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πŸ“™ A guide to unit testing in JavaScript and Typescript

This is a living document, new ideas are always welcome. Contribute: fork, clone, branch, commit, push, pull request!

All the information provided has been compiled & adapted from the references cited at the end of the document. The guidelines are illustrated by the Original Source examples and Whitebox examples.

πŸ“– Table of contents

General principles

Guidelines

Typescript Specific Considerations

Resources

General principles

Unit tests

Unit = Unit of work

This could involve multiple methods and classes invoked by some public API that can:

  • Return a value or throw an exception
  • Change the state of the system
  • Make 3rd party calls (API, database, ...)

A unit test should test the behaviour of a unit of work: for a given input, it expects an end result that can be any of the above.

Unit tests are isolated and independent of each other

  • Any given behaviour should be specified in one and only one test
  • The execution/order of execution of one test cannot affect the others

The code is designed to support this independence (see "Design principles" below).

Unit tests are lightweight tests

  • Repeatable
  • Fast
  • Consistent
  • Easy to write and read

Unit tests are code too

They should meet the same level of quality as the code being tested. They can be refactored as well to make them more maintainable and/or readable.

β€’ Back to ToC β€’

Design principles

The key to good unit testing is to write testable code. Applying simple design principles can help, in particular:

  • Use a good naming convention and comment your code (the "why?" not the "how"), keep in mind that comments are not a substitute for bad naming or bad design
  • DRY: Don't Repeat Yourself, avoid code duplication
  • Single responsibility: each object/function must focus on a single task
  • Keep a single level of abstraction in the same component (for example, do not mix business logic with lower-level technical details in the same method)
  • Minimize dependencies between components: encapsulate, interchange less information between components
  • Support configurability rather than hard-coding, this prevents having to replicate the exact same environment when testing (e.g.: markup)
  • Apply adequate design patterns, especially dependency injection that allows separating an object's creation responsibility from business logic
  • Avoid global mutable state

β€’ Back to ToC β€’

Guidelines

The goal of these guidelines is to make your tests:

  • Readable
  • Maintainable
  • Trustworthy

These are the 3 pillars of good unit testing.

All the following examples assume the usage of the Jasmine framework.

β€’ Back to ToC β€’

Consider using TDD

TDD is a design process, not a testing process. TDD is a robust way of designing software components ("units") interactively so that their behaviour is specified through unit tests.

How? Why?

Test-first cycle

  1. Write a simple failing test
  2. Make the test pass by writing the minimum amount of code, don't bother with code quality
  3. Refactor the code by applying design principles/patterns

Consequences of the test-first cycle

  • Writing a test first makes the code design testable de facto
  • Writing just the amount of code needed to implement the required functionality makes the resulting codebase minimal, thus more maintainable
  • The codebase can be enhanced using refactoring mechanisms, the tests give you confidence that the new code is not modifying the existing functionalities
  • Cleaning the code in each cycle makes the codebase more maintainable, it is much cheaper to change the code frequently and in small increments
  • Fast feedback for the developers, you know that you don't break anything and that you are evolving the system in a good direction
  • Generates confidence to add features, fix bugs, or explore new designs

Note that code written without a test-first approach is often very hard to test.

β€’ Back to ToC β€’

Structure your tests properly

Don't hesitate to nest your suites to structure logically your tests in subsets.

:(

describe('A set of functionalities', () => {
  it('a set of functionalities should do something nice', () => {
  });

  it('a subset of functionalities should do something great', () => {
  });

  it('a subset of functionalities should do something awesome', () => {
  });

  it('another subset of functionalities should also do something great', () => {
  });
});

:)

describe('A set of functionalities', () => {
  it('should do something nice', () => {
  });

  describe('A subset of functionalities', () => {
    it('should do something great', () => {
    });

    it('should do something awesome', () => {
    });
  });

  describe('Another subset of functionalities', () => {
    it('should also do something great', () => {
    });
  });
});

β€’ Back to ToC β€’

Name your tests properly

Tests names should be concise, explicit, descriptive and in correct English. Read the output of the spec runner and verify that it is understandable! Keep in mind that someone else will read it too. Tests can be the live documentation of the code.

:(

// Classes are PascalCase
describe('NameOfClass', () => {
	// functions are camelCase
	describe('nameOfFunction', () => {
		// behavior is in the queen's english
		it('it should do somethings with all of the success', () => {
		});
    });

	// getters and setters are also camelCase
	describe('nameOfGetterOrSetter', () => {
		it('init set correct property when called', () => {
		});
	});
});

β€’ Back to ToC β€’

Don't comment out tests

Never. Ever. Tests have a reason to be or not.

