how to mock a specific function of a core node module using jest in nodejs

Question

I want to mock specific methods of the "crypto" module of nodejs.

I am testing the end-points in my code using jest framework.

Here is how my end-point code looks like for email-verification:

/* engnr_cntrlr.js */

exports.engineeremail_verifcation = async(req, res) => {
    try {

        // 3. i want to mock the next statement and return a mock-value to hashedToken variable

        const hashedToken = crypto
          .createHash('sha256')
          .update(req.params.token)
          .digest('hex');

        const engnr = await Engineer.findOne({
         engnrToken : hashedToken
        }).orFail(new Error ('engnr not found'));

        engnr.emailVerify = true 
        await engnr.save()

        return res.status(202).send({ message: 'email verified' });

      } catch (error) {
        res.status(400).send({ error: error.message });
      }
    };

test script :

    /* tests/engineer.test.js */

     test('engineer verifies his email', async done => {

           // 1. i am fetching an engnr using an email id

          engnr = await Engineer.findOne({
               email: engineerOne.email
        }); 

        try {
          const response = await request(app)
            .put(`/api/engineer/confirm_mail/${engnr.token}`) //2. i am sending a hashed token as a req.params
            .send();

          expect(response.statusCode).toBe(202);
          expect(response.body).toMatchObject({
            message: 'email verified'
          });

          done();
        } catch (error) {
          done(error);
        }

  });

The problem i am facing is to mock the crypto implementation in my 'engineeremail_verification' (just below comment no.3). I am using other crypto methods in other parts of my code and I do not want to mock them. I just want to mock this specific implementation of crypto module with a mock value being returned. How do I do that? Thank You for going through my question. Appreciate any sort of help.

Answer 1

You can use jest.spyOn(object, methodName) to mock crypto.createHash separately.

E.g.

app.js:

const crypto = require('crypto');
const express = require('express');

const app = express();

app.put('/api/engineer/confirm_mail/:token', async (req, res) => {
  try {
    const hashedToken = crypto.createHash('sha256').update(req.params.token).digest('hex');
    console.log(hashedToken);
    return res.status(202).send({ message: 'email verified' });
  } catch (error) {
    res.status(400).send({ error: error.message });
  }
});

app.put('/example/:token', async (req, res) => {
  const hashedToken = crypto.createHash('sha256').update(req.params.token).digest('hex');
  console.log(hashedToken);
  res.sendStatus(200);
});

module.exports = app;

app.test.js:

const app = require('./app');
const request = require('supertest');
const crypto = require('crypto');

describe('61368162', () => {
  it('should verify email', async () => {
    const hashMock = {
      update: jest.fn().mockReturnThis(),
      digest: jest.fn().mockReturnValueOnce('encrypt 123'),
    };
    const createHashMock = jest.spyOn(crypto, 'createHash').mockImplementationOnce(() => hashMock);
    const logSpy = jest.spyOn(console, 'log');
    const engnr = { token: '123' };
    const response = await request(app).put(`/api/engineer/confirm_mail/${engnr.token}`).send();
    expect(createHashMock).toBeCalledWith('sha256');
    expect(hashMock.update).toBeCalledWith('123');
    expect(hashMock.digest).toBeCalledWith('hex');
    expect(logSpy).toBeCalledWith('encrypt 123');
    expect(response.statusCode).toBe(202);
    expect(response.body).toMatchObject({ message: 'email verified' });
    createHashMock.mockRestore();
    logSpy.mockRestore();
  });

  it('should restore crypto methods', async () => {
    const logSpy = jest.spyOn(console, 'log');
    const response = await request(app).put('/example/123');
    expect(jest.isMockFunction(crypto.createHash)).toBeFalsy();
    expect(response.statusCode).toBe(200);
    expect(logSpy).toBeCalledWith(expect.any(String));
    logSpy.mockRestore();
  });
});

integration test results with coverage report:

 PASS  stackoverflow/61368162/app.test.js (13.621s)
  61368162
    ✓ should verify email (44ms)
    ✓ should restore crypto methods (5ms)

  console.log node_modules/jest-environment-enzyme/node_modules/jest-mock/build/index.js:866
    encrypt 123

  console.log node_modules/jest-environment-enzyme/node_modules/jest-mock/build/index.js:866
    a665a45920422f9d417e4867efdc4fb8a04a1f3fff1fa07e998e86f7f7a27ae3

----------|---------|----------|---------|---------|-------------------
File      | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s 
----------|---------|----------|---------|---------|-------------------
All files |   93.75 |      100 |     100 |   92.86 |                   
 app.js   |   93.75 |      100 |     100 |   92.86 | 12                
----------|---------|----------|---------|---------|-------------------
Test Suites: 1 passed, 1 total
Tests:       2 passed, 2 total
Snapshots:   0 total
Time:        15.707s

source code: https://github.com/mrdulin/react-apollo-graphql-starter-kit/tree/master/stackoverflow/61368162