Trying to learn how to write tests, how do I write when I have a promise?

Question

I created a function that connects to an API and returns a promise.. I wanted to test that function so created a mock function in Jest to handle it, however I am not really sure I am doing this right and I am finding it very hard to find good resources on how to go about writing good unit tests..

export const sbConnect = (userId) => {
  return new Promise((resolve, reject) => {
    const sb = new SendBird({appId: APP_ID});
    sb.connect(userId, API_Token, (user, error) => {
      if (error) {
        reject(error);
      } else {
        resolve(user);
      }
    });
  });
};

This is the function I am trying to test. I have created a test and tried this so far..

import {sbConnect} from '../helpers/sendBirdSetupActions';

const SendBird = jest.fn();

describe('Connects to SendBird API', () => {
  it('Creates a new SB instance', () => {
    let userId = 'testUser';
    let APP_ID = 'Testing_App_ID_001';
    sbConnect(userId);
    expect(SendBird).toBeCalled();
  });
});

Answer 1

You can use jest.mock(moduleName, factory, options) to mock sendbird module manually.

Mock SendBird constructor, instance, and its methods.

E.g.

index.ts:

import SendBird from 'sendbird';

const API_Token = 'test api token';
const APP_ID = 'test api id';

export const sbConnect = (userId) => {
  return new Promise((resolve, reject) => {
    const sb = new SendBird({ appId: APP_ID });
    sb.connect(userId, API_Token, (user, error) => {
      if (error) {
        reject(error);
      } else {
        resolve(user);
      }
    });
  });
};

index.test.ts:

import { sbConnect } from './';
import SendBird from 'sendbird';

const mSendBirdInstance = {
  connect: jest.fn(),
};
jest.mock('sendbird', () => {
  return jest.fn(() => mSendBirdInstance);
});

describe('65220363', () => {
  afterAll(() => {
    jest.resetAllMocks();
  });
  it('should get user', async () => {
    const mUser = { name: 'teresa teng' };
    mSendBirdInstance.connect.mockImplementationOnce((userId, API_Token, callback) => {
      callback(mUser, null);
    });
    const actual = await sbConnect('1');
    expect(actual).toEqual(mUser);
    expect(SendBird).toBeCalledWith({ appId: 'test api id' });
    expect(mSendBirdInstance.connect).toBeCalledWith('1', 'test api token', expect.any(Function));
  });

  it('should handle error', async () => {
    const mError = new Error('network');
    mSendBirdInstance.connect.mockImplementationOnce((userId, API_Token, callback) => {
      callback(null, mError);
    });
    await expect(sbConnect('1')).rejects.toThrow(mError);
    expect(SendBird).toBeCalledWith({ appId: 'test api id' });
    expect(mSendBirdInstance.connect).toBeCalledWith('1', 'test api token', expect.any(Function));
  });
});

unit test result:

 PASS  src/stackoverflow/65220363/index.test.ts (11.115s)
  65220363
    ✓ should get user (6ms)
    ✓ should get user (3ms)

----------|----------|----------|----------|----------|-------------------|
File      |  % Stmts | % Branch |  % Funcs |  % Lines | Uncovered Line #s |
----------|----------|----------|----------|----------|-------------------|
All files |      100 |      100 |      100 |      100 |                   |
 index.ts |      100 |      100 |      100 |      100 |                   |
----------|----------|----------|----------|----------|-------------------|
Test Suites: 1 passed, 1 total
Tests:       2 passed, 2 total
Snapshots:   0 total
Time:        12.736s

source code: https://github.com/mrdulin/jest-codelab/tree/master/src/stackoverflow/65220363