Don't comment them out because they are too slow, too complex or produce false negatives. Instead, make them fast, simple and trustworthy. If not, remove them completely.

β€’ Back to ToC β€’

Avoid logic in your tests

Always use simple statements. Don't use loops and/or conditionals. If you do, you add a possible entry point for bugs in the test itself:

  • Conditionals: you don't know which path the test will take
  • Loops: you could be sharing state between tests

:(

it('should properly sanitize strings', () => {
  let result;
  const testValues = {
    'Avion'         : 'Avi' + String.fromCharCode(243) + 'n',
    'The-space'     : 'The space',
    'Weird-chars-'  : 'Weird chars!!',
    'file-name.zip' : 'file name.zip',
    'my-name.zip'   : 'my.name.zip'
  };

  for (result in testValues) {
    expect(sanitizeString(testValues[result])).toBe(result);
  }
});

:)

it('should properly sanitize strings', () => {
  expect(sanitizeString('Avi'+String.fromCharCode(243)+'n')).toBe('Avion');
  expect(sanitizeString('The space')).toBe('The-space');
  expect(sanitizeString('Weird chars!!')).toBe('Weird-chars-');
  expect(sanitizeString('file name.zip')).toBe('file-name.zip');
  expect(sanitizeString('my.name.zip')).toBe('my-name.zip');
});

Better: Use it.each

:) :)

it.each`
    stringInput         | expectedResult
    ${'Avion'}          | ${'Avi'+String.fromCharCode(243)+'n'}
    ${'The-space'}      | ${'The space'}
    ${'Weird-chars-'}   | ${'Weird chars!!'}
    ${'file-name.zip'}  | ${'file name.zip'}
    ${'my-name.zip'}    | ${'my.name.zip'}
`(`should properly sanitize '${stringInput}' to '${expectedResult}'`, ({ stringInput, expectedResult }) => {
	expect(sanitizeString(stringInput)).toBe(expectedResult);
});

β€’ Back to ToC β€’

Don't test implementation details

Remember, unit tests are a design specification of how a certain behaviour should work, not a list of observations of everything the code happens to do.

It is acceptable to test implementation details when we are "leaving" the system under test. That is to say, if we are making an ajax request or querying the database, it is acceptable to make sure that functions were invoked with specific parameters or called x number of times.

:(

it('should multiply the number passed as parameter and subtract one', () => {
  const multiplySpy = spyOn(Calculator, 'multiple').and.callThrough();
  const subtractSpy = spyOn(Calculator, 'subtract').and.callThrough();

  const result = Calculator.compute(21.5);

  expect(multiplySpy).toHaveBeenCalledWith(21.5, 2);
  expect(subtractSpy).toHaveBeenCalledWith(43, 1);
  expect(result).toBe(42);
});

:)

it('should multiply the number passed as parameter and subtract one', () => {
  const result = Calculator.compute(21.5);
  expect(result).toBe(42);
});

This will improve maintainability. Your test is no longer tied to implementation details.

β€’ Back to ToC β€’

Properly setup the actions that apply to all the tests involved

:(

describe('Saving the user profile', () => {
  let profileModule;
  let notifyUserSpy;
  let mockOnComplete;

  beforeEach(() => {
    profileModule = new ProfileModule();
    notifyUserSpy = jest.spyOn<any, any>(profileModule, 'notifyUser');
    mockOnComplete = jest.fn();
  });

  it('should send the updated profile data to the server', () => {
    profileModule.save();

    expect(notifyUserSpy).toHaveBeenCalledWith({
        url: '/profiles/1',
        method: 'POST',
        data: {
        	username: 'mawrkus',
        }
    });
  });

  it('should notify the user', () => {
    profileModule.save();
    expect(notifyUserSpy).toHaveBeenCalledTimes(1);
  });

  it('should properly execute the callback passed as parameter', () => {
    profileModule.save(onCompleteSpy);
    expect(onCompleteSpy).toHaveBeenCalledTimes(1);
  });
});

The setup code should apply to all the tests:

:)

describe('Saving the user profile', () => {
  let profileModule;

  beforeEach(() => {
    profileModule = new ProfileModule();
  });

  it('should send the updated profile data to the server', () => {
    const result = profileModule.save();
    expect(result.username).toBe('theUserName');
    expect(result.description).toBe('this is a description');

  });

  it('should notify the user', () => {
    jest.spyOn(profileModule, 'notifyUser');
    profileModule.save();

    expect(profileModule.notifyUser).toHaveBeenCalled();
  });

  it('should properly execute the callback passed as parameter', () => {
    const onCompleteSpy = jest.fn();

    profileModule.save(onCompleteSpy);

    expect(onCompleteSpy).toHaveBeenCalled();
  });
});

Consider keeping the setup code minimal to preserve readability and maintainability.

