Sometimes, we want to mock an exported const in Jest.

In this article, we’ll look at how to mock an exported const in Jest.

How to mock an exported const in Jest?

To mock an exported const in Jest, we can use the jest.mock method.

For instance, we write:

const mockTrue = {
  ENABLED: true
};
const mockFalse = {
  ENABLED: false
};

describe('allowThrough', () => {
  beforeEach(() => {
    jest.resetModules();
  });

  test('success', () => {
    jest.mock('./constants', () => mockTrue)
    const {
      ENABLED
    } = require('./constants');
    const {
      allowThrough
    } = require('./allowThrough');

    expect(ENABLED).toBe(true);
    expect(allowThrough({
      value: 1
    })).toBe(true);
  });

  test('fail, ENABLED === false', () => {
    jest.mock('./constants', () => mockFalse)
    const {
      ENABLED
    } = require('./constants');
    const {
      allowThrough
    } = require('./allowThrough');

    expect(ENABLED).toBe(false);
    expect(allowThrough({
      value: 1
    })).toBe(false);
  });
});

to mock the ./constants with a function that returns the mocked exports.

Then can use the require the ./constants with require and get the ENABLED exported member.

Jest will use the mocked version of that for the test since we called jest.mock to mock it.

We use expect to check that the mocked ENABLED value it what we expect.

Conclusion

To mock an exported const in Jest, we can use the jest.mock method.