Mocking Segment Analytics with Jest in TS

Question

I'm trying to write a unit test that checks if the .track method of the Analytics is called. For some reason the test keeps failing, although calling the function via http does trigger the call. I'm not sure if I've mocked it wrong, or what the issue could be?

index.ts:

import { Request } from "../types"
import { getSecret } from "../src/secrets"
import Analytics from "analytics-node"

const logger = (req: Request) => {
    const analytics = new Analytics(<string>process.env.WRITE_KEY);
    return analytics.track({
        userId: req.userId
    });
}

export default logger

index.test.ts:

jest.mock('analytics-node');
import { Request } from "../types"
import logger from "./index"
import Analytics from "analytics-node"

const mockAnalytics = new Analytics(process.env.WRITE_KEY = 'test');

describe('Logger tests', () => {
    it(`Should call analytics.track`, () => {
        const request: Request = {
            userId: 23
        }
        return logger(request).then(() => {
            expect(mockAnalytics.track).toHaveBeenCalled()
        });
    });
});

Answer 1

You are using Automatic mock by calling jest.mock('analytics-node').

Calling jest.mock('analytics-node') returns a useful "automatic mock" you can use to spy on calls to the class constructor and all of its methods. It replaces the ES6 class with a mock constructor and replaces all of its methods with mock functions that always return undefined. Method calls are saved in theAutomaticMock.mock.instances[index].methodName.mock.calls.

E.g.

index.ts:

import Analytics from 'analytics-node';

export interface Request {
  userId: string | number;
}

const logger = (req: Request) => {
  const analytics = new Analytics(<string>process.env.WRITE_KEY);
  return analytics.track({
    userId: req.userId,
    anonymousId: 1,
    event: '',
  });
};

export default logger;

index.test.ts:

import logger, { Request } from './';
import Analytics from 'analytics-node';

jest.mock('analytics-node');
const mockAnalytics = Analytics as jest.MockedClass<typeof Analytics>;

describe('Logger tests', () => {
  afterAll(() => {
    jest.resetAllMocks();
  });
  it(`Should call analytics.track`, () => {
    const WRITE_KEY = process.env.WRITE_KEY;
    process.env.WRITE_KEY = 'test key';
    const request: Request = {
      userId: 23,
    };
    logger(request);
    expect(mockAnalytics).toBeCalledWith('test key');
    expect(mockAnalytics.mock.instances[0].track).toHaveBeenCalled();
    process.env.WRITE_KEY = WRITE_KEY;
  });
});

unit test result:

 PASS  examples/65412302/index.test.ts
  Logger tests
    ✓ Should call analytics.track (4 ms)

----------|---------|----------|---------|---------|-------------------
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:       1 passed, 1 total
Snapshots:   0 total
Time:        5.954 s