fix: Use nullish coalescing operator in useParameter(..)

The `useParameter(..)` hook accepts a key and a default value. It then
provides the default value if the current story has not provided a
desired value for the relevant `parameters` object.

This change updates the logic of when to fall back to the default. The
old logic used a boolean operator (`||`), which worked as long as the
provided value wasn't falsey. Instead, account for this by using the
nullish coalescing operator (`??`).

lib/addons/src/hooks.test.js

@@ -0,0 +1,57 @@
+import { useParameter, useStoryContext } from './hooks';
+
+const { window: globalWindow } = global;
+
+describe('addons/hooks', () => {
+  beforeEach(() => {
+    globalWindow.STORYBOOK_HOOKS_CONTEXT = undefined;
+  });
+
+  afterEach(() => {
+    globalWindow.STORYBOOK_HOOKS_CONTEXT = undefined;
+  });
+
+  describe('useStoryContext', () => {
+    test('should throw', () => {
+      expect(() => useStoryContext()).toThrowError(
+        'Storybook preview hooks can only be called inside decorators and story functions.'
+      );
+    });
+  });
+
+  describe('useParameter', () => {
+    beforeEach(() => {
+      globalWindow.STORYBOOK_HOOKS_CONTEXT = {
+        currentContext: {
+          parameters: {
+            'undefined key': undefined,
+            'null key': null,
+            'false key': false,
+            'zero key': 0,
+            'object key': { defined: true },
+          },
+        },
+      };
+    });
+
+    test('undefined key', () => {
+      expect(useParameter('undefined key', 'undefined default')).toEqual('undefined default');
+    });
+
+    test('null key', () => {
+      expect(useParameter('null key', 'null default')).toEqual('null default');
+    });
+
+    test('false key', () => {
+      expect(useParameter('false key', 'false default')).toEqual(false);
+    });
+
+    test('zero key', () => {
+      expect(useParameter('zero key', 'zero default')).toEqual(0);
+    });
+
+    test('object key', () => {
+      expect(useParameter('object key', 'object default')).toMatchObject({ defined: true });
+    });
+  });
+});

lib/addons/src/hooks.ts

@@ -418,7 +418,7 @@ export function useStoryContext<TFramework extends AnyFramework>(): StoryContext
 export function useParameter<S>(parameterKey: string, defaultValue?: S): S | undefined {
   const { parameters } = useStoryContext();
   if (parameterKey) {
-    return parameters[parameterKey] || (defaultValue as S);
+    return parameters[parameterKey] ?? (defaultValue as S);
   }
   return undefined;
 }