β€’ Back to ToC β€’

Consider using factory functions in the tests

Factories can:

  • help reduce the setup code, especially if you use dependency injection
  • make each test more readable, since the creation is a single function call that can be in the test itself instead of the setup
  • provide flexibility when creating new instances (setting an initial state, for example)

There's a trade-off to find here between applying the DRY principle and readability.

:(

describe('User profile module', () => {
  let profileModule;
  let pubSub;

  beforeEach(() => {
    const element = document.getElementById('my-profile');
    pubSub = new PubSub({ sync: true });

    profileModule = new ProfileModule({
      element,
      pubSub,
      likes: 0
    });
  });

  it('should publish a topic when a new "like" is given', () => {
    jest.spyOn(pubSub, 'notify');
    profileModule.incLikes();
    expect(pubSub.notify).toHaveBeenCalledWith('likes:inc', { count: 1 });
  });

  it('should retrieve the correct number of likes', () => {
    profileModule.incLikes();
    profileModule.incLikes();
    expect(profileModule.getLikes()).toBe(2);
  });
});

:)

describe('User profile module', () => {
  function createProfileModule({
    element = document.getElementById('my-profile'),
    likes = 0,
    pubSub = new PubSub({ sync: true })
  }) {
    return new ProfileModule({ element, likes, pubSub });
  }

  it('should publish a topic when a new "like" is given', () => {
    const pubSub = {
    	notify: jest.fn(),
    };
    const profileModule = createProfileModule({ pubSub });

    profileModule.incLikes();

    expect(pubSub.notify).toHaveBeenCalledWith('likes:inc');
  });

  it('should retrieve the correct number of likes', () => {
    const profileModule = createProfileModule({ likes: 40 });

    profileModule.incLikes();
    profileModule.incLikes();

    expect(profileModule.getLikes()).toBe(42);
  });
});

β€’ Back to ToC β€’

Know your testing framework API

The API documentation of the testing framework/library should be your bedside book!

Having a good knowledge of the API can help you in reducing the size/complexity of your test code and, in general, help you during development. A simple example:

:(

it('should call a method with the proper arguments', () => {
  const foo = {
    bar: jest.fn(),
    baz: jest.fn(),
  };

  foo.bar('qux');

  expect(foo.bar).toHaveBeenCalled();
  expect(foo.bar.calls[0][0]).toEqual('qux');
});

/*it('should do more but not now', () => {
});

it('should do much more but not now', () => {
});*/

:)

it.only('should call once a method with the proper arguments', () => {
	const foo = {
		bar: jest.fn(),
		baz: jest.fn(),
	};

  foo.bar('baz');

  expect(foo.bar).toHaveBeenCalledWith('baz');
});

it('should do something else but not now', () => {
});

it('should do something else but not now', () => {
});

Note

The handy .only function used in the example above allows you to execute only one test without having to comment out all the tests below. describe.only does the same for test suites. This could help save a lot of time when developing.

β€’ Back to ToC β€’

Don't test multiple concerns in the same test

If a method has several end results, each one should be tested separately. Whenever a bug occurs, it will help you locate the source of the problem.

You can have multiple expects as long as they address the same concern.

:(

it('should process the order and save it to the database and trigger the webhook', () => {
    const response = api.processOrders(order);
  
    // here we are checking both that data was inserted into the database AND to the webhook was triggered
    expect(mockOrderWebhook.pushUpdate).toHaveBeenCalledWith(order);
    expect(mockLogInfo).toHaveBeenCalledWith(`Publishing webhook for ${order.id}`);
    
    expect(orderFetcher.getOrder(order.id)).toEqual(order);
});

:)

it('should trigger the process orders webhook', () => {
	const response = api.processOrders(order);
	expect(mockOrderWebhook.pushUpdate).toHaveBeenCalledWith(order);
	expect(mockLogInfo).toHaveBeenCalledWith(`Publishing webhook for ${order.id}`);
});

it('should insert the order record into the database', () => {
	const response = api.processOrders(order);
	expect(orderFetcher.getOrder(order.id)).toEqual(order);
});

Beware that writing "AND" or "OR" when naming your test smells bad...

