Unit testing in Typescript often involves the practice of mocking

Question

Unit testing in Typescript often involves the practice of mocking

One challenge with mocking in Typescript arises when dealing with complex objects, as is the case with any strongly-typed language. Sometimes additional elements need to be mocked just to ensure code compilation, such as using AutoFixture in C#. In contrast, Javascript being a dynamic language allows for only specific parts of an object to be mocked when needed for testing.

In Typescript unit testing, dependencies can be declared using the any type to facilitate easy mocking. Are there any potential drawbacks to this approach?

let userServiceMock: MyApp.Services.UserService = {
    // lots of things to mock
}

vs

let userServiceMock: any = {
    user: {
         setting: {
             showAvatar: true
         }
    }
}

javascript unit-testing typescript mocking

Answer 1

Answer №1

In my experience with unit tests in TypeScript, I have found that it is crucial to ensure all mock objects are typed. If mocks are left with a type of 'any,' it can lead to issues during renaming processes. The IDE may struggle to identify which instances of the 'user' or 'settings' parameter need to be updated. However, creating mock objects manually with complete interfaces can be quite tedious.

Luckily, there are two TypeScript tools available for generating type-safe mock objects: ts-mockito (inspired by Java mockito) and typemoq (inspired by C# Moq).

Answer 2

In my experience with unit tests in TypeScript, I have found that it is crucial to ensure all mock objects are typed. If mocks are left with a type of 'any,' it can lead to issues during renaming processes. The IDE may struggle to identify which instances of the 'user' or 'settings' parameter need to be updated. However, creating mock objects manually with complete interfaces can be quite tedious.

Luckily, there are two TypeScript tools available for generating type-safe mock objects: ts-mockito (inspired by Java mockito) and typemoq (inspired by C# Moq).

Answer 3

Answer №2

With TypeScript 3 now released, the power of full strong typing can finally be fully utilized! I recently converted NSubstitute to TypeScript to take advantage of this.

You can access it here: https://www.npmjs.com/package/@fluffy-spoon/substitute

I also did a comparison with other popular frameworks, which you can find here: https://medium.com/@mathiaslykkegaardlorenzen/with-typescript-3-and-substitute-js-you-are-already-missing-out-when-mocking-or-faking-a3b3240c4607

Take note of how it enables the creation of fakes from interfaces while maintaining full strong typing throughout!

Answer 4

With TypeScript 3 now released, the power of full strong typing can finally be fully utilized! I recently converted NSubstitute to TypeScript to take advantage of this.

You can access it here: https://www.npmjs.com/package/@fluffy-spoon/substitute

I also did a comparison with other popular frameworks, which you can find here: https://medium.com/@mathiaslykkegaardlorenzen/with-typescript-3-and-substitute-js-you-are-already-missing-out-when-mocking-or-faking-a3b3240c4607

Take note of how it enables the creation of fakes from interfaces while maintaining full strong typing throughout!

Answer 5

Answer №3

Highlighted by @Terite, using the any type in mocks can lead to a lack of connection between the mock and its actual type/implementation. A better approach would be to cast partially-mocked objects to the mocked type:

export interface UserService {
    getUser: (id: number) => User;
    saveUser: (user: User) => void;
    // ... other methods/fields
}

.......

let userServiceMock: UserService = <UserService> {
    saveUser(user: User) { console.log("save user"); }
}
spyOn(userServiceMock, 'getUser').andReturn(new User());
expect(userServiceMock.getUser).toHaveBeenCalledWith(expectedUserId);

It's important to note that Typescript doesn't allow casting objects with additional members (a superset or derived type), meaning your partial mock will be of the base type to the UserService and can be safely cast. For example:

// Error: Neither type '...' nor 'UserService' is assignable to the other.
let userServiceMock: UserService = <UserService> {
     saveUser(user: User) { console.log("save user"); },
     extraFunc: () => { } // not available in UserService
}

Answer 6

Highlighted by @Terite, using the any type in mocks can lead to a lack of connection between the mock and its actual type/implementation. A better approach would be to cast partially-mocked objects to the mocked type:

export interface UserService {
    getUser: (id: number) => User;
    saveUser: (user: User) => void;
    // ... other methods/fields
}

.......

let userServiceMock: UserService = <UserService> {
    saveUser(user: User) { console.log("save user"); }
}
spyOn(userServiceMock, 'getUser').andReturn(new User());
expect(userServiceMock.getUser).toHaveBeenCalledWith(expectedUserId);

It's important to note that Typescript doesn't allow casting objects with additional members (a superset or derived type), meaning your partial mock will be of the base type to the UserService and can be safely cast. For example:

// Error: Neither type '...' nor 'UserService' is assignable to the other.
let userServiceMock: UserService = <UserService> {
     saveUser(user: User) { console.log("save user"); },
     extraFunc: () => { } // not available in UserService
}

Answer 7

Answer №4

When it comes to utilizing functional objects, there are options like mock libraries that offer support for TypeScript or JavaScript libraries with type definitions. However, these types only exist during the design phase. For instance, JasmineJS provides Spy functionality which can be used in a type-safe manner as shown below:

spyOn(SomeTypescriptClass, "SomeTypescriptClassProperty");

Both the IDE and TypeScript compiler will handle this properly. One limitation though is that parameters are not supported. If you require type support for parameters, TypeScript mock libraries like moq.ts can be utilized.

For DTO objects, an alternative approach can be taken:

export type IDataMock<T> = {
  [P in keyof T]?: IDataMock<T[P]>;
};

export function dataMock<T>(instance: IDataMock<T>): T {
  return instance as any;
}

// Usage example
const obj = dataMock<SomeBigType>({onlyOneProperty: "some value"});

Note that IDataMock might be replaceable with the standard Partial interface from TypeScript.

Answer 8

When it comes to utilizing functional objects, there are options like mock libraries that offer support for TypeScript or JavaScript libraries with type definitions. However, these types only exist during the design phase. For instance, JasmineJS provides Spy functionality which can be used in a type-safe manner as shown below:

spyOn(SomeTypescriptClass, "SomeTypescriptClassProperty");

Both the IDE and TypeScript compiler will handle this properly. One limitation though is that parameters are not supported. If you require type support for parameters, TypeScript mock libraries like moq.ts can be utilized.

For DTO objects, an alternative approach can be taken:

export type IDataMock<T> = {
  [P in keyof T]?: IDataMock<T[P]>;
};

export function dataMock<T>(instance: IDataMock<T>): T {
  return instance as any;
}

// Usage example
const obj = dataMock<SomeBigType>({onlyOneProperty: "some value"});

Note that IDataMock might be replaceable with the standard Partial interface from TypeScript.

Unit testing in Typescript often involves the practice of mocking

Answer №1

Answer №2

Answer №3

Answer №4

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