From b697fe6456c0161f6566f4beece5c1a8176deb7a Mon Sep 17 00:00:00 2001
From: Tom Coleman <tom@thesnail.org>
Date: Fri, 30 Oct 2020 16:50:22 +1100
Subject: [PATCH] Combine args with basic object spread semantics

Fixes #12697
---
 lib/client-api/src/story_store.test.ts | 20 ++++++++++++++++++++
 lib/client-api/src/story_store.ts      |  5 ++++-
 2 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/lib/client-api/src/story_store.test.ts b/lib/client-api/src/story_store.test.ts
index 5000fcd66f43..1532feea38fa 100644
--- a/lib/client-api/src/story_store.test.ts
+++ b/lib/client-api/src/story_store.test.ts
@@ -157,6 +157,26 @@ describe('preview.story_store', () => {
   });
 
   describe('args', () => {
+    it('composes component-level and story-level args, favoring story-level', () => {
+      const store = new StoryStore({ channel });
+      store.addKindMetadata('a', {
+        parameters: { args: { arg1: 1, arg2: 2, arg3: 3, arg4: { complex: 'object' } } },
+      });
+      addStoryToStore(store, 'a', '1', () => 0, {
+        args: {
+          arg1: 4,
+          arg2: undefined,
+          arg4: { other: 'object ' },
+        },
+      });
+      expect(store.getRawStory('a', '1').args).toEqual({
+        arg1: 4,
+        arg2: undefined,
+        arg3: 3,
+        arg4: { other: 'object ' },
+      });
+    });
+
     it('is initialized to the value stored in parameters.args[name] || parameters.argType[name].defaultValue', () => {
       const store = new StoryStore({ channel });
       addStoryToStore(store, 'a', '1', () => 0, {
diff --git a/lib/client-api/src/story_store.ts b/lib/client-api/src/story_store.ts
index 3387ea5958a2..a3501f8cbcbc 100644
--- a/lib/client-api/src/story_store.ts
+++ b/lib/client-api/src/story_store.ts
@@ -445,7 +445,10 @@ export default class StoryStore {
     };
 
     // Pull out parameters.args.$ || .argTypes.$.defaultValue into initialArgs
-    const passedArgs: Args = combinedParameters.args;
+    const passedArgs: Args = {
+      ...this._kinds[kind].parameters.args,
+      ...storyParameters.args,
+    };
     const defaultArgs: Args = Object.entries(
       argTypes as Record<string, { defaultValue: any }>
     ).reduce((acc, [arg, { defaultValue }]) => {