β€’ Back to ToC β€’

Cover the general case and the edge cases

"Strange behaviour" usually happens at the edges... Remember that your tests can be the live documentation of your code.

:(

it('should properly calculate a RPN expression', () => {
  const result = RPN('5 1 2 + 4 * - 10 /');
  expect(result).toBe(-0.7);
});

:)

describe('The RPN expression evaluator', () => {
  it('should return null when the expression is an empty string', () => {
    const result = RPN('');
    expect(result).toBeNull();
  });

  it('should return the same value when the expression holds a single value', () => {
    const result = RPN('42');
    expect(result).toBe(42);
  });

  it('should properly calculate an expression', () => {
    const result = RPN('5 1 2 + 4 * - 10 /');
    expect(result).toBe(-0.7);
  });

  it('should throw an error whenever an invalid expression is passed', () => {
    const compute = () => RPN('1 + - 1');
    expect(compute).toThrow();
  });
});

β€’ Back to ToC β€’

When applying TDD (not required), always start by writing the simplest failing test

:(

it('should suppress all chars that appear multiple times', () => {
  expect(keepUniqueChars('Hello Fostonic !!')).toBe('HeFstnic');
});

:)

it('should return an empty string when passed an empty string', () => {
  expect(keepUniqueChars('')).toBe('');
});

From there, start building the functionalities incrementally.

β€’ Back to ToC β€’

When applying TDD (not required), always make small steps in each test-first cycle

Build your tests suite from the simple case to the more complex ones. Keep in mind the incremental design. Deliver software fast, incrementally, and in short iterations.

:(

it('should return null when the expression is an empty string', () => {
  const result = RPN('');
  expect(result).toBeNull();
});

it('should properly calculate a RPN expression', () => {
  const result = RPN('5 1 2 + 4 * - 10 /');
  expect(result).toBe(-0.7);
});

:)

describe('The RPN expression evaluator', () => {
  it('should return null when the expression is an empty string', () => {
    const result = RPN('');
    expect(result).toBeNull();
  });

  it('should return the same value when the expression holds a single value', () => {
    const result = RPN('42');
    expect(result).toBe(42);
  });

  describe('Additions-only expressions', () => {
    it('should properly calculate a simple addition', () => {
      const result = RPN('41 1 +');
      expect(result).toBe(42);
    });

    it('should properly calculate a complex addition', () => {
      const result = RPN('2 9 + 15 3 + + 7 6 + +');
      expect(result).toBe(42);
    });
  });

  // ...

  describe('Complex expressions', () => {
    it('should properly calculate an expression containing all 4 operators', () => {
      const result = RPN('5 1 2 + 4 * - 10 /');
      expect(result).toBe(-0.7);
    });
  });
});

β€’ Back to ToC β€’

Test the behaviour, not the internal implementation

:(

it('should add a user in memory', () => {
  userManager.addUser('Dr. Falker', 'Joshua');

  expect(userManager._users[0].name).toBe('Dr. Falker');
  expect(userManager._users[0].password).toBe('Joshua');
});

A better approach is to test at the same level of the API:

:)

it('should add a user in memory', () => {
  userManager.addUser('Dr. Falker', 'Joshua');

  expect(userManager.loginUser('Dr. Falker', 'Joshua')).toBe(true);
});

Pro:

  • Changing the internal implementation of a class/object will not necessarily force you to refactor the tests

Con:

  • If a test is failing, we might have to debug to know which part of the code needs to be fixed

Here, a balance has to be found, unit-testing some key parts can be beneficial.

