根据 object 中另一个键的值强制执行特定类型的键（可区分联合）

Question

Maybe the title of the question wasn't so explicit what about I want, but what I want is something called discrimnated unions , I think. It's documented here: https://www.typescriptlang.org/docs/handbook/unions-and-intersections.html#discriminating-unions

A have a type A, with two fields, B and C, the type of the field B is well-defined set of string literals and I want that the type of C be in reason of the value from the field B.

Some code to better explain:

type Field1Values = "a" | "b" | "c";

type TypeForValueA = {
   ...
}

type TypeForValueB = {
   ...
}

type TypeForValueC = {
   ...
}

type RootType = {
    field1: Field1Values,
    field2: <something here that I didn't know exactly what to put>
}


const objA: RootType = {
    field1: "a",
    field2: {
        // Only the values from the type TypeForValueA are allowed
    }

}

const objB: RootType = {
    field1: "b",
    field2: {
        // Only the values from the type TypeForValueB are allowed
    }

}

How can I achieve the effect that I want?

Thanks

Edit:

I already tried like in the docs

type NetworkState =
  | NetworkLoadingState
  | NetworkFailedState
  | NetworkSuccessState;

And the lint shows me all the fields from all the types, I want that only the fields from the correct type be shown

Answer 1

I've created a playground link demonstrating an answer .

The problem with your original approach is that it doesn't communicate any mapping between the discriminating field's set of possible values and the discriminated field's possible set of types. All I did was adapt your example to follow the TypeScript docs you linked.

// type Field1Values = "a" | "b" | "c"; // no longer needed

type TypeForValueA = {
   fieldForA: string;
}

type TypeForValueB = {
   fieldForB: number;
}

type TypeForValueC = {
   fieldForC: boolean;
}

type RootA = {
    field1: "a";
    field2: TypeForValueA;
}
type RootB = {
    field1: "b";
    field2: TypeForValueB;
}
type RootC = {
    field1: "c";
    field2: TypeForValueC;
}
type RootType = RootA | RootB | RootC;


const objA: RootType = {
    field1: "a",
    // field2: press `ctrl+space` for suggestions
}
const objB: RootType = {
    field1: "b",
    // field2: press `ctrl+space` for suggestions
}
const objC: RootType = {
    field1: "c",
    // field2: press `ctrl+space` for suggestions
}