Kinda

For when you want TypeScript to stop yelling at you because your mock is only "kinda" like the real thing.

This takes the built-in TypeScript Partial<T> type to the next level by making it recursive.

Example

Let's say we have some interfaces:

interface A {
  name: string;
  id: number;
  b: B;
}
interface B {
  color: string;
  isActive: boolean;
  c: C;
}
interface C {
  foo: "bar";
}

If you try to create a mock version of A, normally you would have to implement everything recursively:

const mockA: A = {
  name: "testName",
  id: 0,
  b: {
    color: "testColor",
    isActive: false,
    c: {
      foo: "bar",
    },
  },
};

This is not too bad in this limited example, but imagine mocking something like a DOM element, with hundreds of properties!

Luckily TypeScript has a built-in type called Partial<T> which is the same as T, but with all the properties marked optional. This is great, except it only works on one level, so when you are building your mocks, you end up with a lot of as casting to and from Partial types to avoid compiler errors.

let mockA: A;

beforeEach(() => {
  mockA = {
    b: {
      c: {} as Partial<C> as C
    } as Partial<B> as B
  } as Partial<A> as A;
});

This greatly harms the readability of your tests, and is generally pretty messy. Why can't we just tell TypeScript to not worry about the type being complete, while still getting type checking and autocompletion for the things you do add to your mocks?

This is where Kinda<T> comes in. Kinda tells TypeScript that everything (recursively) is optional. This means that the previous example can now be simplified greatly:

let mockA: A;

beforeEach(() => {
  mockA = {
    b: {
      c: {}
    }
  } as Kinda<A> as A;
});

Now you can focus on your testing and not how to make TypeScript stop yelling at your mocks.

Happy testing!

Acknowledgement

The implementation of Kinda<T> was largely based on the work of Pavel Husakouski from this StackOverflow post: https://stackoverflow.com/a/64060332/1396477. Renamed to Kinda from RecursivePartial for brevity.