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fix: type inference of type #833

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fix: type inference of type #833

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46 changes: 42 additions & 4 deletions src/operators/of-type.spec.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -3,10 +3,17 @@ import { ofType } from './of-type';
import { IEvent } from '../interfaces';

describe('operators/ofType', () => {
class A implements IEvent {}
class B implements IEvent {}
class A implements IEvent {
keyAOrB: string = 'keyAOrB';
keyAOnly: string = 'keyAOnly';
}
class B implements IEvent {
keyAOrB: string = 'keyAOrB';
}
class SubA extends A {}
class C implements IEvent {}
class C implements IEvent {
keyCOptional?: string;
}

let stream: Subject<any>;
let output: IEvent[];
Expand All @@ -32,7 +39,7 @@ describe('operators/ofType', () => {
expectedResults.push(new A());

stream.next(new B());
stream.next(...expectedResults);
expectedResults.forEach((event) => stream.next(event));
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This was failing tsc before because of

A spread argument must either have a tuple type or be passed to a rest parameter.

stream.next(new Date());

expect(output).toEqual(expectedResults);
Expand All @@ -46,4 +53,35 @@ describe('operators/ofType', () => {

expect(output).toEqual(expectedResults);
});

// TypeScript TSC test, if this compiles it passes
it('should infer return types of similar unions unions', (done) => {
stream.pipe(ofType(A, B)).subscribe((event) => {
event.keyAOrB;
// @ts-expect-error -- A | B cannot reference a B only key
event.keyBOnly;
done();
});

stream.next(new A());
});

// TypeScript TSC test, if this compiles it passes
it('should infer return types of disparate unions', (done) => {
stream.pipe(ofType(A, C)).subscribe((event) => {
// @ts-expect-error -- A | C cannot reference a key that is not on C
event.keyAOnly;

if (event instanceof A) {
event.keyAOnly;
}

if (event instanceof C) {
event.keyCOptional;
}
done();
});

stream.next(new A());
});
});
10 changes: 6 additions & 4 deletions src/operators/of-type.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -11,11 +11,13 @@ import { IEvent } from '../interfaces';
*
* @return A stream only emitting the filtered instances.
*/
export function ofType<TInput extends IEvent, TOutput extends IEvent>(
...types: Type<TOutput>[]
export function ofType<TInput extends IEvent, T extends Type<IEvent>[]>(
...types: T
) {
const isInstanceOf = (event: IEvent): event is TOutput =>
type InstanceOf<T> = T extends Type<infer I> ? I : never;
const isInstanceOf = (event: IEvent): event is InstanceOf<T[number]> =>
!!types.find((classType) => event instanceof classType);
return (source: Observable<TInput>): Observable<TOutput> =>

return (source: Observable<TInput>): Observable<InstanceOf<T[number]>> =>
source.pipe(filter(isInstanceOf));
}