jestjs - Jest - getting error when mocking FS modules and calling config module

Question

I'm writing unit tests with Jest trying to test a module which uses FS.

The module file:

import fs from 'fs';
import logger from './logger.utils';

export const getNumberOfFiles = async (targetDir: string): Promise<number> => {
  // get number of folders
  logger.info(`getNumberOfFiles from ${targetDir}/${fileName}`);
  const numberOfFiles = await fs.readdirSync(targetDir);
  return numberOfFiles.length;
};

Test file

import fs from 'fs';
import { getNumberOfFiles } from '../../src/utils/fs.utils';

jest.mock('fs');

describe('fs.utils', () => {
  describe('getNumberOfFiles', () => {
    it('Should return number', async () => {
      fs.readdirSync = jest.fn();
      const readdirSyncMock = fs.readdirSync = jest.fn();
      readdirSyncMock.mockResolvedValue([1, 2, 3]);

      const result = await getNumberOfFiles('targetDir');
      expect(result).toEqual(3);
      expect(readdirSyncMock.mock.calls.length).toEqual(1);
    });
  });
});

When I run the test file, I get the following error:

Config file ..../config/runtime.json cannot be read. Error code is: undefined. Error message is: Cannot read property 'replace' of undefined

  1 | const cheggLogger = require('@chegg/logger');
  2 | import loggingContext from './loggingContext';
> 3 | import config from 'config';
    | ^
  4 | import os from 'os';
  5 | import constants from '../../config/constants';
  6 | 

  at Config.Object.<anonymous>.util.parseFile (node_modules/config/lib/config.js:789:13)
  at Config.Object.<anonymous>.util.loadFileConfigs (node_modules/config/lib/config.js:666:26)
  at new Config (node_modules/config/lib/config.js:116:27)
  at Object.<anonymous> (node_modules/config/lib/config.js:1459:31)
  at Object.<anonymous> (src/utils/logger.utils.ts:3:1)

Content of logger.utils.ts

const internalLogger = require('internalLogger');
import loggingContext from './loggingContext';
import config from 'config';
import os from 'os';
import constants from '../../config/constants';

const logger = internalLogger.createLogger({
  level: config.get(constants.LOG_LEVEL)
});

export default logger;

I assume that config is using FS, and once I mock the module, it fails. How can I resolve this? Please advise

question from:https://stackoverflow.com/questions/65871990/jest-getting-error-when-mocking-fs-modules-and-calling-config-module

Answer 1

I'm guessing the problem comes from config also using the fs api but you are now mock entire module fs which makes all methods should be mocked before using.

But I have an idea for you by using jest.doMock which you can provide a factory for each test and just mock only method we need. Here is a draft idea:

describe('fs.utils', () => {
  describe('getNumberOfFiles', () => {
    it('Should return number', async () => {

      jest.doMock('fs', () => ({
        // Keep other methods still working so `config` or others can use
        // so make sure we don't break anything
        ...jest.requireActual('fs'),
        readdirSync: jest.fn(pathUrl => {
          // Mock for our test path since `config` also uses this method :(
          return pathUrl === 'targetDir' ? Promise.resolve([1, 2, 3]) : jest.requireActual('fs').readdirSync(pathUrl)
        })
      }));
      
      // One of the thing we should change is to switch `require` here
      // to make sure the mock is happened before we actually require the code
      // we can also use `import` here but requires us do a bit more thing
      // so I keep thing simple by using `require`
      const {getNumberOfFiles} = require('../../src/utils/fs.utils');
  
      const result = await getNumberOfFiles('targetDir');
      expect(result).toEqual(3);
      // you might stop assert this as well
      // expect(readdirSyncMock.mock.calls.length).toEqual(1);
    });
  });
});

Just also want to check, if you created a config file as described here: https://www.npmjs.com/package/config#quick-start