How can I resolve the error "Unable to use 'K extends X[T] ? keyof K : never' to access type for nested objects"?

Question

How can I resolve the error "Unable to use 'K extends X[T] ? keyof K : never' to access type for nested objects"?

I am currently working on developing a function with specific parameters. The first parameter, 'data', should be an array consisting of objects of type Product. The second parameter, 'element', can be any property within Product. Additionally, the third and fourth parameters specify properties of the 'element' we are targeting.

For example, if the 'Product' object includes a property named 'Customer', my goal is to extract the 'Id' and 'Name' attributes from the Customer section.

Below is the code snippet I have been using:

function getFilterItem<T extends keyof Product, K extends Product[T]>(
  data: Product[],
  element: T,
  id: K extends Product[T] ? keyof K : never,
  name: K extends Product[T] ? keyof K : never,
): FilterItem[] {
  // functionality removed for simplicity
}

The desired way to call this function would look like:

getFilterItem(data, 'customer', 'id', 'name');

While the current implementation correctly identifies parameters, there is an issue when attempting to access the specified property. An error arises stating "Type 'K extends Product[T] ? keyof K : never' cannot be used to index type".

This error might occur due to the fact that Product[T] can be of any type within Product. For instance, if the 'Product' object contains a numerical property like 'price', trying to call

getFilterItem(data, 'price', 'id', 'name);

would lead TypeScript to struggle with indexing using the third and fourth parameters.

Given the full code block provided below (with certain logic omitted), how can we enable 'id' as a key of 'Product[T]'?

For reference, I've included a functional example where we aim to obtain 'id' and 'autonomyLevel' from a nested property within Product:

sandbox

javascript typescript

Answer 1

Answer №1

If I understand correctly, your code looks like this:

interface Product {
  customer: Customer
}

interface Customer {
  id: number
  name: string
}

You are trying to access the third and fourth arguments from the keys of an object obtained by accessing the first argument using the second argument.

declare const data: Product
getFilterItem([data], 'customer', 'id', 'name');

In that case, you only need one generic type parameter for the "customer" key. The "id" and "name" keys can be inferred from that.

function getFilterItem<T extends keyof Product>(
  data: Product[],
  element: T,
  id: keyof Product[T],
  name: keyof Product[T],
) {
  data.forEach((product) => {
    const value = product[element];
    if (value) {
      const valueId = value[id]; // works
    }
  })
}

See Playground

The properties may be null

In such a scenario, my solution returns "never" for the secondary keys because the type of Product[T] is something like AutonomyLevel | never. To address this issue, you can use Exclude to remove undefined from the type:

function getFilterItem<T extends keyof Product>(
  data: Product[],
  element: T,
  id: keyof Exclude<Product[T], undefined>,
  name: keyof Exclude<Product[T], undefined>,
) {

Playground

However, a new problem arises:

function getFilterItem<T extends keyof Product>(
  data: Product[],
  element: T,
  id: keyof Exclude<Product[T], undefined>,
  name: keyof Exclude<Product[T], undefined>,
) {
  data.forEach((product) => {
    const value = product[element];
    if (value) {
      const valueId = value[id];
      // Type 'keyof Exclude<Product[T], undefined>' cannot be used to index type 'string | number | ProductType | AutonomyLevel | ProductStatus | Industry[] | LifeCycle[]'.(2536)
    }
  })
}

The TypeScript compiler struggles to match up different derived generic types in this situation. Although we have ensured type safety, TypeScript does not recognize it.

As a workaround, you can add a runtime check and mark it as an expected error:

  data.forEach((product) => {
    const value = product[element];
    if (value && id in value) {
      // @ts-expect-error The compiler has a hard time understanding
      // that `id` is guaranteed to be a valid key of `value`.
      const valueId: Product[T] = value[id];
    }
  })

This approach is acceptable because:

It is limited to the function scope and does not impact the overall types in your program.
It includes necessary runtime checks to ensure the validity of the key "id" in the "value" object.

While turning off the type checker is not ideal, this solution provides a somewhat safe option in this particular scenario.

