From 5bb5f44b2761c998993640e5a7b3c867a966274b Mon Sep 17 00:00:00 2001
From: webdevsamran <webdevsamran@users.noreply.github.com>
Date: Thu, 27 Aug 2026 14:37:35 +0500
Subject: [PATCH 1/4] fix(docs): prevent source transform from being applied
 twice on docs pages

When dynamic story snippets are rendered, emitTransformCode applies
parameters.docs.source.transform before emitting SNIPPET_RENDERED to
the source context. When the docs <Source /> block rendered the snippet,
useCode re-applied sourceParameters.transform unconditionally, causing
the transform to run twice on docs pages while running only once on
the individual story Code tab.

Fix:
- In Source.tsx: only apply sourceParameters.transform when not already
  using an emitted snippet, or when explicitly overridden via transformFromProps.
- In Source.test.tsx: add unit tests verifying single execution on snippets,
  prop transform override, and originalSource transform fallback.

Fixes #35889
---
 .../docs/src/blocks/blocks/Source.test.tsx    | 78 +++++++++++++++++++
 code/addons/docs/src/blocks/blocks/Source.tsx |  5 +-
 2 files changed, 82 insertions(+), 1 deletion(-)
 create mode 100644 code/addons/docs/src/blocks/blocks/Source.test.tsx

diff --git a/code/addons/docs/src/blocks/blocks/Source.test.tsx b/code/addons/docs/src/blocks/blocks/Source.test.tsx
new file mode 100644
index 000000000000..d2a720261d90
--- /dev/null
+++ b/code/addons/docs/src/blocks/blocks/Source.test.tsx
@@ -0,0 +1,78 @@
+import { describe, expect, it, vi } from 'vitest';
+import { SourceType } from 'storybook/internal/docs-tools';
+import { useCode } from './Source.tsx';
+
+describe('useCode', () => {
+  it('does not apply sourceParameters.transform twice when snippet is already provided (issue #35889)', () => {
+    const transform = vi.fn((code: string) => /*transformed*/ );
+
+    const result = useCode({
+      snippet: '/*transformed*/ <Button />', // already transformed by emitTransformCode
+      serviceSnippet: '',
+      storyContext: {
+        parameters: {
+          __isArgsStory: true,
+          docs: {
+            source: {
+              type: SourceType.DYNAMIC,
+              transform,
+            },
+          },
+        },
+      } as any,
+      typeFromProps: SourceType.DYNAMIC,
+    });
+
+    expect(transform).not.toHaveBeenCalled();
+    expect(result).toBe('/*transformed*/ <Button />');
+  });
+
+  it('applies transformFromProps if explicitly provided on props', () => {
+    const propTransform = vi.fn((code: string) => /*prop*/ );
+
+    const result = useCode({
+      snippet: '<Button />',
+      serviceSnippet: '',
+      storyContext: {
+        parameters: {
+          __isArgsStory: true,
+          docs: {
+            source: {
+              type: SourceType.DYNAMIC,
+            },
+          },
+        },
+      } as any,
+      typeFromProps: SourceType.DYNAMIC,
+      transformFromProps: propTransform,
+    });
+
+    expect(propTransform).toHaveBeenCalledWith('<Button />', expect.anything());
+    expect(result).toBe('/*prop*/ <Button />');
+  });
+
+  it('applies sourceParameters.transform when using originalSource fallback', () => {
+    const transform = vi.fn((code: string) => /*transformed*/ );
+
+    const result = useCode({
+      snippet: '',
+      serviceSnippet: '',
+      storyContext: {
+        parameters: {
+          __isArgsStory: false,
+          docs: {
+            source: {
+              type: SourceType.CODE,
+              originalSource: '<Button />',
+              transform,
+            },
+          },
+        },
+      } as any,
+      typeFromProps: SourceType.CODE,
+    });
+
+    expect(transform).toHaveBeenCalledWith('<Button />', expect.anything());
+    expect(result).toBe('/*transformed*/ <Button />');
+  });
+});
diff --git a/code/addons/docs/src/blocks/blocks/Source.tsx b/code/addons/docs/src/blocks/blocks/Source.tsx
index 37d725e05d52..91685db64a41 100644
--- a/code/addons/docs/src/blocks/blocks/Source.tsx
+++ b/code/addons/docs/src/blocks/blocks/Source.tsx
@@ -91,7 +91,10 @@ const useCode = ({
     (type === SourceType.AUTO && staticSnippet && isArgsStory);
 
   const code = useSnippet ? staticSnippet : sourceParameters.originalSource || '';
-  const transformer = transformFromProps ?? sourceParameters.transform;
+  // When using snippet (emitted via SNIPPET_RENDERED), emitTransformCode has already applied sourceParameters.transform.
+  // Avoid re-applying it unless explicitly overridden via transformFromProps.
+  const transformer =
+    transformFromProps ?? (useSnippet && snippet ? undefined : sourceParameters.transform);
 
   const transformedCode = transformer ? useTransformCode(code, transformer, storyContext) : code;
 

