如何用玩笑测试异步等待 pg 连接？

Question

I am trying to test a function which makes a connection to pg, using async/await ,

import pg from 'pg';

module.exports.test = async (event, context, callback) => {
  const client = new pg.Client(someConnectionString);
  try {
    await client.connect();
  } catch (e) {
    return callback(e);
  }
  try {
    await client.query(await someAsyncFunction(test));
    client.end();
    return callback(null, 'success');
  } catch (e) {
    client.end();
    return callback(e);
  }
};

Don't really understand how I would mock this using jest? Any ideas?

Answer 1

first of all, in jest you can return a promise to test async code:

import pg from 'pg';

module.exports.test = async (event, context) => {
  const client = new pg.Client(someConnectionString);
  try {
    await client.connect();
    await client.query(await someAsyncFunction(test));
  } finally {
    client.end();
  }
};

As any async function returns a promise you should not have to handle manually passing the error to a callback.

Now to the question about how to mock pg, you can create a fake pg object, but that's rarely ideal, I'll just show an example to answer the actual question.

You should try to setup a test db before running your tests, and then clean it up after, this way you don't have to mock anything and can use the real pg this would improve a lot the quality of the tests.

// mock example:
class MockClient {
  async connect() {
  }
  async query() {
  }
}

Answer 2

Here is the unit test solution:

index.js :

import pg from 'pg';
import { someAsyncFunction } from './someAsyncFunction';

const someConnectionString = 'someConnectionString';
const test = 'test';

module.exports = async (event, context, callback) => {
  const client = new pg.Client(someConnectionString);
  try {
    await client.connect();
  } catch (e) {
    return callback(e);
  }
  try {
    await client.query(await someAsyncFunction(test));
    client.end();
    return callback(null, 'success');
  } catch (e) {
    client.end();
    return callback(e);
  }
};

someAsyncFunction.js :

export async function someAsyncFunction() {}

index.test.js :

import pg from 'pg';
import fn from './';
import { someAsyncFunction } from './someAsyncFunction';

jest.mock('./someAsyncFunction', () => {
  return { someAsyncFunction: jest.fn() };
});
jest.mock('pg', () => {
  const mClient = { connect: jest.fn(), query: jest.fn(), end: jest.fn() };
  return { Client: jest.fn(() => mClient) };
});

describe('46152048', () => {
  afterEach(() => {
    jest.clearAllMocks();
  });
  afterAll(() => {
    jest.resetAllMocks();
  });
  it('should query success', async (done) => {
    someAsyncFunction.mockResolvedValueOnce('select 1;');
    const mClient = new pg.Client();
    mClient.connect.mockResolvedValueOnce();
    await fn({}, {}, (err, result) => {
      expect(pg.Client).toBeCalledWith('someConnectionString');
      expect(someAsyncFunction).toBeCalledWith('test');
      expect(mClient.query).toBeCalledWith('select 1;');
      expect(mClient.end).toBeCalledTimes(1);
      expect(err).toBeNull();
      expect(result).toBe('success');
      done();
    });
  });

  it('should handle error if connect database failed', async () => {
    const mError = new Error('network');
    const mClient = new pg.Client();
    mClient.connect.mockRejectedValueOnce(mError);
    await fn({}, {}, (err, result) => {
      expect(err.message).toBe('network');
      expect(result).toBeUndefined();
    });
  });

  it('should handle error if query failed', async () => {
    const mError = new Error('network');
    const mClient = new pg.Client();
    mClient.connect.mockResolvedValueOnce();
    mClient.query.mockRejectedValueOnce(mError);
    await fn({}, {}, (err, result) => {
      expect(err.message).toBe('network');
      expect(mClient.end).toBeCalledTimes(1);
      expect(result).toBeUndefined();
    });
  });
});

unit test result with coverage report:

 PASS  src/stackoverflow/46152048/index.test.js
  46152048
    ✓ should query success (11ms)
    ✓ should handle error if connect database failed (1ms)
    ✓ should handle error if query failed (1ms)

----------|----------|----------|----------|----------|-------------------|
File      |  % Stmts | % Branch |  % Funcs |  % Lines | Uncovered Line #s |
----------|----------|----------|----------|----------|-------------------|
All files |      100 |      100 |      100 |      100 |                   |
 index.js |      100 |      100 |      100 |      100 |                   |
----------|----------|----------|----------|----------|-------------------|
Test Suites: 1 passed, 1 total
Tests:       3 passed, 3 total
Snapshots:   0 total
Time:        4.103s, estimated 10s

source code: https://github.com/mrdulin/jest-codelab/tree/master/src/stackoverflow/46152048