Narrow error type for ignore()

Author: kanongil

lib/index.d.ts

@@ -63,6 +63,10 @@ export function ignore<E extends Error, D extends Decoration>(err: any, types: T
 export function ignore<T extends object, D extends Decoration>(err: T, types: TypeRule | TypeRule[], options: BounceOptions & { return: true, decorate: D }): (T & D) | undefined;
 export function ignore<E extends Error>(err: any, types: TypeRule | TypeRule[], options: BounceOptions & { return: true, override: E }): E | undefined;
 export function ignore<T>(err: T, types: TypeRule | TypeRule[], options: BounceOptions & { return: true }): T | undefined;
+export function ignore(err: any, types: 'boom', options: { return?: false | undefined }): asserts err is Boom;
+export function ignore(err: any, types: 'boom'): asserts err is Boom;
+export function ignore(err: any, types: TypeRule | TypeRule[], options: { return?: false | undefined }): asserts err is Error;
+export function ignore(err: any, types: TypeRule | TypeRule[]): asserts err is Error;
 export function ignore(err: any, types: TypeRule | TypeRule[], options?: BounceOptions): void;
 
 type Action = 'rethrow' | 'ignore';

test/index.ts

@@ -29,6 +29,20 @@ expect.type<TypeError | undefined>(Bounce.ignore(new TypeError(), 'boom', { retu
 expect.type<RangeError | undefined>(Bounce.ignore(null, 'boom', { return: true, override: new RangeError() }));
 expect.type<(TypeError & { prop: string }) | undefined>(Bounce.ignore(new TypeError(), 'boom', { return: true, decorate: { prop: 'ok' } }));
 
+// Narrows the error type
+
+{
+    const error = new TypeError() as any;
+    Bounce.ignore(error, TypeError);
+    expect.type<Error>(error);
+}
+
+{
+    const error = new Boom.Boom() as any;
+    Bounce.ignore(error, 'boom');
+    expect.type<Boom.Boom>(error);
+}
+
 expect.error(Bounce.ignore(new Error()));
 expect.error(Bounce.ignore(new Error(), 'unknown'));
 expect.error(Bounce.ignore(new Error(), 'boom', true));