Answer 2

If I understand correctly, your code looks like this:

interface Product {
  customer: Customer
}

interface Customer {
  id: number
  name: string
}

You are trying to access the third and fourth arguments from the keys of an object obtained by accessing the first argument using the second argument.

declare const data: Product
getFilterItem([data], 'customer', 'id', 'name');

In that case, you only need one generic type parameter for the "customer" key. The "id" and "name" keys can be inferred from that.

function getFilterItem<T extends keyof Product>(
  data: Product[],
  element: T,
  id: keyof Product[T],
  name: keyof Product[T],
) {
  data.forEach((product) => {
    const value = product[element];
    if (value) {
      const valueId = value[id]; // works
    }
  })
}

See Playground

The properties may be null

In such a scenario, my solution returns "never" for the secondary keys because the type of Product[T] is something like AutonomyLevel | never. To address this issue, you can use Exclude to remove undefined from the type:

function getFilterItem<T extends keyof Product>(
  data: Product[],
  element: T,
  id: keyof Exclude<Product[T], undefined>,
  name: keyof Exclude<Product[T], undefined>,
) {

Playground

However, a new problem arises:

function getFilterItem<T extends keyof Product>(
  data: Product[],
  element: T,
  id: keyof Exclude<Product[T], undefined>,
  name: keyof Exclude<Product[T], undefined>,
) {
  data.forEach((product) => {
    const value = product[element];
    if (value) {
      const valueId = value[id];
      // Type 'keyof Exclude<Product[T], undefined>' cannot be used to index type 'string | number | ProductType | AutonomyLevel | ProductStatus | Industry[] | LifeCycle[]'.(2536)
    }
  })
}

The TypeScript compiler struggles to match up different derived generic types in this situation. Although we have ensured type safety, TypeScript does not recognize it.

As a workaround, you can add a runtime check and mark it as an expected error:

  data.forEach((product) => {
    const value = product[element];
    if (value && id in value) {
      // @ts-expect-error The compiler has a hard time understanding
      // that `id` is guaranteed to be a valid key of `value`.
      const valueId: Product[T] = value[id];
    }
  })

This approach is acceptable because:

It is limited to the function scope and does not impact the overall types in your program.
It includes necessary runtime checks to ensure the validity of the key "id" in the "value" object.

While turning off the type checker is not ideal, this solution provides a somewhat safe option in this particular scenario.

Answer 3

Answer №2

If you are looking to retrieve the array element id value mentioned in the code snippet, there is no need to pass the prop name as an argument to the function. The common props for all types shared in Product should be defined as T, rather than the keys themselves. In this case, it appears that only the id key is being referenced.

In situations where the behavior of TypeScript may not seem logical, the explanation provided can clarify any uncertainties. The usage of

id: keyof Exclude<Product[T], undefined>

allows for calling the getFilterItem function with the 'id' parameter, especially when optional arguments exist within the Product type definition.

The essence of the issue lies in trying to obtain the index of a type Product[T] using

keyof Exclude<Product[T], undefined>

. These represent distinct types, where one may contain undefined values due to optionality while the other does not. This discrepancy is highlighted in the playground example. Omitted the id and image references due tothe implications of keyof and Lookup Types.

From a perspective of maintaining type safety, excluding undefined for the id argument in getFilterItem, and subsequently reintroducing it for the index check of Product[T] with T potentially containing undefined values from optional arguments, could lead to inconsistencies.

An additional consideration relates to the potential variations in accessing properties using object.prop versus object.[prop] due tokeyof and Lookup Types.

For resolution, focus on refining the type definitions to address scenarios involving optional arguments and property access concerns.

Answer 4

If you are looking to retrieve the array element id value mentioned in the code snippet, there is no need to pass the prop name as an argument to the function. The common props for all types shared in Product should be defined as T, rather than the keys themselves. In this case, it appears that only the id key is being referenced.

In situations where the behavior of TypeScript may not seem logical, the explanation provided can clarify any uncertainties. The usage of

id: keyof Exclude<Product[T], undefined>

allows for calling the getFilterItem function with the 'id' parameter, especially when optional arguments exist within the Product type definition.

The essence of the issue lies in trying to obtain the index of a type Product[T] using

keyof Exclude<Product[T], undefined>

. These represent distinct types, where one may contain undefined values due to optionality while the other does not. This discrepancy is highlighted in the playground example. Omitted the id and image references due tothe implications of keyof and Lookup Types.

From a perspective of maintaining type safety, excluding undefined for the id argument in getFilterItem, and subsequently reintroducing it for the index check of Product[T] with T potentially containing undefined values from optional arguments, could lead to inconsistencies.

An additional consideration relates to the potential variations in accessing properties using object.prop versus object.[prop] due tokeyof and Lookup Types.

For resolution, focus on refining the type definitions to address scenarios involving optional arguments and property access concerns.

How can I resolve the error "Unable to use 'K extends X[T] ? keyof K : never' to access type for nested objects"?

Answer №1

Answer №2

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