Introducing @mockapi/msw, mock an API server for your entities without writing any code with Mock Service Worker

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Jan 14

MockAPI

The problem

Inspired by Kent C Dodds's blog post Stop mocking fetch, I started to
use Mock Service Worker to mock an API server both for unit testing with jest and for the
Browser.

However, just like developing a real API server, writing every endpoint is tedious, and it can get messy very quickly.

@mockapi/msw was born to solve this problem. With @mockapi/msw, you can mock a whole set of standard CRUD endpoints
for any entity without any code. All you have to do is configure your base URL and wire it up with msw.

Endpoints Provided

GET: baseUrl/entity, get all entities
GET: baseUrl/entity/:id, get an entity by id
POST: baseUrl/entity, create an entity
PUT: baseUrl/entity/:id, update an entity
DELETE: baseUrl/entity/:id, delete an entity

You can override the above endpoints by providing handlers with identical signatures.

You can also build your own set of CRUD endpoints by building on top of or replacing the handlerfactory(introduced
below) using the provided repository methods for data access.

You can extend the endpoints by wiring up your endpoints.

Setup

Install `@mockapi/msw`

yarn add --dev @mockapi/msw

npm install --dev @mockapi/msw

Configure `@mockapi/msw`

1. Create a typescript file with the following content:

// server.ts
import {configMockApi} from '@mockapi/msw';

export const baseUrl = 'http://localhost:5000/api/v1'; // change to your own base url

export const {handlerFactory, repositoryFactory, clearAllData} = configMockApi({
  baseUrl: baseUrl,
});

handlerFactory is the function to generate and wire up all the endpoints.
repositoryFactory is the function to access mocked data in your custom endpoints and tests.
configMockApi takes a store of type Store<T extends BaseEntity> as an argument to configure what storage the mocked data will be stored. By default, it uses localStorage. You can also use sessionStorage or IndexedDB to store the data.

// Store.ts
export type Store<T extends BaseEntity> = {
  setItem: (key: string, value: T[]) => void;
  getItem: (key: string) => T[] | null;
  clearAll: () => void;
};

// BaseEntity.ts
export interface BaseEntity extends Record<string, unknown> {
  id: string | number;
}

2. Create your mock entity:

// hero.ts
import {repositoryFactory} from '../server/server';

export type Hero = {
  id: string;
  name: string;
};

export const heroKey = 'Hero';

export const heroSeeds: Hero[] = [
  {
    id: '1',
    name: 'Iron Man',
  },
  {
    id: '2',
    name: 'Spider Man',
  },
  {
    id: '3',
    name: 'Ant Man',
  },
];

export function seedHeroes() {
  const repository = repositoryFactory<Hero>(heroKey);
  repository.seed(heroSeeds);
}

3. Optional: extend endpoints and override provided endpoints. You can skip this step if you don't need to extend or override the endpoints.

The handlerFactory takes a second parameter of type RestHandler[], which will take precedence over the provided
endpoints.

// heroHandlers.ts
import { errorResponseFactory } from '@mockapi/msw';

import { Hero, heroKey } from './hero';
import { rest } from 'msw';
import { baseUrl, repositoryFactory } from '../server/server';

const repository = repositoryFactory<Hero>(heroKey);

export const heroHandlers = [
  rest.get(`${baseUrl}/${heroKey}`, (req, res, ctx) => {
    const { data: items = [], error } = repository.getItems();
    if (error) {
      return errorResponseFactory(res, ctx)(error);
    }
    const search = req.url.searchParams.get('search');
    const result = items.filter((hero) =>
      search ? hero.name.toLowerCase().includes(search?.toLowerCase()) : true
    );

    return res(ctx.status(200), ctx.json(result));
  }),

  rest.get(`${baseUrl}/${heroKey}/getByName/:name`, (req, res, ctx) => {
    const { data: items = [], error } = repository.getItems();
    if (error) {
      return errorResponseFactory(res, ctx)(error);
    }

    const { name } = req.params;

    const result = items.find((hero) => hero.name === name);

    return res(ctx.status(200), ctx.json(result));
  }),
];

4. Wire up with Mock Service Work for `jest`. For browser integration, read the msw doc here.

//starup.ts
import {setupServer} from 'msw/node';
import {
  heroHandlers,
  heroKey,
  seedHeroes
} from '../entity';
import {clearAllData, handlerFactory} from './server';

const handlers = [
  ...handlerFactory(heroKey, heroHandlers),
  // other endpoints, for example:
  //...handlerFactory(todoKey),
];

export const server = setupServer(...handlers);

beforeAll(() => server.listen());

beforeEach(() => {
  seedHeroes();
  // other data seeding, for example:
  //seedTodos();
});

afterEach(() => {
  clearAllData();
  server.resetHandlers();
});

afterAll(() => server.close());

5. Wire up with `jest`

//jest.setup.ts
import './src/lib/server/startup';

//jest.config.js
module.exports = {
  //other configurations
  //...
  setupFilesAfterEnv: ['./jest.setup.ts']
};

Usage

If set up correctly, you can use the mock data in your tests like the below example. For more examples, please check
out /packages/tests.

// getAll.spec.ts
import {baseUrl} from '../server/server';
import {todoKey, todoSeeds} from '../entities';
import axios from 'axios';

describe('Get Items Endpoint', function () {
  it('should get todos', async function () {
    const data = await axios.get(`${baseUrl}/${todoKey}`);
    expect(data.data).toMatchObject(todoSeeds);
  });
});

Error Response

The best way to handle errors is to override the target endpoint using server.use in the tests. For more details, read the doc here

Repository

The library provides a repository for each entity, the interface is as follows:

export type Repository<T extends BaseEntity> = {
  getItems: () => Result<T[]>;
  getItemById: (id: string | number) => Result<T>;
  addItem: (item: T) => Result<never>;
  deleteItem: (id: string | number) => Result<never>;
  editItem: (item: T) => Result<never>;
  seed: (items: T[]) => void;
};

Result

Most of the repository methods return a Result<T> type, which contains either the data or the error.

This Result is originated from Valdimir Kudinov's Result class described here: Functional C#: Handling failures, input errors.

export interface Result<T> {
  error?: { statusCode: number; message: string };
  data?: T;
}

To get a repository for an entity:

const repository = repositoryFactory<Hero>(heroKey);