In Jest, how do you mock a TypeScript class with dependencies?

user_6451457 0
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Given this relatively contrived class that gets a single record from Elasticsearch (or any datastore for that matter):

export default class UserRepoImpl implements UserRepo {
  constructor(private readonly esClient: ElasticsearchClient) {}

  public async getUser(id: string): Promise<User> {
    const response = await this.esClient.request('GET', `/users/_doc/${id}`);

    return response._source;
  }
}

How do I write a test that will instantiate a UserRepoImpl for me but insert a mocked ElasticsearchClient as the argument to the constructor?

ts-jest jestjs typescript
2 Answers

You can create a mock client within your test like this:

const client: jest.Mocked<ElasticsearchClient> = {
  request: jest.fn()
}

which will statically check the type of the resolved value when passed:

example test:

describe('UserRepoImpl', () => {
  describe('given a client', () => {
    const client: jest.Mocked<ElasticsearchClient> = {
      request: jest.fn()
    }
    describe('getUser', () => {
      beforeAll(() => {
        client.request.mockResolvedValue({
          _source: {
            id: '3'
          }
        })
      })

      it('should return user by Id', async () => {
        const userRepo = new UserRepoImpl(client)
        await expect(userRepo.getUser("3")).resolves.toEqual({
          id: '3'
        })
      })
    })
  })
})
user_6373670

Here is the unit test solution:

userRepoImpl.ts:

interface User {}
interface UserRepo {
  getUser(id: string): Promise<User>;
}
interface ElasticsearchClient {
  request(method: string, endpoint: string): Promise<{ _source: any }>;
}
export default class UserRepoImpl implements UserRepo {
  constructor(private readonly esClient: ElasticsearchClient) {}

  public async getUser(id: string): Promise<User> {
    const response = await this.esClient.request('GET', `/users/_doc/${id}`);

    return response._source;
  }
}

userRepoImpl.test.ts:

import UserRepoImpl from './userRepoImpl';

describe('62603645', () => {
  it('should pass', async () => {
    const mUser = { name: 'wen' };
    const mEsClient = { request: jest.fn().mockResolvedValueOnce({ _source: mUser }) };
    const userRepoImpl = new UserRepoImpl(mEsClient);
    const actual = await userRepoImpl.getUser('1');
    expect(actual).toEqual(mUser);
    expect(mEsClient.request).toBeCalledWith('GET', '/users/_doc/1');
  });
});

unit test result with coverage report:

 PASS  stackoverflow/62603645/userRepoImpl.test.ts (10.988s)
  62603645
    ✓ should pass (5ms)

-----------------|---------|----------|---------|---------|-------------------
File             | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s 
-----------------|---------|----------|---------|---------|-------------------
All files        |     100 |      100 |     100 |     100 |                   
 userRepoImpl.ts |     100 |      100 |     100 |     100 |                   
-----------------|---------|----------|---------|---------|-------------------
Test Suites: 1 passed, 1 total
Tests:       1 passed, 1 total
Snapshots:   0 total
Time:        12.27s
user_64635580
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