How to Effectively Mock Functions Within the Same Module Using Jest?

Mocking Functions in the Same Module Using Jest

When mocking functions within the same module, it's crucial to understand the limitations of Jest's default mocking mechanism.

Problem:

Importing the module and directly mocking its exported function, as in the example below, can lead to unexpected behavior.

// module.js
export function bar() { return 'bar'; }
export function foo() { return `I am foo. bar is ${bar()}`; }

// module.test.js
import * as module from '../src/module';

describe('module', () => {
  let barSpy;

  beforeEach(() => {
    barSpy = jest.spyOn(module, 'bar').mockImplementation(jest.fn());
  });

  it('foo', () => {
    module.bar.mockReturnValue('fake bar');
    expect(module.foo()).toEqual('I am foo. bar is fake bar');
    // Fails: Expected "I am foo. bar is fake bar" but received "I am foo. bar is bar"
  });
});

Explanation:

In the above example, foo makes a direct call to the original bar function during import time. Even though barSpy is correctly updated during test setup, foo still references the unmocked bar function, resulting in incorrect output.

Solution 1:

Import the Module into its Own Code File:

To circumvent this issue, the module can be imported into its own code file, ensuring that all exported entities are imported from the same instance.

// module.js
export function bar() { return 'bar'; }
export function foo() { return `I am foo. bar is ${thisModule.bar()}`; }

// module.test.js
import * as module from './module';

describe('module', () => {
  it('foo', () => {
    spyOn(module, 'bar').and.returnValue('fake bar');
    expect(module.foo()).toEqual('I am foo. bar is fake bar');
  });
});

In this modified example, foo now accesses bar through the imported instance module.bar, allowing for easy mocking.

Solution 2:

Manually Mock the Imports:

Alternatively, the imports can be manually mocked in the test setup, creating a new instance of the module that is isolated from the unmocked version.

// module.test.js
jest.mock('../src/module');
const * as mockedModule = require('../src/module');

describe('module', () => {
  beforeEach(() => {
    mockedModule.bar.mockImplementation(jest.fn());
  });

  it('foo', () => {
    mockedModule.bar.mockReturnValue('fake bar');
    expect(mockedModule.foo()).toEqual('I am foo. bar is fake bar');
  });
});

In this approach, the mocked instance mockedModule is used for testing, preventing interference from the unmocked imports.

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