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Frédéric Leroy
Frédéric Leroy

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Typescript type grouping a union type of objects by any property discriminating these objects.

#typescript #cheatsheet

MappingByDiscriminator

Example:

type Car = {
  kind: "car",
  ...
};

type Truck = {
  kind: "truck",
  ...
};

type Vehicle = Car | Truck;
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Then

type VehiclesByKind = MappingByDiscriminator<Vehicle,"kind">;
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Will produce :

type VehiclesByKind = {
    car: Car[],
    truck: Truck[],
}
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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 | number ?
   {
      [d in GetPropertyType<E, D>]: UnionMemberByDiscriminator<E, D, d>[];
    }
  : never;
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Will map types in a union by a discriminator.

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;
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Example:

type Entity = {
   kind: 'human' | 'animal',
   size: 'microscopic' | 'small' | 'big' | 'giant',
}
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Then

type SizeType = GetProperty<Entity,"size">
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Will produce

type SizeType = 'microscopic' | 'small' | 'big' | 'giant';
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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 | number
> = E extends {[key in D]:DValue} ? E : never;
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Example:

type Car = {
  kind: "car",
  ...
};

type Truck = {
  kind: "truck",
  ...
};

type Vehicle = Car | Truck;

type InferredCar = 
   UnionMemberByDiscriminator<Vehicle, "kind", "car">
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Will produce

type InferredCar = Car;
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This is useful in a generic context (see above)

Top comments (2)

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loriel_alade_f298650d4f69 profile image
Loriel Alade • • Edited

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alex_ander_1bcc72456032a7 profile image
Alex Ander • • Edited

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