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

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

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