considered: method, url and query.

Mocker supports proxying requests to API that allows to mock only part of the API and not a whole backend

Routing

If mocker is running as a standalone server, for example on 4000 port, then it will be accessible at url http://localhost:4000/.

For every api from settings options.apis will be created a nester router, e.g. for first-api it will be http://localhost:4000/first-api/.

Routes for api are getting created based on mocks, in which key is a method + url of the request, e.g. mock GET /endpoint will be accessible at http://localhost:4000/first-api/endpoint for GET requests.

FileSystem mocks

Mocker reads mocks using Repository, that allows to store mocks in FileSystem or on the another server.

FileSystemMockRepository works with FileSystem and supports next kinds of mocks:

mock.json

{
  "api": "first-api",
  "mocks": {
    "GET /foo": {
      "status": 200,
      "headers": {},
      "payload": {
        "fake": "true"
      }
    }
  }
}

mock.js

module.exports = {
  api: 'first-api',
  mocks: {
    'GET /bar': {
      status: 200,
      headers: {},
      payload: {
        fake: 'true',
      },
    },
    'POST /bar': (req, res) => {
      res.status(200);
      res.set('X-Mock-Server', 'true');
      res.json({ fake: 'true' });
    },
  },
};

API

Mocker

interface Mocker {
  new (params: { options: MockerOptions; repository: MockRepository; logger: Logger }): Mocker;

  init(): Promise<void>;

  update(): Promise<void>;

  use(req: IncomingMessage, res: ServerResponse): express.Express;

  start(port: number, callback?: (...args: any[]) => void): Server;
}

Mocker.init - resolve mocks using MockRepository, routing initialization. Must be called before using server with mocks.

Mocker.update - updating mocks using MockRepository, routing update. Might be called in runtime.

Mocker.use - express middleware. Might be used for adding mocker on the existing server.

Mocker.start - run mocker as a standalone http-server.

MockerOptions

interface MockerOptions {
  apis: Record<string, { target: string }>;
  passUnhandledRequests?: boolean;
  apiRoutePrefix?: string;
}

MockerOptions.apis - list of APIs for mocking, key target points to the source API.

MockerOptions.passUnhandledRequests - when enabled, all of the request without according mock will be proxied to the target, otherwise fail the request.

MockerOptions.apiRoutePrefix - if mocker is used in existing server on nested route, e.g. /mocker, this option might be used to pass this route as apiRoutePrefix for proper routing.

MockRepository

interface MockRepository {
  get(api: string, endpoint: string): Promise<Mock>;

  getAll(api: string): Promise<Record<string, Mock>>;

  add(api: string, endpoint: string, mock: Mock): Promise<void>;

  delete(api: string, endpoint: string): Promise<void>;
}

MockRepository.getAll - get all mocks for specified API.

MockRepository.get - get specific mock for specific API.

MockRepository.add - add new mock for specific API.

MockRepository.delete - remove specific mock for specific API.

How to

How to mock request with specific query parameters?

Mocker allows to specify query parameters for mocks.

In the example below request to /endpoint?foo=bar will be mocked with first mock, and request to /endpoint?foo=baz will be mocked with second. All of the other requests with\without query will be proxied to source API (if passUnhandledRequests is enabled).

module.exports = {
  api: 'api',
  mocks: {
    'GET /endpoint?foo=bar': {
      status: 200,
      headers: {},
      payload: 'mocked bar response',
    },
    'GET /endpoint?foo=baz': {
      status: 200,
      headers: {},
      payload: 'mocked baz response',
    },
  },
};

How to proxy specific request to source API?

It might be useful if option passUnhandledRequests is disabled. In that case you may pass option pass: true to mock:

module.exports = {
  api: 'api',
  mocks: {
    'ALL /endpoint': {
      pass: true,
    },
  },
};