TypeScript: conditional type inside the same type/interface

Question

Im trying to get some conditional typing to work and not sure if what Im thinking is possible to do or not. Here it goes:

type AParams = {
    hello: string

}
type BParams = {
    world: string
}

type Data = {
    name: "a" | "b",
    data: AParams | BParams
}

Depending on name being a or b I want to enforce the data to either have type AParams or BParams , respectively. Any way this can be accomplished with TypeScript?

Answer 1

You can use two different interfaces for the two cases like:

interface AData {
    name: "a",
    data: AParams
}

interface BData {
    name: "b",
    data: BParams
}

and then set Data using union types like:

type Data = AData | BData;

Answer 2

Your Data type can be next instead:

type Data = {
    name: "a"
    data: AParams
} | {
    name: "b"
    data: BParams
}