From b6fe1fed618983f97e3583c815692b2aab88ff88 Mon Sep 17 00:00:00 2001
From: Samran Asif <samranwebdev2000@gmail.com>
Date: Mon, 14 Sep 2026 11:25:59 +0500
Subject: [PATCH 2/4] fix(docs): complete vi.fn mock bodies and guard
 service-snippet transform

- Source.test.tsx: three vi.fn arrow functions used a block comment
  (/*transformed*/) as the function body, which evaluates to void and
  causes Biome parse errors and makes all transform-called assertions
  fail. Replace each with a proper string-returning body that matches
  the toBe() assertion at the end of each test case:
    - test 1 and 3: (code) => '/*transformed*/ ' + code
    - test 2:       (code) => '/*prop*/ ' + code

- Source.tsx (useCode): the transformer-selection guard previously
  skipped sourceParameters.transform whenever useSnippet && snippet was
  truthy. This incorrectly skipped the transform when a raw
  serviceSnippet was selected as staticSnippet (serviceSnippet is not
  pre-transformed by emitTransformCode). The guard now requires
  !serviceSnippet so transforms still apply for raw service snippets.

Signed-off-by: Samran Yousaf <webdevsamran@gmail.com>
Signed-off-by: Samran Asif <samranwebdev2000@gmail.com>
---
 code/addons/docs/src/blocks/blocks/Source.test.tsx | 6 +++---
 code/addons/docs/src/blocks/blocks/Source.tsx      | 2 +-
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/code/addons/docs/src/blocks/blocks/Source.test.tsx b/code/addons/docs/src/blocks/blocks/Source.test.tsx
index d2a720261d90..897c868ca625 100644
--- a/code/addons/docs/src/blocks/blocks/Source.test.tsx
+++ b/code/addons/docs/src/blocks/blocks/Source.test.tsx
@@ -4,7 +4,7 @@ import { useCode } from './Source.tsx';
 
 describe('useCode', () => {
   it('does not apply sourceParameters.transform twice when snippet is already provided (issue #35889)', () => {
-    const transform = vi.fn((code: string) => /*transformed*/ );
+    const transform = vi.fn((code: string) => '/*transformed*/ ' + code);
 
     const result = useCode({
       snippet: '/*transformed*/ <Button />', // already transformed by emitTransformCode
@@ -28,7 +28,7 @@ describe('useCode', () => {
   });
 
   it('applies transformFromProps if explicitly provided on props', () => {
-    const propTransform = vi.fn((code: string) => /*prop*/ );
+    const propTransform = vi.fn((code: string) => '/*prop*/ ' + code);
 
     const result = useCode({
       snippet: '<Button />',
@@ -52,7 +52,7 @@ describe('useCode', () => {
   });
 
   it('applies sourceParameters.transform when using originalSource fallback', () => {
-    const transform = vi.fn((code: string) => /*transformed*/ );
+    const transform = vi.fn((code: string) => '/*transformed*/ ' + code);
 
     const result = useCode({
       snippet: '',
diff --git a/code/addons/docs/src/blocks/blocks/Source.tsx b/code/addons/docs/src/blocks/blocks/Source.tsx
index 91685db64a41..82a43a7240d4 100644
--- a/code/addons/docs/src/blocks/blocks/Source.tsx
+++ b/code/addons/docs/src/blocks/blocks/Source.tsx
@@ -94,7 +94,7 @@ const useCode = ({
   // When using snippet (emitted via SNIPPET_RENDERED), emitTransformCode has already applied sourceParameters.transform.
   // Avoid re-applying it unless explicitly overridden via transformFromProps.
   const transformer =
-    transformFromProps ?? (useSnippet && snippet ? undefined : sourceParameters.transform);
+    transformFromProps ?? (useSnippet && !serviceSnippet && snippet ? undefined : sourceParameters.transform);
 
   const transformedCode = transformer ? useTransformCode(code, transformer, storyContext) : code;
 