β€’ Back to ToC β€’

Don't mock everything

:(

describe('when the user has already visited the page', () => {
  // storage.getItem('page-visited', '1') === '1'
  describe('when the survey is not disabled', () => {
    // storage.getItem('survey-disabled') === null
    it('should display the survey', () => {
      const storage = jasmine.createSpyObj('storage', ['setItem', 'getItem']);
      storage.getItem.and.returnValue('1'); // ouch.

      const surveyManager = new SurveyManager(storage);
      spyOn(surveyManager, 'display');

      surveyManager.start();

      expect(surveyManager.display).toHaveBeenCalled();
    });
  });

  // ...
});

This test fails, because the survey is considered disabled. Let's fix this:

:)

describe('when the user has already visited the page', () => {
  // storage.getItem('page-visited', '1') === '1'
  describe('when the survey is not disabled', () => {
    // storage.getItem('survey-disabled') === null
    it('should display the survey', () => {
      const storage = {
      	setItem: jest.fn(),
        getItem: jest.fn(),
      };
      
      storage.getItem.mockImplementation((key) => {
        switch (key) {
          case 'page-visited':
            return '1';

          case 'survey-disabled':
            return null;
        }

        return null;
      });

      const surveyManager = new SurveyManager(storage);
      spyOn(surveyManager, 'display');

      surveyManager.start();

      expect(surveyManager.display).toHaveBeenCalled();
    });
  });

  // ...
});

This will work... but needs a lot of code. Let's try a simpler approach:

:(

describe('when the user has already visited the page', () => {
  // storage.getItem('page-visited', '1') === '1'
  describe('when the survey is not disabled', () => {
    // storage.getItem('survey-disabled') === null
    it('should display the survey', () => {
      const storage = window.localStorage; // ouch.
      storage.setItem('page-visited', '1');

      const surveyManager = new SurveyManager();
      spyOn(surveyManager, 'display');

      surveyManager.start();

      expect(surveyManager.display).toHaveBeenCalled();
    });
  });

  // ...
});

We created a permanent storage of data. What happens if we do not properly clean it? We might affect the other tests. Let's fix this:

Try to consider the effort involved in "cleaning up after yourself" and the ramifications if you don't. Sometimes there are better ways to achieve the same result.

Also, whenever possible, avoid using/mutating global variables or setting anything globally.

:) :)

describe('when the user has already visited the page', () => {
  // storage.getItem('page-visited', '1') === '1'
  describe('when the survey is not disabled', () => {
    // storage.getItem('survey-disabled') === null
    it('should display the survey', () => {
      const storage = new MemoryStorage(); // see https://github.com/tatsuyaoiw/webstorage
      storage.setItem('page-visited', '1');

      const surveyManager = new SurveyManager(storage);
      spyOn(surveyManager, 'display');

      surveyManager.start();

      expect(surveyManager.display).toHaveBeenCalled();
    });
  });
});

The MemoryStorage used here does not persist data. Nice and easy, with no side effects.

Takeaway

The idea to keep in mind is that dependencies can still be "real" objects. Don't mock everything because you can. In particular, consider using the "real" version of the objects if:

  • it leads to a simple, nice and easy tests setup
  • it does not create a shared state between the tests, causing unexpected side effects
  • the code being tested does not make AJAX requests, API calls or browser page reloads
  • the speed of execution of the tests stays within the limits you fixed

β€’ Back to ToC β€’

Keep mock implementations of dependencies within the describe blocks

When mocking dependencies, the jest.mock should only be the initial mock of that specific dependency. It should not contain any further implementation details/mocks.

:(

import { myFunction } from './my-function';
import { Foo } from 'dependency';
import { mocked } from 'ts-jest/utils';

const mockBar = jest.fn().mockReturnValue(true);
jest.mock('dependency', () => ({
  Foo: jest.fn().mockImplementation(() => ({
    bar: mockBar
  })),
}));

describe('myFunction', () => {
  it('should do a thing.', () => {
    // assume the internal implementation of myFunction simply returns Foo.bar
    expect(myFunction()).toBe(true);
  });
});

:)

import { myFunction } from './my-function';
import { Foo } from 'dependency';
import { mocked } from 'ts-jest/utils';

jest.mock('dependency', () => ({
  Foo: jest.fn(),
}));

const mockedFooClass = mocked(Foo);

describe('myFunction', () => {
  const mockFooInstance = { bar: jest.fn() }
  
  beforeEach(() => {
    mockFooInstance.bar.mockReturnValue(true);
    mockedFooClass.mockImplentation(() => mockFooInstance);
  });

  it('should do a thing.', () => {
    // assume the internal implementation of myFunction simply returns Foo.bar
    expect(myFunction()).toBe(true);
  });
});

When mocking a class, use the mocked instance instead of mocking methods directly

When mocking a class and needing to mock a method on that class that is later used in a test, use a mock object to be the instance of the class, rather than mocking the specific method name.

