feat(filter): improve type inference for filter(Boolean)

ReactiveX · Oct 15, 2020 · 051ad34 · 051ad34
1 parent adbe65e
commit 051ad34
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Showing 3 changed files with 10 additions and 3 deletions.
diff --git a/api_guard/dist/types/operators/index.d.ts b/api_guard/dist/types/operators/index.d.ts
@@ -109,7 +109,7 @@ export declare function expand<T, R>(project: (value: T, index: number) => Obser
 export declare function expand<T, R>(project: (value: T, index: number) => ObservableInput<R>, concurrent: number | undefined, scheduler: SchedulerLike): OperatorFunction<T, R>;
 
 export declare function filter<T, S extends T>(predicate: (value: T, index: number) => value is S, thisArg?: any): OperatorFunction<T, S>;
-export declare function filter<T>(predicate: BooleanConstructor): OperatorFunction<T | null | undefined, NonNullable<T>>;
+export declare function filter<T>(predicate: BooleanConstructor): OperatorFunction<T, T extends null | undefined | false | 0 | -0 | 0n | '' ? never : T>;
 export declare function filter<T>(predicate: (value: T, index: number) => boolean, thisArg?: any): MonoTypeOperatorFunction<T>;
 
 export declare function finalize<T>(callback: () => void): MonoTypeOperatorFunction<T>;

diff --git a/spec-dtslint/operators/filter-spec.ts b/spec-dtslint/operators/filter-spec.ts
@@ -43,6 +43,12 @@ it('should support Boolean as a predicate', () => {
   const o = of(1, 2, 3).pipe(filter(Boolean)); // $ExpectType Observable<number>
   const p = of(1, null, undefined).pipe(filter(Boolean)); // $ExpectType Observable<number>
   const q = of(null, undefined).pipe(filter(Boolean)); // $ExpectType Observable<never>
+  const r = of(true).pipe(filter(Boolean)); // $ExpectType Observable<true>
+  const s = of(false as const).pipe(filter(Boolean)); // $ExpectType Observable<never>
+  const t = of(0 as const, -0 as const, 1 as const).pipe(filter(Boolean)); // $ExpectType Observable<1>
+  const u = of(0 as const, -0 as const).pipe(filter(Boolean)); // $ExpectType Observable<never>
+  const v = of('' as const, "foo" as const, "bar" as const).pipe(filter(Boolean)); // $ExpectType Observable<"foo" | "bar">
+  const w = of('' as const).pipe(filter(Boolean)); // $ExpectType Observable<never>
 });
 
 // I've not been able to effect a failing dtslint test for this situation and a

diff --git a/src/internal/operators/filter.ts b/src/internal/operators/filter.ts
@@ -5,8 +5,9 @@ import { OperatorSubscriber } from './OperatorSubscriber';
 
 /* tslint:disable:max-line-length */
 export function filter<T, S extends T>(predicate: (value: T, index: number) => value is S, thisArg?: any): OperatorFunction<T, S>;
-// NOTE(benlesh): T|null|undefined solves the issue discussed here: https://github.com/ReactiveX/rxjs/issues/4959#issuecomment-520629091
-export function filter<T>(predicate: BooleanConstructor): OperatorFunction<T | null | undefined, NonNullable<T>>;
+export function filter<T>(
+  predicate: BooleanConstructor
+): OperatorFunction<T, T extends null | undefined | false | 0 | -0 | 0n | '' ? never : T>;
 export function filter<T>(predicate: (value: T, index: number) => boolean, thisArg?: any): MonoTypeOperatorFunction<T>;
 /* tslint:enable:max-line-length */