From e7470653926e31aba915e7c1a7e2c84d8273cc72 Mon Sep 17 00:00:00 2001
From: Samran Asif <samranwebdev2000@gmail.com>
Date: Sat, 17 Oct 2026 08:33:47 -0700
Subject: [PATCH 3/4] test(docs): rewrite Source tests using renderHook and
 polish sourceKind selection (#36061)

Signed-off-by: Samran Asif <samranwebdev2000@gmail.com>
---
 .../docs/src/blocks/blocks/Source.test.tsx    | 355 +++++++++++++++---
 code/addons/docs/src/blocks/blocks/Source.tsx |  22 +-
 .../modules/preview-web/emitTransformCode.ts  |   1 +
 3 files changed, 320 insertions(+), 58 deletions(-)

diff --git a/code/addons/docs/src/blocks/blocks/Source.test.tsx b/code/addons/docs/src/blocks/blocks/Source.test.tsx
index 897c868ca625..6591f9516774 100644
--- a/code/addons/docs/src/blocks/blocks/Source.test.tsx
+++ b/code/addons/docs/src/blocks/blocks/Source.test.tsx
@@ -1,78 +1,325 @@
-import { describe, expect, it, vi } from 'vitest';
+// @vitest-environment happy-dom
+import { renderHook } from '@testing-library/react';
+import { describe, expect, it } from 'vitest';
+
 import { SourceType } from 'storybook/internal/docs-tools';
-import { useCode } from './Source.tsx';
+import type { PreparedStory } from 'storybook/internal/types';
 
-describe('useCode', () => {
-  it('does not apply sourceParameters.transform twice when snippet is already provided (issue #35889)', () => {
-    const transform = vi.fn((code: string) => '/*transformed*/ ' + code);
+import type { DocsContextProps } from './DocsContext';
+import type { SourceProps } from './Source';
+import { useSourceProps } from './Source';
+import type { SourceContextProps } from './SourceContainer';
+import { UNKNOWN_ARGS_HASH } from './SourceContainer';
 
-    const result = useCode({
-      snippet: '/*transformed*/ <Button />', // already transformed by emitTransformCode
-      serviceSnippet: '',
-      storyContext: {
-        parameters: {
-          __isArgsStory: true,
-          docs: {
-            source: {
-              type: SourceType.DYNAMIC,
-              transform,
-            },
+type ReducedStoryContext = ReturnType<DocsContextProps['getStoryContext']>;
+
+const createMockDocsContext = (
+  story: PreparedStory,
+  storyContext: ReducedStoryContext
+): DocsContextProps =>
+  ({
+    storyById: () => story,
+    getStoryContext: () => storyContext,
+  }) as unknown as DocsContextProps;
+
+describe('useSourceProps — source transform handling (issue #35889)', () => {
+  it('does not apply parameter transform twice when snippet arrives on later render for auto args story', () => {
+    const storyContext: ReducedStoryContext = {
+      id: 'story--primary',
+      parameters: {
+        __isArgsStory: true,
+        docs: {
+          source: {
+            type: SourceType.AUTO,
+            originalSource: '<Button />',
+            transform: (code: string) => `T:${code}`,
+          },
+        },
+      },
+      args: {},
+      initialArgs: {},
+      unmappedArgs: {},
+    } as unknown as ReducedStoryContext;
+    const story: PreparedStory = {
+      id: 'story--primary',
+      parameters: storyContext.parameters,
+    } as unknown as PreparedStory;
+    const docsContext = createMockDocsContext(story, storyContext);
+
+    const { result, rerender } = renderHook(
+      ({ sourceContext, serviceSnippet, props }) =>
+        useSourceProps(props, docsContext, sourceContext, serviceSnippet),
+      {
+        initialProps: {
+          sourceContext: { sources: {} } as SourceContextProps,
+          serviceSnippet: '',
+          props: {} as SourceProps,
+        },
+      }
+    );
+
+    // Initial render before snippet arrives falls back to originalSource transformed
+    expect(result.current.code).toBe('T:<Button />');
+
+    // Snippet arrives after initial render (already transformed by emitTransformCode)
+    rerender({
+      sourceContext: {
+        sources: {
+          'story--primary': {
+            [UNKNOWN_ARGS_HASH]: { code: 'T:<Button />' },
           },
         },
-      } as any,
-      typeFromProps: SourceType.DYNAMIC,
+      },
+      serviceSnippet: '',
+      props: {},
     });
 
