Tips for handling a jest mock function of a module and TypeScript types

Question

Tips for handling a jest mock function of a module and TypeScript types

When it comes to writing my testing files in TypeScript, I rely on ts-jest and jest.

I'm facing some confusion regarding how to type the mock function of a module.

Let's take a look at the code:

./module.ts:

import {IObj} from '../interfaces';

const obj: IObj = {
  getMessage() {
    return `Her name is ${this.genName()}, age is ${this.getAge()}`;
  },

  genName() {
    return 'novaline';
  },

  getAge() {
    return 26;
  }
};

export default obj;

./module.test.ts:

import * as m from './module';

describe('mock function test suites', () => {

  it('t-1', () => {
    // Unsure about the correct jest.Mock<string> type here.
    m.genName: jest.Mock<string> = jest.fn(() => 'emilie'); 
    expect(jest.isMockFunction(m.genName)).toBeTruthy();
    expect(m.genName()).toBe('emilie');
    expect(m.getMessage()).toEqual('Her name is emilie, age is 26');
    expect(m.genName).toHaveBeenCalled(); 

  });

});

How can I properly type the mock function genName of module m?

TypeScript throws an error in this line:

Error:(8, 7) TS2540:Cannot assign to 'genName' because it is a constant or a read-only property.

javascript typescript jestjs

Answer 1

Answer №1

Here is my approach to tackling the same issue and how I handle mocking and spying in my current workflow.

import * as m from './module';

describe('testing scenario', () => {
  let mockGenName;

  beforeEach(() => {
    mockGenName = jest.spyOn(m, 
      'genName').mockImplementation(() => 'franc');
  })

  afterEach(() => {
    mockGenName.mockRestore();
  })


  test('description of the test case', () => {
    // perform an action that triggers the genName function
    expect(mockGenName).toHaveBeenCalledTimes(1);
  })

})

By setting it up this way, you have the flexibility to modify the mock's implementation based on different tests, validate the function calls and parameters, and reset the mock between tests and at the conclusion of all tests.

Answer 2

Here is my approach to tackling the same issue and how I handle mocking and spying in my current workflow.

import * as m from './module';

describe('testing scenario', () => {
  let mockGenName;

  beforeEach(() => {
    mockGenName = jest.spyOn(m, 
      'genName').mockImplementation(() => 'franc');
  })

  afterEach(() => {
    mockGenName.mockRestore();
  })


  test('description of the test case', () => {
    // perform an action that triggers the genName function
    expect(mockGenName).toHaveBeenCalledTimes(1);
  })

})

By setting it up this way, you have the flexibility to modify the mock's implementation based on different tests, validate the function calls and parameters, and reset the mock between tests and at the conclusion of all tests.

Answer 3

Answer №2

Check out this helpful resource - https://jestjs.io/docs/mock-function-api#typescript

To simplify, there are only three possible strategies to consider:

Completely mock the imported module and access the mocked function for manipulation (jest.Mock(), jest.MockedFunction)
Partially mock the imported module and access the mocked function for manipulation using a factory method (jest.Mock() with factory, jest.MockedFunction)
Import the module as is and then spy on the function that needs to be mocked (jest.spy())

Answer 4

Check out this helpful resource - https://jestjs.io/docs/mock-function-api#typescript

To simplify, there are only three possible strategies to consider:

Completely mock the imported module and access the mocked function for manipulation (jest.Mock(), jest.MockedFunction)
Partially mock the imported module and access the mocked function for manipulation using a factory method (jest.Mock() with factory, jest.MockedFunction)
Import the module as is and then spy on the function that needs to be mocked (jest.spy())

Answer 5

Answer №3

If you wish to create a mock for the module and modify the function that is exported from it, you can achieve this by using the following code snippet. This will effectively override your initial import statement.

jest.mock('./module', () => ({
    genName: jest.fn().mockImplementation(() => 'emilie')
}))

Answer 6

If you wish to create a mock for the module and modify the function that is exported from it, you can achieve this by using the following code snippet. This will effectively override your initial import statement.

jest.mock('./module', () => ({
    genName: jest.fn().mockImplementation(() => 'emilie')
}))

Answer 7

Answer №4

The reason for encountering the error:

It seems that the properties of a module object foo (import * as foo from 'foo') behave similarly to those of a frozen object.

For further details, refer to In ES6, imports are live read-only views on exported values

Solving the issue involved changing from import * as m from './module' to import m from './module';.

Package configurations:

"typescript": "^3.6.4"
"ts-jest": "^24.1.0"
"jest": "^24.9.0",

jest.config.js:

module.exports = {
  preset: 'ts-jest/presets/js-with-ts',
  //...
}

tsconfig.json:

"compilerOptions": {
    "target": "es6",
    "module": "commonjs",
    //...
}

Answer 8

The reason for encountering the error:

It seems that the properties of a module object foo (import * as foo from 'foo') behave similarly to those of a frozen object.

For further details, refer to In ES6, imports are live read-only views on exported values

Solving the issue involved changing from import * as m from './module' to import m from './module';.

Package configurations:

"typescript": "^3.6.4"
"ts-jest": "^24.1.0"
"jest": "^24.9.0",

jest.config.js:

module.exports = {
  preset: 'ts-jest/presets/js-with-ts',
  //...
}

tsconfig.json:

"compilerOptions": {
    "target": "es6",
    "module": "commonjs",
    //...
}

Answer 9

Answer №5

import * as m from './module'

jest.mock('./module', () => ({
    genName: jest.fn().mockImplementation(() => 'emilie')
    // This will ensure that "emilie" is returned for all tests
}))

it('returns franc', () => {
  m.genName.mockImplementationOnce(() => 'franc')
  // Only for this test, "franc" will be returned 
})

Answer 10

import * as m from './module'

jest.mock('./module', () => ({
    genName: jest.fn().mockImplementation(() => 'emilie')
    // This will ensure that "emilie" is returned for all tests
}))

it('returns franc', () => {
  m.genName.mockImplementationOnce(() => 'franc')
  // Only for this test, "franc" will be returned 
})

Answer 11

Answer №6

I'm imitating a certain process like this:

jest.mock('./module')
const {genName} = require('./module')

and during my test:

 genName.mockImplementationOnce(() => 'franc')

This approach works perfectly for me without any TypeScript errors.

Answer 12

I'm imitating a certain process like this:

jest.mock('./module')
const {genName} = require('./module')

and during my test:

 genName.mockImplementationOnce(() => 'franc')

This approach works perfectly for me without any TypeScript errors.

Tips for handling a jest mock function of a module and TypeScript types

Answer №1

Answer №2

Answer №3

Answer №4

Answer №5

Answer №6

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