Understand the utility of mocks with Jest - jestjs

Question

I can't understand at all the utility of mockings. See, I have the next module:
function Code() {
this.generate = () => {
const result = 'code124';
return result;
};
}
module.exports = Code;
Now, I want to test it with jest:
const Code = require('../lib/code');
jest.mock('../lib/code', () => {
return jest.fn().mockImplementation(() => {
return {
generate: () => [1, 2, 3]
};
});
});
describe('Code', () => {
test('returns the code "code123"', () => {
const code = new Code();
expect(code.generate()).toBe('code123');
});
});
So... This test will be fine but... My code ain't so... what's the point about mocking if I can set a correct result even though my code is wrong?

Answer 1

You're NOT supposed to mock the unit you're testing. You're supposed to mock it's dependencies.
for example:
whenever you have a dependency in the implementation:
const dependency = require('dependency');
function Code() {
this.generate = () => {
const result = 'code' + dependency.getNumber();
return result;
};
}
module.exports = Code;
you'll be able to modify it's results to be able to test all scenarios without using the actual implementation of your dependency:
const dependency = require('dependency');
const Code = require('../lib/code');
jest.mock('dependency');
describe('Code', () => {
describe('when dependency returns 123', () => {
beforeAll(() => {
dependency.getNumber.mockReturnValue('123');
});
it('should generate code123', () => {
const code = new Code();
expect(code.generate()).toEqual('code123');
});
});
describe('when dependency returns 124', () => {
beforeAll(() => {
dependency.getNumber.mockReturnValue('124');
});
it('should generate code123', () => {
const code = new Code();
expect(code.generate()).toEqual('code124');
});
});
});