-    expect(transform).not.toHaveBeenCalled();
-    expect(result).toBe('/*transformed*/ <Button />');
+    // Should not apply transform a second time (would be T:T:<Button />)
+    expect(result.current.code).toBe('T:<Button />');
   });
 
-  it('applies transformFromProps if explicitly provided on props', () => {
-    const propTransform = vi.fn((code: string) => '/*prop*/ ' + code);
+  it('does not apply parameter transform twice when snippet arrives on later render for dynamic story', () => {
+    const storyContext: ReducedStoryContext = {
+      id: 'story--primary',
+      parameters: {
+        __isArgsStory: true,
+        docs: {
+          source: {
+            type: SourceType.DYNAMIC,
+            originalSource: '<Button />',
+            transform: (code: string) => `T:${code}`,
+          },
+        },
+      },
+      args: {},
+      initialArgs: {},
+      unmappedArgs: {},
+    } as unknown as ReducedStoryContext;
+    const story: PreparedStory = {
+      id: 'story--primary',
+      parameters: storyContext.parameters,
+    } as unknown as PreparedStory;
+    const docsContext = createMockDocsContext(story, storyContext);
+
+    const { result, rerender } = renderHook(
+      ({ sourceContext, serviceSnippet, props }) =>
+        useSourceProps(props, docsContext, sourceContext, serviceSnippet),
+      {
+        initialProps: {
+          sourceContext: { sources: {} } as SourceContextProps,
+          serviceSnippet: '',
+          props: {} as SourceProps,
+        },
+      }
+    );
+
+    expect(result.current.code).toBe('');
 
-    const result = useCode({
-      snippet: '<Button />',
+    // Snippet arrives after initial render
+    rerender({
+      sourceContext: {
+        sources: {
+          'story--primary': {
+            [UNKNOWN_ARGS_HASH]: { code: 'T:<Button />' },
+          },
+        },
+      },
       serviceSnippet: '',
-      storyContext: {
-        parameters: {
-          __isArgsStory: true,
-          docs: {
-            source: {
-              type: SourceType.DYNAMIC,
+      props: {},
+    });
+
+    expect(result.current.code).toBe('T:<Button />');
+  });
+
+  it('applies transform when raw service snippet arrives even if emitted snippet is present', () => {
+    const storyContext: ReducedStoryContext = {
+      id: 'story--primary',
+      parameters: {
+        __isArgsStory: true,
+        docs: {
+          source: {
+            type: SourceType.AUTO,
+            originalSource: '<Button />',
+            transform: (code: string) => `T:${code}`,
+          },
+        },
+      },
+      args: {},
+      initialArgs: {},
+      unmappedArgs: {},
+    } as unknown as ReducedStoryContext;
+    const story: PreparedStory = {
+      id: 'story--primary',
+      parameters: storyContext.parameters,
+    } as unknown as PreparedStory;
+    const docsContext = createMockDocsContext(story, storyContext);
+
+    const { result, rerender } = renderHook(
+      ({ sourceContext, serviceSnippet, props }) =>
+        useSourceProps(props, docsContext, sourceContext, serviceSnippet),
+      {
+        initialProps: {
+          sourceContext: {
+            sources: {
+              'story--primary': {
+                [UNKNOWN_ARGS_HASH]: { code: 'T:<Button />' },
+              },
             },
+          } as SourceContextProps,
+          serviceSnippet: '',
+          props: {} as SourceProps,
+        },
+      }
+    );
+
+    expect(result.current.code).toBe('T:<Button />');
+
+    // Raw service snippet arrives: not pre-transformed, so transform must be applied
+    rerender({
+      sourceContext: {
+        sources: {
+          'story--primary': {
+            [UNKNOWN_ARGS_HASH]: { code: 'T:<Button />' },
           },
         },
-      } as any,
-      typeFromProps: SourceType.DYNAMIC,
-      transformFromProps: propTransform,
+      },
+      serviceSnippet: '<Svc />',
+      props: {},
     });
 
-    expect(propTransform).toHaveBeenCalledWith('<Button />', expect.anything());
-    expect(result).toBe('/*prop*/ <Button />');
+    expect(result.current.code).toBe('T:<Svc />');
   });
 
