MappingByDiscriminator
Mapping union type on discriminator
using the UnionMemberByDiscriminator and GetProperty described below,
type MappingByDiscriminator< E extends object, D extends keyof E > =
GetPropertyType<E,D> extends string ?
{
[d in GetPropertyType<E, D>]: UnionMemberByDiscriminator<E, D, d>[];
}
: never;
GetPropertyType<E,D> extends string ?
{
[d in GetPropertyType<E, D>]: UnionMemberByDiscriminator<E, D, d>[];
}
: never;
Will map types in a union by a discriminator.
Example:
type Car = {
kind: “car“,
…
};
kind: “car“,
…
};
type Truck = {
kind: “truck“,
…
};
type Vehicle = Car | Truck;
Then
type VehiclesByKind = MappingByDiscriminator<Vehicle,“kind“>;
Will produce :
type VehiclesByKind = {
car: Car[],
truck: Truck[],
}
car: Car[],
truck: Truck[],
}
GetProperty
Get the type of an object property with generics
type GetProperty<E extends object, P extends keyof E> =
E extends { [key in P]: infer V } ? V : never;
E extends { [key in P]: infer V } ? V : never;
Example:
type Entity = {
kind: ‘human‘ | ‘animal‘,
size: ‘microscopic‘ | ‘small‘ | ‘big‘ | ‘giant‘,
}
kind: ‘human‘ | ‘animal‘,
size: ‘microscopic‘ | ‘small‘ | ‘big‘ | ‘giant‘,
}
Then
type SizeType = GetProperty<Entity,“size“>
Will produce
type SizeType = ‘microscopic‘ | ‘small‘ | ‘big‘ | ‘giant‘;
UnionMemberByDiscriminator
Extract a specific type from a union for a given discriminator value (can be used with generics)
type UnionMemberByDiscriminator<
E extends object,
D extends keyof E,
DValue extends string
> = E extends {[key in D]:DValue} ? E : never;
E extends object,
D extends keyof E,
DValue extends string
> = E extends {[key in D]:DValue} ? E : never;
Example:
type Car = {
kind: “car“,
…
};
kind: “car“,
…
};
type Truck = {
kind: “truck“,
…
};
type Vehicle = Car | Truck;
type InferredCar =
UnionMemberByDiscriminator<Vehicle, “kind“, “car“>
Will produce
type InferredCar = Car;
This is useful in a generic context (see above)
