Can inner function calls be mimicked?

Question

Can inner function calls be mimicked?

Consider this scenario where a module is defined as follows:

// utils.ts
function innerFunction() {
  return 28;
}

function testing() {
  return innerFunction();
}

export {testing}

To write a unit test for the testing function and mock the return value of

innerFunction</code, you can specify a fixed value for any call to <code>innerFunction

. This can be achieved using the following code snippet:

jest.mock('../utils', () => {
  const originalModule = jest.requireActual('../utils');

  return {
    ...originalModule,
    innerFunction: jest.fn().mockReturnValue(33),
  };
});

import { testing } from '../utils';

it('should validate properly', () => {
  expect(testing()).toBe(33);
});

Despite the expectation that jest.requireActual would read all functions and set

innerFunction: jest.fn().mockReturnValue(33)</code to always return <code>33

, it doesn't seem to work in practice based on the provided experiment.

In reality, innerFunction returns 28, but within Jest environment, we hope to control innerFunction to resolve to the desired value.

javascript typescript jestjs

Answer 1

Answer №1

Choice 1

Consider passing innerFunction as a parameter to function testing

utils.ts

function testing(innerFunction: () => number)  {
  return () => innerFunction();
}

export {testing}

test.ts

import { testing } from '../utils'  
  it('everything should work fine', () => {
    const mock = () => 33

    // Also known as SUT (System under test)
    const sut = testing(mock)

    expect(sut()).toBe(33);
  });

Unit tests can be simplified without jest, and are also more reliable...

Choice 2

Relocate innerFunction into a separate file, and utilize jest.mock

innerFunction.ts

export function innerFunction()  {
      return 33
}

utils.ts

import { innerFunction } from './innerFunction.ts'

function testing()  {
    return innerFunction();
}

export { testing }

test.ts

jest.mock('./innerFunction', () => {
  return () => 33
});

import { testing } from '../utils'

  it('everything should work fine', () => {
    expect(testing()).toBe(33);
  });

Answer 2

Choice 1

Consider passing innerFunction as a parameter to function testing

utils.ts

function testing(innerFunction: () => number)  {
  return () => innerFunction();
}

export {testing}

test.ts

import { testing } from '../utils'  
  it('everything should work fine', () => {
    const mock = () => 33

    // Also known as SUT (System under test)
    const sut = testing(mock)

    expect(sut()).toBe(33);
  });

Unit tests can be simplified without jest, and are also more reliable...

Choice 2

Relocate innerFunction into a separate file, and utilize jest.mock

innerFunction.ts

export function innerFunction()  {
      return 33
}

utils.ts

import { innerFunction } from './innerFunction.ts'

function testing()  {
    return innerFunction();
}

export { testing }

test.ts

jest.mock('./innerFunction', () => {
  return () => 33
});

import { testing } from '../utils'

  it('everything should work fine', () => {
    expect(testing()).toBe(33);
  });

Can inner function calls be mimicked?

Answer №1

Choice 1

utils.ts

test.ts

Choice 2

innerFunction.ts

utils.ts

test.ts

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