-  it('applies sourceParameters.transform when using originalSource fallback', () => {
-    const transform = vi.fn((code: string) => '/*transformed*/ ' + code);
+  it('transforms originalSource once for type code even when snippet is present', () => {
+    const storyContext: ReducedStoryContext = {
+      id: 'story--primary',
+      parameters: {
+        __isArgsStory: true,
+        docs: {
+          source: {
+            type: SourceType.CODE,
+            originalSource: '<Button />',
+            transform: (code: string) => `T:${code}`,
+          },
+        },
+      },
+      args: {},
+      initialArgs: {},
+      unmappedArgs: {},
+    } as unknown as ReducedStoryContext;
+    const story: PreparedStory = {
+      id: 'story--primary',
+      parameters: storyContext.parameters,
+    } as unknown as PreparedStory;
+    const docsContext = createMockDocsContext(story, storyContext);
+
+    const { result } = renderHook(
+      ({ sourceContext, serviceSnippet, props }) =>
+        useSourceProps(props, docsContext, sourceContext, serviceSnippet),
+      {
+        initialProps: {
+          sourceContext: {
+            sources: {
+              'story--primary': {
+                [UNKNOWN_ARGS_HASH]: { code: 'T:<Snippet />' },
+              },
+            },
+          } as SourceContextProps,
+          serviceSnippet: '',
+          props: {} as SourceProps,
+        },
+      }
+    );
 
-    const result = useCode({
-      snippet: '',
-      serviceSnippet: '',
-      storyContext: {
-        parameters: {
-          __isArgsStory: false,
-          docs: {
-            source: {
-              type: SourceType.CODE,
-              originalSource: '<Button />',
-              transform,
+    expect(result.current.code).toBe('T:<Button />');
+  });
+
+  it('transforms originalSource once for auto non-args story even when snippet is present', () => {
+    const storyContext: ReducedStoryContext = {
+      id: 'story--primary',
+      parameters: {
+        __isArgsStory: false,
+        docs: {
+          source: {
+            type: SourceType.AUTO,
+            originalSource: '<Button />',
+            transform: (code: string) => `T:${code}`,
+          },
+        },
+      },
+      args: {},
+      initialArgs: {},
+      unmappedArgs: {},
+    } as unknown as ReducedStoryContext;
+    const story: PreparedStory = {
+      id: 'story--primary',
+      parameters: storyContext.parameters,
+    } as unknown as PreparedStory;
+    const docsContext = createMockDocsContext(story, storyContext);
+
+    const { result } = renderHook(
+      ({ sourceContext, serviceSnippet, props }) =>
+        useSourceProps(props, docsContext, sourceContext, serviceSnippet),
+      {
+        initialProps: {
+          sourceContext: {
+            sources: {
+              'story--primary': {
+                [UNKNOWN_ARGS_HASH]: { code: 'T:<Snippet />' },
+              },
             },
+          } as SourceContextProps,
+          serviceSnippet: '',
+          props: {} as SourceProps,
+        },
+      }
+    );
+
+    expect(result.current.code).toBe('T:<Button />');
+  });
+
+  it('applies transformFromProps to emitted snippet when explicitly provided on props', () => {
+    const storyContext: ReducedStoryContext = {
+      id: 'story--primary',
+      parameters: {
+        __isArgsStory: true,
+        docs: {
+          source: {
+            type: SourceType.DYNAMIC,
+            transform: (code: string) => `T:${code}`,
           },
         },
-      } as any,
-      typeFromProps: SourceType.CODE,
-    });
+      },
+      args: {},
+      initialArgs: {},
+      unmappedArgs: {},
+    } as unknown as ReducedStoryContext;
+    const story: PreparedStory = {
+      id: 'story--primary',
+      parameters: storyContext.parameters,
+    } as unknown as PreparedStory;
+    const docsContext = createMockDocsContext(story, storyContext);
+
+    const { result } = renderHook(
+      ({ sourceContext, serviceSnippet, props }) =>
+        useSourceProps(props, docsContext, sourceContext, serviceSnippet),
+      {
+        initialProps: {
+          sourceContext: {
+            sources: {
+              'story--primary': {
+                [UNKNOWN_ARGS_HASH]: { code: 'T:<Button />' },
+              },
+            },
+          } as SourceContextProps,
+          serviceSnippet: '',
+          props: {
+            transform: (code: string) => `/*prop*/ ${code}`,
+          } as SourceProps,
+        },
+      }
+    );
 
