What is the best way to simulate a specific file from an external package using Jest?

Question

What is the best way to simulate a specific file from an external package using Jest?

Currently, I am developing a project in Typescript that relies on an external package (written in JavaScript) through npm. During testing, I only want to mock one specific JS file from that package.

Is there a way to achieve this using Jest?

The package.js file includes:

  "dependencies": {
    ...
    "@company/product": "1.8.0"
    ...

Within someFile.ts:

import * as sdk from "@company/product";

I attempted to create a mock like this:

In someFile.spec.ts:

...

const prefs: IPrefs = {
  some_property: -1
};

jest.mock(
  "@company/product/sdk/api/prefs.js",
  (): { Prefs: IPrefs } => {
    return {
      Prefs: prefs
    };
  }
);

This is the original "prefs.js" file from the external package:


import ... // I can see in the failed test stack trace

export let Prefs = {
  ...
};

Despite my efforts, it does not effectively mock the file since I am still seeing the stack trace from the original "prefs.js" provided by the external package.

javascript typescript jestjs mocking

Answer 1

Answer №1

import * as lib from "@organization/service";


const mockSettings = {
  custom_setting: -1
};


jest.mock('@organization/service/lib/api/settings.js', () => ({
  Settings: mockSettings
}));

// Start testing module after setting up the mock
import { functionToTest } from './someFile';

describe('Testing someFile Module', () => {
  it('should utilize the mocked settings', () => {
    const output = functionToTest();
    expect(output).toEqual(/* expected result based on mockSettings */);
  });
});

Answer 2

import * as lib from "@organization/service";


const mockSettings = {
  custom_setting: -1
};


jest.mock('@organization/service/lib/api/settings.js', () => ({
  Settings: mockSettings
}));

// Start testing module after setting up the mock
import { functionToTest } from './someFile';

describe('Testing someFile Module', () => {
  it('should utilize the mocked settings', () => {
    const output = functionToTest();
    expect(output).toEqual(/* expected result based on mockSettings */);
  });
});

Answer 3

Answer №2

Establish a __mocks__ folder next to the node_modules
Within the __mocks__ directory, mirror the structure of the package you wish to mock.

In the

__mocks__/company/product/sdk/api/prefs.js

file, create the mock implementation for your function:

export const someFunction = jest.fn(() => {
  // Add logic here;
});

Using jest.fn(), you can customize a mock function for your testing purposes.

Answer 4

Establish a __mocks__ folder next to the node_modules
Within the __mocks__ directory, mirror the structure of the package you wish to mock.

In the

__mocks__/company/product/sdk/api/prefs.js

file, create the mock implementation for your function:

export const someFunction = jest.fn(() => {
  // Add logic here;
});

Using jest.fn(), you can customize a mock function for your testing purposes.

What is the best way to simulate a specific file from an external package using Jest?

Answer №1

Answer №2

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