What are the steps to effectively utilize a jest mock within a typed test fake when working with typescript?

Question

What are the steps to effectively utilize a jest mock within a typed test fake when working with typescript?

I am facing an issue where I have defined an interface that requires certain functions to be implemented by objects. For instance:

interface MyInterface {
    someFunc: () => void,
    /* ... other details */
}

Now, I need to create a test to ensure that these functions are called under specific conditions.

To achieve this, I utilized test fakes in the following manner:

const obj: MyInterface = {
    ...fakeObj,
    someFunc: jest.fn(() => {})
};

/* ... actual testing code */

expect(obj.someFunc.mock.calls.length).toBe(1);

Initially, everything was working fine until I explicitly assigned a type to the obj variable. This change resulted in the error message:

Error: Property 'mock' does not exist on type '() => void'.

My intention behind adding types to my objects was to enable smooth usage of refactoring tools without the hassle of manually correcting all tests.

How can I effectively write a test to verify function calls while implementing explicit types?

typescript mocking jestjs

Answer 1

Answer №1

If you want an effortless way to handle mocking in your tests, the mocked test helper is the solution for you.

By using the mocked test helper, you can leverage typings on your mocked modules and their deep methods, all based on the types defined in its source. This tool takes advantage of the latest TypeScript features, offering argument type completion directly in your IDE (instead of jest.MockInstance).

For example:

index.ts:

export interface MyInterface {
  someFunc: () => void;
}

export function main(obj: MyInterface) {
  return obj.someFunc();
}

index.test.ts:

import { main, MyInterface } from './';
import { mocked } from 'ts-jest/utils';

describe('63478184', () => {
  it('should pass', () => {
    const obj: MyInterface = {
      someFunc: jest.fn().mockReturnValueOnce('mocked value'),
    };
    main(obj);
    expect(mocked(obj.someFunc).mock.calls.length).toBe(1);
  });
});

Upon running the unit tests and generating coverage reports:

 PASS  src/stackoverflow/63478184/index.test.ts (12.902s)
  63478184
    ✓ should pass (8ms)

----------|----------|----------|----------|----------|-------------------|
File      |  % Stmts | % Branch |  % Funcs |  % Lines | Uncovered Line #s |
----------|----------|----------|----------|----------|-------------------|
All files |      100 |      100 |      100 |      100 |                   |
 index.ts |      100 |      100 |      100 |      100 |                   |
----------|----------|----------|----------|----------|-------------------|
Test Suites: 1 passed, 1 total
Tests:       1 passed, 1 total
Snapshots:   0 total
Time:        14.759s

Find the complete source code at: https://github.com/mrdulin/jest-codelab/tree/master/src/stackoverflow/63478184

Answer 2

If you want an effortless way to handle mocking in your tests, the mocked test helper is the solution for you.

By using the mocked test helper, you can leverage typings on your mocked modules and their deep methods, all based on the types defined in its source. This tool takes advantage of the latest TypeScript features, offering argument type completion directly in your IDE (instead of jest.MockInstance).

For example:

index.ts:

export interface MyInterface {
  someFunc: () => void;
}

export function main(obj: MyInterface) {
  return obj.someFunc();
}

index.test.ts:

import { main, MyInterface } from './';
import { mocked } from 'ts-jest/utils';

describe('63478184', () => {
  it('should pass', () => {
    const obj: MyInterface = {
      someFunc: jest.fn().mockReturnValueOnce('mocked value'),
    };
    main(obj);
    expect(mocked(obj.someFunc).mock.calls.length).toBe(1);
  });
});

Upon running the unit tests and generating coverage reports:

 PASS  src/stackoverflow/63478184/index.test.ts (12.902s)
  63478184
    ✓ should pass (8ms)

----------|----------|----------|----------|----------|-------------------|
File      |  % Stmts | % Branch |  % Funcs |  % Lines | Uncovered Line #s |
----------|----------|----------|----------|----------|-------------------|
All files |      100 |      100 |      100 |      100 |                   |
 index.ts |      100 |      100 |      100 |      100 |                   |
----------|----------|----------|----------|----------|-------------------|
Test Suites: 1 passed, 1 total
Tests:       1 passed, 1 total
Snapshots:   0 total
Time:        14.759s

Find the complete source code at: https://github.com/mrdulin/jest-codelab/tree/master/src/stackoverflow/63478184

What are the steps to effectively utilize a jest mock within a typed test fake when working with typescript?

Answer №1

Similar questions

Managing asynchronous variable assignment in Angular: tips and tricks

Utilizing React Typescript Discriminating Unions to choose between two different types based solely on props

Testing the download functionality of a JavaScript unit

Exploring TypeScript interfaces with optional properties and returning types

When receiving JSON and attempting to store the data in a variable, I encounter an issue where it returns the error message "undefined is not iterable (cannot read property Symbol

VSC is throwing a type error, but is it still possible to compile the code?

Tips on utilizing storage.set() within google.maps.events.addListener(marker, 'dragend', function() { }); in Ionic 3

When the page hosted on Vercel is reloaded, `getStaticProps` does not function as expected

Arrange Angular code in WebStorm by placing decorated variables after non-decorated ones

React-select or react-testing-library: Target change causing onChange not to fire in Jest mock

Issue with logging messages using console.log in Knex migration script

Custom claims fetched from Admin SDK are returning empty values

I attempt to transfer information from one of my components to another component

Error in TypeScript when compiling Mongoose document middleware

Having trouble with an unexpected value in your Angular2 Service? Don't forget to add

Having difficulty retrieving an angular file from a location outside of the asset folder

AWS CDK Pipeline not initiating on commit to Bitbucket

Provide initial values to a custom TypeScript hook

ESLint refuses to be turned off for a particular file

Having trouble locating the type definition file for 'cucumber' when using the Protractor framework with Cucumber and Typescript