Incorrectly asserting the data type of a union

Question

Incorrectly asserting the data type of a union

I am having trouble getting the type assertion to work in this specific scenario.

type Letter = "A" | "B"

type Useless = {}

type Container<T> =
    Useless |
    {
        type: "container"
        value: T
    }

function transform<X extends Letter, Y extends Letter>(container: Container<X | Y>): asserts container is Container<X> {
    // custom logic
}

let container: Container<"A" | "B"> = undefined as any

// Asserts no B
transform<"A", "B">(container)

container // Container<"A" | "B">
          // Container<"A"> when Useless is commented
          // I want Container<"A">

I want container to be of type Container<"A">.

Removing type Useless altogether makes it work as expected.

Replacing type Useless with type Useless = { key: string } results in container being of type Useless instead of Container<"A">.

How can I achieve the desired type of Container<"A">?

typescript typescript-typings type-assertion

Answer 1

Answer №1

It is somewhat unclear to me why TypeScript encounters issues in this scenario. My assumption is that the union with a generic, where T has no effect, can sometimes cause confusion.

When the code is separated, it seems to function correctly for me. I believe that in this context, the Useless type always asserts itself?

type Letter = "A" | "B"

type Useless = {}

type Container<T> =
    {
        type: "container"
        value: T
    }

type Input<T> = Useless | Container<T>

function transform<X extends Letter, Y extends Letter>(container: Input<X | Y>): asserts container is Container<X> {
    // custom logic
}

let container: Input<"A" | "B"> = undefined as any

// Asserts no B
transform<"A", "B">(container)

container // Container<"A" | "B">
          // Container<"A"> when Useless is commented out
          // I expect Container<"A">

Playground

Answer 2

It is somewhat unclear to me why TypeScript encounters issues in this scenario. My assumption is that the union with a generic, where T has no effect, can sometimes cause confusion.

When the code is separated, it seems to function correctly for me. I believe that in this context, the Useless type always asserts itself?

type Letter = "A" | "B"

type Useless = {}

type Container<T> =
    {
        type: "container"
        value: T
    }

type Input<T> = Useless | Container<T>

function transform<X extends Letter, Y extends Letter>(container: Input<X | Y>): asserts container is Container<X> {
    // custom logic
}

let container: Input<"A" | "B"> = undefined as any

// Asserts no B
transform<"A", "B">(container)

container // Container<"A" | "B">
          // Container<"A"> when Useless is commented out
          // I expect Container<"A">

Playground

Incorrectly asserting the data type of a union

Answer №1

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