Mock fs function with jest
The error there is because it is looking for a method called fs.mkdirSync
on your logger
object, which doesn't exist. If you had access to the fs
module in your test then you would spy on the mkdirSync
method like this:
jest.spyOn(fs, 'mkdirSync');
However, I think you need to take a different approach.
Your createLogDir
function is a static method - meaning that it can only be called on the class, and not on an instance of that class (new Logger()
is an instance of the class Logger
). Therefore, in order to test that function you need to export the class and not an instance of it, i.e.:
module.exports = Logger;
Then you could have the following tests:
const Logger = require('./logger');
const fs = require('fs');
jest.mock('fs') // this auto mocks all methods on fs - so you can treat fs.existsSync and fs.mkdirSync like you would jest.fn()
it('should create a new log directory if one doesn\'t already exist', () => {
// set up existsSync to meet the `if` condition
fs.existsSync.mockReturnValue(false);
// call the function that you want to test
Logger.createLogDir('test-path');
// make your assertion
expect(fs.mkdirSync).toHaveBeenCalled();
});
it('should NOT create a new log directory if one already exists', () => {
// set up existsSync to FAIL the `if` condition
fs.existsSync.mockReturnValue(true);
Logger.createLogDir('test-path');
expect(fs.mkdirSync).not.toHaveBeenCalled();
});
Note: it looks like you're mixing CommonJS and es6 module syntax (export default
is es6) - I would try to stick to one or the other
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Oyabi
Updated on July 09, 2022Comments
-
Oyabi almost 2 years
First of all, I'm new to
es6
andjest
.I have a
Logger
class for instantiatewinston
and I would like to test it.Here my code :
const winston = require('winston'); const fs = require('fs'); const path = require('path'); const config = require('../config.json'); class Logger { constructor() { Logger.createLogDir(Logger.logDir); this.logger = winston.createLogger({ level: 'info', format: winston.format.json(), transports: [ new (winston.transports.Console)({ format: winston.format.combine( winston.format.colorize({ all: true }), winston.format.simple(), ), }), new (winston.transports.File)({ filename: path.join(Logger.logDir, '/error.log'), level: 'error', }), new (winston.transports.File)({ filename: path.join(Logger.logDir, '/info.log'), level: 'info', }), new (winston.transports.File)({ filename: path.join(Logger.logDir, '/combined.log'), }), ], }); } static get logDir() { return (config.logDir == null) ? 'log' : config.logDir; } static createLogDir(logDir) { if (!fs.existsSync(logDir)) { // Create the directory if it does not exist fs.mkdirSync(logDir); } } } exports.logger = new Logger().logger; export default new Logger();
I would like to test my function
createLogDir()
. I my head, I think it's a good idea to test the state of fs.existsSync. Iffs.existsSync
returnfalse
,fs.mkdirSync
must be called. So I try to write somejest
test :describe('logDir configuration', () => { test('default path must be used', () => { const logger = require('./logger'); jest.mock('fs'); fs.existsSync = jest.fn(); fs.existsSync.mockReturnValue(false); const mkdirSync = jest.spyOn(logger, 'fs.mkdirSync'); expect(mkdirSync).toHaveBeenCalled(); }); });
However, I've got an error :
● logDir configuration › default path must be used Cannot spy the fs.mkdirSync property because it is not a function; undefined given instead 18 | fs.existsSync = jest.fn(); 19 | fs.existsSync.mockReturnValue(true); > 20 | const mkdirSync = jest.spyOn(logger, 'fs.mkdirSync'); 21 | expect(mkdirSync).toHaveBeenCalled(); 22 | }); 23 | }); at ModuleMockerClass.spyOn (node_modules/jest-mock/build/index.js:590:15) at Object.test (src/logger.test.js:20:28)
Can you help me to debug and test my function please ?
Regards.
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Tom over 5 yearsI think this pattern doesn't allow for the real FS methods to be restored. This permanently alters the test environment in ways that could affect other tests. You'll need to use
jest.spyOn(fs, 'mkdirSync')
so you can later dofs.mkdirSync.mockRestore()
. -
Learner over 5 years@Billy winston will automatically create new folder do we need to write extra code for that one
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Learner over 5 years@Billy i was also implementing the above one can you check this one stackoverflow.com/questions/54250372/… , will be a help for me
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Billy Reilly about 5 years@Tom you're right, have changed to auto mocking so it wouldn't affect other tests