feat: narrow down Result type after isOk or isErr

src/__tests__/result.test.ts

@@ -32,6 +32,15 @@ describe(`Test method \`${RustlikeResult.name}.prototype.${RustlikeResult.protot
         expect(Err('Some error message').isOk()).toBe(false);
     });
 
+    // eslint-disable-next-line jest/expect-expect
+    it('should narrow down result type', () => {
+        const result = Ok<number, string>(1);
+        if (result.isOk()) {
+            // eslint-disable-next-line @typescript-eslint/no-unused-vars
+            const expected: Result<number, never> = result;
+        }
+    });
+
     it('should have correct examples doc', () => {
         function examples() {
             const x: Result<number, string> = Ok(2);
@@ -153,6 +162,15 @@ describe(`Test method \`${RustlikeResult.name}.prototype.${RustlikeResult.protot
         expect(Err('Some error message').isErr()).toBe(true);
     });
 
+    // eslint-disable-next-line jest/expect-expect
+    it('should narrow down result type', () => {
+        const result = Err<number, string>('Some error message');
+        if (result.isErr()) {
+            // eslint-disable-next-line @typescript-eslint/no-unused-vars
+            const expected: Result<never, string> = result;
+        }
+    });
+
     it('should have correct examples doc', () => {
         function examples() {
             const x: Result<number, string> = Ok(-3);

src/result.ts

@@ -56,7 +56,7 @@ export class RustlikeResult<T, E> implements Result<T, E> {
      *
      * ref: https://doc.rust-lang.org/std/result/enum.Result.html#method.is_ok
      */
-    isOk(): boolean {
+    isOk(): this is Result<T, never> {
         return this._type === 'ok';
     }
 
@@ -125,7 +125,7 @@ export class RustlikeResult<T, E> implements Result<T, E> {
      *
      * ref: https://doc.rust-lang.org/std/result/enum.Result.html#method.is_err
      */
-    isErr(): boolean {
+    isErr(): this is Result<never, E> {
         return this._type === 'err';
     }
 

src/types.ts

@@ -28,7 +28,7 @@ export interface Result<T, E> {
      *
      * ref: https://doc.rust-lang.org/std/result/enum.Result.html#method.is_ok
      */
-    isOk(): boolean;
+    isOk(): this is Result<T, never>;
 
     /**
      * Returns `true` if the result is `Ok` and the value inside of it matches a predicate.
@@ -91,7 +91,7 @@ export interface Result<T, E> {
      *
      * ref: https://doc.rust-lang.org/std/result/enum.Result.html#method.is_err
      */
-    isErr(): boolean;
+    isErr(): this is Result<never, E>;
 
     /**
      * Returns `true` if the result is `Err` and the value inside of it matches a predicate.