Jest - mocking zlib function which is wrapped in promisify does not work

Question

I'm trying to test a file which uses zlib which is wrapped in promisify but when the test reaches the line in the code which uses zlib I get a jest timeout error.

: Timeout - Async callback was not invoked within the 30000 ms timeout specified by jest.setTimeout.Timeout - Async callback was not invoked within the 30000 ms timeout specified by jest.setTimeout.Error:

Module file:

import zlib from 'zlib';
import util from 'util';
const zlibGunzip = util.promisify(zlib.gunzip);

async function unzipObjectContent(objectBody): Promise<string> {
    const buff = objectBody as Buffer;
  
    try {
      const data = await zlibGunzip(buff);
      const utf8String = data.toString('utf8');
      const parsedJson = JSON.parse(utf8String);
  
      return JSON.stringify(parsedJson);
    } catch (error) {
      logger.error(`unzipObjectContent -> failed to unzip file ${error}`);
      throw error;
    }
  }

Test File

jest.mock('zlib');
import zlib from 'zlib';

let gunzipMock: jest.SpyInstance;
gunzipMock = jest.spyOn(zlib, 'gunzip');
gunzipMock.mockResolvedValue(JSON.stringify(problemZipContent));

When I debug the test, I see that it reaches the call to await zlibGunzip(buff); but then an error is thrown. It does not reaches the catch block either.

Please advise on how can I test this. Thank you

Answer 1

Mock the zlib.gunzip() method and its implementation, since the second argument of this method is a Node.js error-first callback, you need to call the callback function manually in your test. So that the promise returned from util.promisify(zlib.gunzip) will be resolved.

E.g.

index.ts:

import zlib from 'zlib';
import util from 'util';

const zlibGunzip = util.promisify(zlib.gunzip);

export async function unzipObjectContent(objectBody): Promise<string> {
  const buff = objectBody as Buffer;

  try {
    const data = await zlibGunzip(buff);
    const utf8String = data.toString('utf8');
    const parsedJson = JSON.parse(utf8String);

    return JSON.stringify(parsedJson);
  } catch (error) {
    console.error(`unzipObjectContent -> failed to unzip file ${error}`);
    throw error;
  }
}

index.test.ts:

import zlib from 'zlib';
import { unzipObjectContent } from './';
import { mocked } from 'ts-jest/utils';

jest.mock('zlib');

const mzlib = mocked(zlib);

describe('67475685', () => {
  afterAll(() => {
    jest.resetAllMocks();
  });
  it('should pass', async () => {
    const problemZipContent = Buffer.from(JSON.stringify({ name: 'teresa teng' }));
    mzlib.gunzip.mockImplementationOnce((buffer, callback: any) => {
      callback(null, problemZipContent);
    });
    const actual = await unzipObjectContent(problemZipContent);
    expect(actual).toEqual('{"name":"teresa teng"}');
    expect(mzlib.gunzip).toBeCalledTimes(1);
  });
});

test result:

 PASS  examples/67475685/index.test.ts (7.156 s)
  67475685
    ✓ should pass (4 ms)

----------|---------|----------|---------|---------|-------------------
File      | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s 
----------|---------|----------|---------|---------|-------------------
All files |   83.33 |      100 |     100 |   83.33 |                   
 index.ts |   83.33 |      100 |     100 |   83.33 | 16-17             
----------|---------|----------|---------|---------|-------------------
Test Suites: 1 passed, 1 total
Tests:       1 passed, 1 total
Snapshots:   0 total
Time:        7.656 s, estimated 8 s