TypeScript Generic Type Guards

generic-typeguard-option-1.ts

interface _typeMap {
  chunky: Chunky;
  bacon: Bacon;
}

type _stuffKind = keyof _typeMap;

export interface Stuff {
  kind: _stuffKind;
  /* ... */
}

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

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

type _mappedType<T extends _stuffKind> = _typeMap[T];

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

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

console.log(
  stuffIs('chunky', chunky),
  stuffIs('chunky', bacon),
  stuffIs('bacon', chunky),
  stuffIs('bacon', bacon),
);

generic-typeguard-option-2.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),
  stuffIs('chunky', bacon),
  stuffIs('bacon', chunky),
  stuffIs('bacon', bacon),
);