:(

const mockedFooClass = mocked(Foo);

describe('myFunction', () => {
  const mockBar = jest.fn()
  
  beforeEach(() => {
    mockedFooClass.mockImplementation(() => ({ bar: mockBar }));
    mockBar.mockReturnValue(true);
  })

  it('should be called with blah', () => {
    myFunction('blah');
    expect(mockBar).toHaveHaveBeenCalledWith('blah');
  });
})

:)

const mockedFooClass = mocked(Foo);

describe('myFunction', () => {
  const mockFooInstance = { bar: jest.fn() };
  
  beforeEach(() => {
    mockedFooClass.mockImplementation(() => mockFooInstance);
    mockFooInstance.bar.mockReturnValue(true);
  })

  it('should be called with blah', () => {
    myFunction('blah');
    expect(mockFooInstance.bar).toHaveHaveBeenCalledWith('blah');
  });
})

Create new tests for every defect (FOR REALSIES NOT KIDDING!!!!!)

Whenever a bug is found, create a test that replicates the problem.

When this is not possible (due to the defect being a sev / unrealistically large effort to test) you MUST enter a tech-debt ticket which should be immediately prioritized.

β€’ Back to ToC β€’

Don't write unit tests for complex user interactions

Examples of complex user interactions:

  • Filling a form, drag and dropping some items then submitting the form
  • Clicking a tab, clicking an image thumbnail then navigating through a gallery of images previously loaded from a database
  • (...)

These interactions might involve many units of work and should be handled at a higher level by functional tests. They will take more time to execute. They could be flaky (false negatives) and they need debugging whenever a failure is reported.

For functional testing, consider using a test automation framework (Selenium, ...) or QA manual testing.

β€’ Back to ToC β€’

Test simple user actions

Example of simple user actions:

  • Clicking on a link that toggles the visibility of a DOM element
  • Submitting a form that triggers the form validation
  • (...)

These actions can be easily tested by simulating DOM events, for example:

describe('clicking on the "Preview profile" link', () => {
  it('should show the profile preview if it is hidden', () => {
    const previewLink = document.createElement('a');
    const profileModule = createProfileModule({ previewLink, previewIsVisible: false });

    spyOn(profileModule, 'showPreview');

    click(previewLink);

    expect(profileModule.showPreview).toHaveBeenCalled();
  });

  it('should hide the profile preview if it is displayed', () => {
    const previewLink = document.createElement('a');
    const profileModule = createProfileModule({ previewLink, previewIsVisible: true });

    spyOn(profileModule, 'hidePreview');

    click(previewLink);

    expect(profileModule.hidePreview).toHaveBeenCalled();
  });
});

Note how simple the test is because the UI (DOM) layer does not mix with the business logic layer:

  • a "click" event occurs
  • a public method is called

The next step could be to test the business logic implemented in "showPreview()" or "hidePreview()".

β€’ Back to ToC β€’

Review test code first (strongly suggested, not required)

When reviewing code, always start by reading the code of the tests. Tests are mini use cases of the code that you can drill into.

It will help you understand the intent of the developer very quickly (could be just by looking at the names of the tests).

β€’ Back to ToC β€’

Practice code katas, learn with pair programming (suggested)

Because experience is the only teacher. Ultimately, greatness comes from practicing; applying the theory over and over again, using feedback to get better every time.

β€’ Back to ToC β€’

Typescript Specific Considerations

Setting up TS mocks

When mocking dependencies in Typescript you will need to use the mocked function from ts-jest/utils in order to tell TS that the function that is being mocked is an actual Jest mock.

// function under test
import { doCoolThing } from './do-cool-thing';

// mocked dependencies
import { coolify } from '@coolify';
// mocking utils
import { mocked } from 'ts-jest/utils';

// jest mock but TS doesn't know that it is a mock yet.
jest.fn('@coolify', () => ({
  coolify: jest.fn(),
}));

// mocked just changes the type for us so TS knows that this is a jest mock.
const mockedCoolify = mocked(coolify);

describe('do cool thing', () => {
  const mockCoolThing = 'cool';

  beforeEach(() => {
    // TS will recognize mockReturnValue since mockedCoolify is of type Jest.Mocked (or whatever the type is)
    mockedCoolify.mockReturnValue(mockCoolThing);
  });

  it('should return a cool thing', () => {
    expect(doCoolThing()).toBe(mockCoolThing);
  });
});

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πŸ“™ A guide to unit testing in Javascript

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