"Exploring the world of mocking module functions in Jest

Question

"Exploring the world of mocking module functions in Jest

I have been working on making assertions with jest mocked functions, and here is the code I am using:

const mockSaveProduct = jest.fn((product) => {
  //some logic
  return
});

jest.mock('./db', () => ({
  saveProduct: mockSaveProduct
}));

The assertion I want to make is:

expect(mockSaveProduct.mock.calls[0][0]).toBe(product)

However, I encountered the following error:

ReferenceError: Cannot access 'mockSaveProduct' before initialization at the line saveProduct: mockSaveProduct

javascript typescript jestjs

Answer 1

Answer №1

jest.mock() gets hoisted. This means the code runs in the following way:

jest.mock('./db', () => {
  return {
    saveProduct: saveProductMock
  }
})

const saveProductMock = jest.fn((product) => {
  //someLogic
  return
});

//WAS HERE

One solution is to include the logic inside jest.mock();.

jest.mock('./db', () => {
    return {
        saveProduct: jest.fn((product) => {
            //someLogic
            return
        })
    }
})

Another option is to mock the entire module and then specify an explicit return value to use.

const db = require('./db');
jest.mock('./db');

it("example", () => {
    db.saveProduct.mockReturnValue(123);
    /** different types of return methods */
    // db.saveProduct.mockResolvedValue(123);

    const product = db.saveProduct();
    const expectedProduct = "xyz";
    expect(product).toBe(expectedProduct)
})

Answer 2

jest.mock() gets hoisted. This means the code runs in the following way:

jest.mock('./db', () => {
  return {
    saveProduct: saveProductMock
  }
})

const saveProductMock = jest.fn((product) => {
  //someLogic
  return
});

//WAS HERE

One solution is to include the logic inside jest.mock();.

jest.mock('./db', () => {
    return {
        saveProduct: jest.fn((product) => {
            //someLogic
            return
        })
    }
})

Another option is to mock the entire module and then specify an explicit return value to use.

const db = require('./db');
jest.mock('./db');

it("example", () => {
    db.saveProduct.mockReturnValue(123);
    /** different types of return methods */
    // db.saveProduct.mockResolvedValue(123);

    const product = db.saveProduct();
    const expectedProduct = "xyz";
    expect(product).toBe(expectedProduct)
})

"Exploring the world of mocking module functions in Jest

Answer №1

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