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 .

Answer 1

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