节点 + 笑话 + mocking 事件处理程序回调

Question

I have a function in my Node.js code which I am trying to test but I want to mock out processStreamData :

async functionToTest(stream) {
  // Run some checks on the stream here
  stream.on('data', async (data) => {
    // Run some checks on the data here
    await this.processStreamData(data);
  }).on('finish', () => {
    // Do some processing here
    resolve();
  });
}

async processStreamData(data) {
  ...
}

Here is my test:

test('functionToTest success', async (testCase) => {
  try {
    let readStream = fs.createReadStream(`${DATA_DIR}/genericCsv.csv`);
    jest.spyOn(myClass, "processStreamData").mockImplementation();

    await expect(myClass.functionToTest(readStream)).resolves.toEqual(undefined);
    await expect(myClass.processStreamData).toBeCalledWith(expect.anything());

    testCase();
  } catch(exception) {
    console.log(exception);
    testCase.fail(new Error(`Test failed${exception}`));
  };
});

It seems like because processStreamData is inside an event handler, the original function (rather than the mock) is called.

I've also tried mocking the function via

myClass.processStreamData = jest.fn.mockImplementation()

which didn't work. The original function was still called.

I've done some googling and am well-aware that jest historically has had limitations around this https://github.com/facebook/jest/issues/207 but does this still apply? If not, can someone hand-hold me through the process of how I should set up my mock so it can be used?

Answer 1

Here is a unit test solution:

index.ts :

export class SomeClass {
  async functionToTest(stream) {
    // Run some checks on the stream here
    stream
      .on('data', async data => {
        // Run some checks on the data here
        await this.processStreamData(data);
      })
      .on('finish', () => {
        // Do some processing here
        Promise.resolve();
      });
  }

  async processStreamData(data) {
    console.log(data);
  }
}

index.spec.ts :

import fs, { ReadStream } from 'fs';
import path from 'path';
import { SomeClass } from './';

const myClass = new SomeClass();
const DATA_DIR = './.tmp';

afterEach(() => {
  jest.restoreAllMocks();
});

test('functionToTest success', async () => {
  const readStream: ReadStream = fs.createReadStream(path.resolve(__dirname, `${DATA_DIR}/genericCsv.csv`));
  const events: { [event: string]: (...args: any[]) => void } = {};
  jest.spyOn(readStream, 'on').mockImplementation((event: string, listener: (...args: any[]) => void) => {
    events[event] = listener;
    return readStream;
  });
  jest.spyOn(myClass, 'processStreamData');

  await expect(myClass.functionToTest(readStream)).resolves.toEqual(undefined);
  events.data('fake data');
  expect(myClass.processStreamData).toBeCalledWith('fake data');
});

test('functionToTest finish', async () => {
  const readStream: ReadStream = fs.createReadStream(path.resolve(__dirname, `${DATA_DIR}/genericCsv.csv`));
  const events: { [event: string]: (...args: any[]) => void } = {};
  jest.spyOn(readStream, 'on').mockImplementation((event: string, listener: (...args: any[]) => void) => {
    events[event] = listener;
    return readStream;
  });
  jest.spyOn(Promise, 'resolve');

  await expect(myClass.functionToTest(readStream)).resolves.toEqual(undefined);
  events.finish();
  expect(Promise.resolve).toBeCalledTimes(1);
});

Unit test result with 100% coverage:

 PASS  src/stackoverflow/58244855/index.spec.ts (6.502s)
  ✓ functionToTest success (14ms)
  ✓ functionToTest finish (1ms)

  console.log src/stackoverflow/58244855/index.ts:3320
    fake data

----------|----------|----------|----------|----------|-------------------|
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:        7.544s

Source code: https://github.com/mrdulin/jest-codelab/tree/master/src/stackoverflow/58244855