Why java script fs.readFileSync is not getting mocked?

Question

i have a fs.readFileSync function that i need to mock using jest. i have tried the below code.

my original file which i want to test.

 const fs = require('fs'); const read_file = (path) => { try { const data = fs.readFileSync(path, 'utf8'); return data; } catch (err) { console.error('Error in read_file', err); throw err; } }; const getSecret = secretName => { try { return read_file(`/etc/secrets/${secretName}.txt`); } catch (err){ throw err; } }; const secretConfig = { kafka_keystore_password: getSecret('kafka_keystore_password') }; module.exports = secretConfig;

here is my test case

const secret = require('./secret'); let fs = require('fs') jest.mock('fs'); describe('secret read files', () => { afterEach(jest.restoreAllMocks); it('should read secret from file', () => { //fs.readFileSync.mockReturnValue("randomPrivateKey"); fs.readFileSync.mockResolvedValue("randomPrivateKey") const secretMessage = secret.kafka_keystore_password; //expect(fs.readFileSync).toHaveBeenCalled(); expect(secretMessage).toEqual('randomPrivateKey'); }) }) describe('getSecretsFromFile', () => { const secret = 'secret'; const secrets = { mySecret: secret }; afterEach(jest.restoreAllMocks); it('returns secrets if file is present', () => { fs.existsSync.mockReturnValue(true); fs.readFileSync.mockReturnValue('randomPrivateKey'); const secretMessage = secret.kafka_keystore_password; expect(secretMessage).toEqual('randomPrivateKey'); }); });

and i get the following error.

FAIL src/config/secret.test.js ● secret read files › should read secret from file

expect(received).toEqual(expected) // deep equality Expected: "randomPrivateKey" Received: undefined 15 | const secretMessage = secret.kafka_keystore_password; 16 | //expect(fs.readFileSync).toHaveBeenCalled(); > 17 | expect(secretMessage).toEqual('randomPrivateKey'); | ^ 18 | 19 | }) 20 | at Object.<anonymous> (src/config/secret.test.js:17:27)

● getSecretsFromFile › returns secrets if file is present

expect(received).toEqual(expected) // deep equality Expected: "randomPrivateKey" Received: undefined 32 | 33 | const secretMessage = secret.kafka_keystore_password; > 34 | expect(secretMessage).toEqual('randomPrivateKey'); | ^ 35 | }); 36 | }); 37 | at Object.<anonymous> (src/config/secret.test.js:34:29)

help me fix this .

If my answer doesn't solve your problem please add a comment below it. If it does then please accept my answer. Happy to help but I might require further information. — K-Dawg
– K-Dawg, Commented Sep 16, 2021 at 7:44

K-Dawg · Accepted Answer · 2021-09-16 07:43:10Z

try:

const fs = { readFileSync: jest.fn(() => ({ message: 'Test'})) }

to mock the fs readFileSync message.

But I don't see how this test is ever going to pass because you're returning value "Test"

but checking for randomPrivateKey

expect(secretMessage).toEqual('randomPrivateKey');

Without seeing what's going on inside the code it's difficult to say but I am assuming you might want to change that line to:

expect(secretMessage).toEqual('test');

As test is the value your mocking alternatively return randomPrivateKey from the mock.

I'm having to make a load of assumptions here because I don't know-how secret.kafka_keystore_password is calculated. I'm assuming it just returns the whatever the fs.readFileSync returns.7

added more details about my original file, also i have fixed my testing condition . now even after trying your suggestion i am getting "Expected: "randomPrivateKey" Received: undefined" error
but it looks like you're still using? fs.readFileSync.mockReturnValue('randomPrivateKey'); If you haven't done so can you please update ALL of the code examples to reflect your implementation of my suggestion?
look at my answer again and see how I am mocking using jest.fn(()=>{}) it's how I use jest and it works for me.
can you post full code to get accepted answer, its not working for me.

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