How to include object keys into Generic types

Question

Hello, I have this issue with tsx. I'm not sure how to compose the type for generics.

In my Options component I have an interface that gets display (display is property name of T, value will be passed as string) that should be a key in T. T will hold an array of PETS or USERS. The extends {id: string} here is to ensure that the PETS or USERS has an ID property.

These links are my reference.

https://www.typescriptlang.org/docs/handbook/advanced-types.html#index-types

https://react-typescript-cheatsheet.netlify.app/docs/advanced/patterns_by_usecase/#generic-components

Thank you for the help.

import React from "react";

interface IOptions<T, K extends keyof T> {
  display: K;
  values: T[];
}

const Options = <Y extends {id: string}, U>({values, display}: IOptions<Y, U>) => {
  return (
    <>
      {values.map((val: Y, index: number) => (
        <option key={index} value={val.id}>{val[display]}</option>
      ))}
    </>
  );
};

export default Options;

also tried this:

<option key={index} value={val.id}>{val[display as keyof Y]}</option>

Answer 1

Type 'U' does not satisfy the constraint 'keyof Y'.

...and:

Type 'U' cannot be used to index type 'Y'.

(Y in your screenshot)

You need to constrain U so that it is compatible with the constraint in IOptions as well:

const Options = <Y extends { id: string }, U extends keyof Y>({ values, display }: IOptions<Y, U>) => {}

This directly solves both current errors, because now U is explicitly a key of Y.

But then you will have another error appearing:

---

Type 'Y[U]' is not assignable to type 'ReactNode'.

That is because you constrain Y for id only, but it can have anything for its other keys, in particular values which are not suitable for rendering, e.g.:

const y = {
  id: "foo",
  bar: function () {} // Not suitable as ReactNode
} satisfies { id: string }

Therefore you will have to further constrain your generic type parameter Y, to ensure that Y[U] can be rendered, for example if all values are ReactNode:

const Options = <Y extends {
  id: string,
  [key: PropertyKey]: React.ReactNode
}, U extends keyof Y>({ values, display }: IOptions<Y, U>) => {}

Playground Link

---

Or we could be more specific, by constraining only the value of the U key to be renderable:

const Options = <
  U extends PropertyKey,
  Y extends {
    [key in U]: React.ReactNode // Only value of U key must be a ReactNode
  } & { id: string }
>({ values, display }: IOptions<Y, U>) => {}

Then you can pass values which other (non displayed) properties can be anything else:

const other = {
  id: "",
  foo: 0,
  content: <b>Bold</b>,
  notRenderable: () => { }
};

<Options
  values={[other]}
  display="content" // Okay with "id", "foo" or "content"
/>

<Options
  values={[other]} // Types of property 'notRenderable' are incompatible. Type '() => void' is not assignable to type 'ReactNode'.
  //       ~~~~~
  display="notRenderable"
/>

Playground Link