TypeScript object set with unique property

Question

i wrote simply interfaces like that:

interface IProduct {
  id: number;
  name: string;
  price: number;
  description?: string;
}

Now i want to id be unique in ReadonlyArray. So when a create a few products, i want to prevent add object with the same id. Array with products will be creating once, in file, and will not be modified.

Have you any idea for that? JS Set will be good solution, but i can't add own comparator to them. Please not provide solution which require additional frameworks etc.

Answer 1

This example uses type system to staticaly validate whether there are duplicates or not.

interface IProduct<Id extends number> {
    id: Id
    name: string;
}

const product = <Id extends number>(id: Id, name: string) => ({ id, name })

type Validation<
    Products extends IProduct<number>[],
    Accumulator extends IProduct<number>[] = []>
    =
    (Products extends []
        // #1 Last call
        ? Accumulator
        // #2 All calls but last
        : (Products extends [infer Head, ...infer Tail]
            ? (Head extends IProduct<number>
                // #3 Check whether [id] property already exists in our accumulator 
                ? (Head['id'] extends Accumulator[number]['id']
                    ? (Tail extends IProduct<number>[]
                        // #4 [id] property is a duplicate, hence we need to replace it with [never] in order to trigger the error
                        ? Validation<Tail, [...Accumulator, { id: never, name: Head['name'] }]>
                        : never)
                    // #5 [id] is not a duplicate, hence we can add to our accumulator whole product
                    : (Tail extends IProduct<number>[]
                        ? Validation<Tail, [...Accumulator, Head]>
                        : never)
                )
                : never)
            : never)
    )

type Ok = Validation<[{ id: 1, name: '1' }, { id: 2, name: '2' }]>
type Fail = Validation<[{ id: 1, name: '1' }, { id: 1, name: '2' }]> // id:never

const builder = <
    Product extends IProduct<number>,
    Products extends Product[]
>(...products: [...Products] & Validation<Products>) => products

builder(product(1, 'John'), product(2, 'Doe'))

Playground

Validation - iterates recursively through all passed into function products. If product[id] already exists in accumulator type - replace id property with never, otherwise just add product to accumulator.

Please see the comments #1, #2 ....

If you dont want to use rest operator, consider this example:

interface IProduct<Id extends number> {
    id: Id
    name: string;
}

const product = <Id extends number>(id: Id, name: string) => ({ id, name })

type Validation<
    Products extends IProduct<number>[],
    Accumulator extends IProduct<number>[] = []>
    =
    (Products extends []
        // #1 Last call
        ? Accumulator
        // #2 All calls but last
        : (Products extends [infer Head, ...infer Tail]
            ? (Head extends IProduct<number>
                // #3 Check whether [id] property already exists in our accumulator 
                ? (Head['id'] extends Accumulator[number]['id']
                    ? (Tail extends IProduct<number>[]
                        // #4 [id] property is a duplicate, hence we need to replace it with [never] in order to trigger the error
                        ? Validation<Tail, [...Accumulator, { id: never, name: Head['name'] }]>
                        : 1)
                    // #5 [id] is not a duplicate, hence we can add to our accumulator whole product
                    : (Tail extends IProduct<number>[]
                        ? Validation<Tail, [...Accumulator, Head]>
                        : 2)
                )
                : 3)
            : Products)
    )


type Ok = Validation<[{ id: 1, name: '1' }, { id: 2, name: '2' }]>
type Fail = Validation<[{ id: 1, name: '1' }, { id: 1, name: '2' }]> // id:never

const builder = <
    Id extends number,
    Product extends IProduct<Id>,
    Products extends Product[]
>(products: Validation<[...Products]>) => products

builder([product(1, 'John'), product(1, 'John')]) // error

Playground

If you are interested in static validation, you can check my article