What is the best approach for conducting unit tests on model interfaces with TypeScript?

export interface Person {
    fullName: string;
}

What is the best way to create unit tests for the above interface and ensure that Karma includes it in the code coverage report?

I attempted to assert properties by creating an object, but it seems that Karma is not recognizing it in the coverage report, even though the test passes.

import { Person } from "./person";

describe('Person', () => {

    it('should have correct name property', () => {
        const person: Person = {
            fullName: "John Doe",
        }
        expect(person.fullName).toEqual("John Doe");
    });

});

Answer №1

It is impossible. There is no coding involved in this scenario as nothing can be executed.

Interfaces are only present during the compilation process and do not have any existence during runtime.

Answer №2

When encountering similar issues in the future, I have developed a unique system for testing interfaces, particularly when dealing with unconventional interfaces that have been autogenerated. This method acts as a workaround and effectively identifies any issues with the interface specification through failed builds.

Initially, in the "test" phase, I recommend casting an object with the expected fields and types into the interface. For example,

interface MyInterface = { 
  id: number;
  createTime: Date;
}

test("MyInterface should have appropriate fields and types", () => {
  ({
    id: 3,
    createTime: new Date(),
  } as MyInterface);
})

Subsequently, I have implemented a build step in the TypeScript compilation process, which triggers an error if there are any changes made to the MyInterface.

tsc --noEmit

It is essential to note that these tests do not include assertions, meaning they do not function as traditional unit tests. Nevertheless, this workaround has proven to be effective in pinpointing issues, thus fulfilling its intended purpose.

Answer №3

Expanding on the insight provided by @JB_Nizet, my response delves into the topic further.

When it comes to test coverage for a report, there are alternative approaches to consider:

1. Separate your interface into its own file and exclude it from the coverage report. I personally utilize the istanbul library for generating code test coverage reports, and it allows for the exclusion of specific files. Simply add the following comment in the file to have it ignored:

/* istanbul ignore file */

2. If ensuring the consistency of your types and interfaces is a priority, you can write dedicated tests for them that won't impact the coverage report. Various methods can be found online, but I find the technique of assigning the tested interface to a variable particularly effective:

interface IResponse {
  success: boolean;
  message: string;
}

describe('IResponse interface', () => {
  it('should contain a boolean property named success', () => {
    const response: IResponse = { success: true, message: 'Test message' };
    expect(response.success).toBeDefined();
    expect(typeof response.success).toBe('boolean');
  });

  it('should include a string property named message', () => {
    const response: IResponse = { success: true, message: 'Test message' };
    expect(response.message).toBeDefined();
    expect(typeof response.message).toBe('string');
  });
});

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