-    expect(transform).toHaveBeenCalledWith('<Button />', expect.anything());
-    expect(result).toBe('/*transformed*/ <Button />');
+    expect(result.current.code).toBe('/*prop*/ T:<Button />');
   });
-});
+});
diff --git a/code/addons/docs/src/blocks/blocks/Source.tsx b/code/addons/docs/src/blocks/blocks/Source.tsx
index 82a43a7240d4..3171c8b73323 100644
--- a/code/addons/docs/src/blocks/blocks/Source.tsx
+++ b/code/addons/docs/src/blocks/blocks/Source.tsx
@@ -90,11 +90,25 @@ const useCode = ({
     // if this is an args story and there's a snippet
     (type === SourceType.AUTO && staticSnippet && isArgsStory);
 
-  const code = useSnippet ? staticSnippet : sourceParameters.originalSource || '';
-  // When using snippet (emitted via SNIPPET_RENDERED), emitTransformCode has already applied sourceParameters.transform.
-  // Avoid re-applying it unless explicitly overridden via transformFromProps.
+  const sourceKind: 'service' | 'emitted' | 'original' = useSnippet
+    ? serviceSnippet
+      ? 'service'
+      : snippet
+        ? 'emitted'
+        : 'original'
+    : 'original';
+
+  const code =
+    sourceKind === 'service'
+      ? serviceSnippet
+      : sourceKind === 'emitted'
+        ? snippet
+        : sourceParameters.originalSource || '';
+
+  // The emitted snippet is already transformed by emitTransformCode,
+  // so only transformFromProps applies to it.
   const transformer =
-    transformFromProps ?? (useSnippet && !serviceSnippet && snippet ? undefined : sourceParameters.transform);
+    transformFromProps ?? (sourceKind === 'emitted' ? undefined : sourceParameters.transform);
 
   const transformedCode = transformer ? useTransformCode(code, transformer, storyContext) : code;
 
diff --git a/code/core/src/preview-api/modules/preview-web/emitTransformCode.ts b/code/core/src/preview-api/modules/preview-web/emitTransformCode.ts
index 93fb269a3e6d..0891d5957f0f 100644
--- a/code/core/src/preview-api/modules/preview-web/emitTransformCode.ts
+++ b/code/core/src/preview-api/modules/preview-web/emitTransformCode.ts
@@ -19,6 +19,7 @@ export async function emitTransformCode(source: string | undefined, context: Red
   const transformed = transform && source ? transform?.(source, context) : source;
   const result = transformed ? await transformed : undefined;
 
+  // Note: The snippet payload emitted via SNIPPET_RENDERED is already transformed.
   addons.getChannel().emit(SNIPPET_RENDERED, {
     id,
     source: result,

From 8d5669793b3d9ef969014c7456c7ba9426587dda Mon Sep 17 00:00:00 2001
From: Samran Asif <samranwebdev2000@gmail.com>
Date: Sat, 17 Oct 2026 22:08:50 -0700
Subject: [PATCH 4/4] fix(docs): preserve hook count and avoid early fallback
 on dynamic Source blocks

Signed-off-by: Samran Asif <samranwebdev2000@gmail.com>
---
 code/addons/docs/src/blocks/blocks/Source.tsx | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/code/addons/docs/src/blocks/blocks/Source.tsx b/code/addons/docs/src/blocks/blocks/Source.tsx
index 3171c8b73323..3fe890bc66c8 100644
--- a/code/addons/docs/src/blocks/blocks/Source.tsx
+++ b/code/addons/docs/src/blocks/blocks/Source.tsx
@@ -93,9 +93,7 @@ const useCode = ({
   const sourceKind: 'service' | 'emitted' | 'original' = useSnippet
     ? serviceSnippet
       ? 'service'
-      : snippet
-        ? 'emitted'
-        : 'original'
+      : 'emitted'
     : 'original';
 
   const code =
@@ -107,10 +105,14 @@ const useCode = ({
 
   // The emitted snippet is already transformed by emitTransformCode,
   // so only transformFromProps applies to it.
-  const transformer =
-    transformFromProps ?? (sourceKind === 'emitted' ? undefined : sourceParameters.transform);
-
-  const transformedCode = transformer ? useTransformCode(code, transformer, storyContext) : code;
+  const hasTransformer = Boolean(transformFromProps ?? sourceParameters.transform);
+  const activeTransformer =
+    transformFromProps ??
+    (sourceKind === 'emitted' ? (rawCode: string) => rawCode : sourceParameters.transform);
+
+  const transformedCode = hasTransformer
+    ? useTransformCode(code, activeTransformer!, storyContext)
+    : code;
 
   if (sourceParameters.code !== undefined) {
     return sourceParameters.code;