fix(core): allow passing value of any type to isSignal function (#50035)

markostanimirovic · AndrewKushnir · commit 7d679bdb5981 · 2023-05-16T09:22:06.000-07:00
Unlike the current signature where the input argument must a function, this change allows an input of any type to be passed to the `isSignal` function. PR Close #50035
diff --git a/goldens/public-api/core/index.md b/goldens/public-api/core/index.md
@@ -838,7 +838,7 @@ export interface InputDecorator {
 export function isDevMode(): boolean;
 
 // @public
-export function isSignal(value: Function): value is Signal<unknown>;
+export function isSignal(value: unknown): value is Signal<unknown>;
 
 // @public
 export function isStandalone(type: Type<unknown>): boolean;
diff --git a/packages/core/src/signals/src/api.ts b/packages/core/src/signals/src/api.ts
@@ -30,10 +30,12 @@ export type Signal<T> = (() => T)&{
 };
 
 /**
- * Checks if the given `value` function is a reactive `Signal`.
+ * Checks if the given `value` is a reactive `Signal`.
+ *
+ * @developerPreview
  */
-export function isSignal(value: Function): value is Signal<unknown> {
-  return (value as Signal<unknown>)[SIGNAL] !== undefined;
+export function isSignal(value: unknown): value is Signal<unknown> {
+  return typeof value === 'function' && (value as Signal<unknown>)[SIGNAL] !== undefined;
 }
 
 /**
diff --git a/packages/core/test/signals/is_signal_spec.ts b/packages/core/test/signals/is_signal_spec.ts
@@ -0,0 +1,36 @@
+/**
+ * @license
+ * Copyright Google LLC All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+import {computed, isSignal, signal} from '@angular/core/src/signals';
+
+describe('isSignal', () => {
+  it('should return true for writable signal', () => {
+    const writableSignal = signal('Angular');
+    expect(isSignal(writableSignal)).toBe(true);
+  });
+
+  it('should return true for readonly signal', () => {
+    const readonlySignal = computed(() => 10);
+    expect(isSignal(readonlySignal)).toBe(true);
+  });
+
+  it('should return false for primitive', () => {
+    const primitive = 0;
+    expect(isSignal(primitive)).toBe(false);
+  });
+
+  it('should return false for object', () => {
+    const object = {name: 'Angular'};
+    expect(isSignal(object)).toBe(false);
+  });
+
+  it('should return false for function', () => {
+    const fn = () => {};
+    expect(isSignal(fn)).toBe(false);
+  });
+});
