TypeScript - Type Guard for Discriminated Unions with Extract

typeguard-discriminated-union.ts

export interface Chunky {
  kind: 'chunky';
  /* ... */
}

export interface Bacon {
  kind: 'bacon';
  /* ... */
}

export type Stuff = Chunky | Bacon;

type StuffKind = Stuff['kind'];

export function stuffIs<T extends StuffKind>(kind: StuffKind, stuff: Stuff): stuff is Extract<Stuff, {kind: T}> {
  return stuff.kind === kind;
}

const chunky : Chunky = {kind: 'chunky'};
const bacon : Bacon = {kind: 'bacon'};

console.log(
  stuffIs('chunky', chunky), /* true */
  stuffIs('chunky', bacon), /* false */
  stuffIs('bacon', chunky), /* false */
  stuffIs('bacon', bacon), /* true */
);

This can be further refined into a point free style like:

export const isKind = <
  TKindable extends {
    kind: string;
  },
  TKind extends TKindable["kind"]
>(
  kind: TKind
) => (item: TKindable): item is Extract<TKindable, { kind: TKind }> => item.kind == kind;