Unleash the power of Partial <T> for mock data perfection!
Tags: javascript, typescript, webdev, testing
Published: 2023-04-15 04:59:46

TypeScript Partial is a utility type that allows you to create a new type based on an existing one, making all of its properties optional. It is particularly useful when working with partial updates or constructing objects incrementally. In the context of testing, Partial can be a valuable tool for creating mock data. 

Lets take an example:

```
interface User {
  id: number;
  firstName: string;
  lastName: string;
  email: string;
}
```

Now lets define a function that creates a mock user for testing:

```
function createMockUser(userPartial: Partial<User>): User {
  const defaultUser: User = {
    id: 1,
    firstName: 'John',
    lastName: 'Doe',
    email: 'john.doe@example.com',
  };

  return { ...defaultUser, ...userPartial };
}
```

The `createMockUser` accepts a `Partial<User>` object and merges it with a `defaultUser` object. This way, you only need to provide the properties relevant to your test, and the function will take care of the rest:

```
const mockUser1 = createMockUser({ firstName: 'Jane', lastName: 'Smith' });

const mockUser2 = createMockUser({ email: 'jane.doe@example.com' });

```

So what are the advantages of this approach:

- **Focussed testing**: You can concentrate on testing specific properties or functionality rather than being overwhelmed by a complex object structure ( which is probably a code smell but a topic for another post ). This helps you write more targeted, efficient tests and makes it easier to identify the purpose of each test.

- **Increased maintainability**: Because you no longer need to specify all properties for your mock data, you minimise the need to update your tests when your interfaces change. This makes your tests more maintainable and less prone to breaking due to changes in the underlying data structures.

- **Less boilerplate**: With Partial, you only need to define the properties relevant to your test case, reducing the amount of boilerplate code and making your tests more focused and easier to understand.

- **Reusable mock data creation**: You can use functions like this that leverage Partial to create mock data easily and consistently across your tests. This promotes reusability and can help you maintain a consistent approach to creating mock data in your test suite. 

- **Enhanced readability**: When using Partial, it becomes clearer which properties of the mocked object are relevant to the test case. This improves the readability of your tests, making it easier for you and your team members to understand and maintain the test suite.

So in summary, Partials can be your best friend when it comes to testing, helping you write clean, consistent, maintainable tests that are more focussed. 

There's one more approach that I have recently come across by none other than our [Typescript Wizard: Matt Pocock](https://twitter.com/mattpocockuk). He's created a library called [shoehorn](https://github.com/total-typescript/shoehorn) that has some utility types. Head over to his article and read more about it. 

[Here's a link to his article](https://www.totaltypescript.com/testing-with-typescript-is-painful-heres-a-solution).

Happy Typescripting :)