Generic types in Typescript offer a range of possibilities

I have been struggling to extract a list of available IDs from an object array. Despite my attempts at trial and error, I am hopeful that someone can offer me some assistance.

Here is an example of the array structure:

const myValues = [
    {id: 'abc', label: 'anyLabelForAbc'},
    {id: 'xyz', label: 'anyLabelForXyz'},
    {id: 'foo', label: 'anyLabelForFoo'},
    {id: 'bar', label: 'anyLabelForBar'},
]

My goal is to retrieve all available IDs such as 'abc' | 'xyz' | 'foo' | 'bar' from a generic type like this:

const availableIds: AvailableIds<typeof myValues > = 'abc' //'abc' | 'xyz' | 'foo' | 'bar'

Despite trying various methods, none of my attempts seem to be working correctly. The current code I have doesn't yield the desired results:

type Item = { id: string; label: string };
type Index<T> = T extends number ? T : never;
type Ids<T extends Item []> = T[Index<T>]['id'];

const items: Item[] = {
    {id: 'abc', label: 'anyLabelForAbc'},
    {id: 'xyz', label: 'anyLabelForXyz'},
    {id: 'foo', label: 'anyLabelForFoo'},
    {id: 'bar', label: 'anyLabelForBar'},
}
const ids: Ids<typeof items> = ''; // not receiving any suggested output

Any help would be greatly appreciated. Thank you.

Answer №1

One crucial step is to include the as const modifier with the items array to maintain the specific literal types for the id property; otherwise, they will be automatically widened to string.

Subsequently, you can utilize an index type query to extract the specific type of the id property from the items within the items array:

const items = [
  { id: 'abc', label: 'anyLabelForAbc' },
  { id: 'xyz', label: 'anyLabelForXyz' },
  { id: 'foo', label: 'anyLabelForFoo' },
  { id: 'bar', label: 'anyLabelForBar' },
] as const

type AvailableIds = typeof items[number]['id']
const ids: AvailableIds[] = items.map(o => o.id);

Link to Playground

Answer №2

Applying a const assertion enables the compiler to deduce literal types in situations where it typically wouldn't (for example, inferring strings only from property values in objects rather than string literals).

You can merge this idea with a restricted identity function to extract desired types and data:

TS Playground

type Item = { id: string; label: string };

function createItems <T extends readonly Item[]>(items: T): T {
  return items;
};

const items = createItems([
  {id: 'abc', label: 'anyLabelForAbc'},
  {id: 'xyz', label: 'anyLabelForXyz'},
  {id: 'foo', label: 'anyLabelForFoo'},
  {id: 'bar', label: 'anyLabelForBar'},
] as const); /*
  ^^^^^^^^
const assertion */

const ids = items.map(({id}) => id);
    //^? const ids: ("abc" | "xyz" | "foo" | "bar")[]

Utilizing a restricted identity function can prevent errors when constructing your data by ensuring that the input value is assignable to the type constraint:

const invalidItems = createItems([
  {id: 'abc', label: 'abc'},
  {id: 'xyz'},
] as const); // Compiler error: ...Property 'label' is missing in type '{ readonly id: "xyz"; }' but required in type 'Item'.(2345)

If you neglect using a constraint, the const assertion won't be beneficial, leading to data that doesn't match the specified type.

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