what's the best way to type a mock store?

Question

Our test code is riddled with code like this in our tests...

const mockState = {
      client: {
        users: {
          results: {
            something: -91893.21,
          },
        },
      },
    };

Obviously mockState is untyped. I can't type it as RootState since that interface has heaps of properties that this test doesn't need.

I looked at Partial and DeepPartial but they don't really help since any json would implement any DeepPartial since every property would be optional. Is there a way to type a json object so that all it's properties match a specific type, but not all properties are required. Confusing question. I want to type mockState to IWhatever so that it won't compile unless IWhatever has a property called client, which has a property called users etc.

Answer 1

If you do the assignment at the same time as you do the variable definitionobject freshness kicks in and will prevent you from adding any additional properties.

Example

type RootState = {
  client: {
    a: {
      something: number
    },
    b: {
      something: number
    }
  },
  server: {
    a: {
      something: number
    },
    b: {
      something: number
    }
  }
}

// Reference DeepPartial
type DeepPartial<T> = {
  [P in keyof T]?: T[P] extends Array<infer U>
    ? Array<DeepPartial<U>>
    : T[P] extends ReadonlyArray<infer U>
      ? ReadonlyArray<DeepPartial<U>>
      : DeepPartial<T[P]>
};


// Okay 
const mockState: DeepPartial<RootState> = {
  client: {
    a: {
      something: 123
    }
  }
}

// Not Okay 
const mockStateBad: DeepPartial<RootState> = {
  client: {
    a: {
      somethings: 123 // ERROR 
    }
  }
}