refactor: extract shared mergeMarkerDeclarations helper

DRY up the marker-merging logic that was duplicated in transform.ts,
run.ts, and the regeneration script. All three now use the shared
mergeMarkerDeclarations() from run.ts.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: skovhus

scripts/regenerate-test-case-outputs.mts

@@ -29,12 +29,14 @@ const [
   { scanCrossFileSelectors },
   { createModuleResolver },
   { Logger },
+  { mergeMarkerDeclarations },
 ] = await Promise.all([
   import("../src/transform.ts"),
   import("../src/__tests__/fixture-adapters.ts"),
   import("../src/internal/prepass/scan-cross-file-selectors.ts"),
   import("../src/internal/prepass/resolve-imports.ts"),
   import("../src/internal/logger.ts"),
+  import("../src/run.ts"),
 ]);
 
 // Suppress diagnostic output during regeneration
@@ -81,18 +83,6 @@ async function listFixtureNames(): Promise<Array<{ name: string; ext: string }>>
     .sort((a, b) => a.name.localeCompare(b.name));
 }
 
-/** Merge marker declarations from `incoming` into `base`, appending only new exports. */
-function mergeMarkerContent(base: string, incoming: string): string {
-  const markerLineRe = /^export const \w+ = stylex\.defineMarker\(\);$/gm;
-  const newMarkers = [...incoming.matchAll(markerLineRe)].map((m) => m[0]);
-  const markersToAdd = newMarkers.filter((line) => !base.includes(line));
-  if (markersToAdd.length === 0) {
-    return base;
-  }
-  const trailingNewline = base.endsWith("\n") ? "" : "\n";
-  return base + trailingNewline + markersToAdd.join("\n") + "\n";
-}
-
 // Accumulate all sidecar files across fixtures for merging (shared marker files)
 const allSidecarFiles = new Map<string, string>();
 
@@ -125,7 +115,10 @@ async function updateFixture(name: string, ext: string) {
   // Accumulate sidecar files for merging after all fixtures are processed
   for (const [sidecarPath, content] of sidecarFiles) {
     const existing = allSidecarFiles.get(sidecarPath);
-    allSidecarFiles.set(sidecarPath, existing ? mergeMarkerContent(existing, content) : content);
+    allSidecarFiles.set(
+      sidecarPath,
+      existing ? mergeMarkerDeclarations(existing, content) : content,
+    );
   }
 
   return outputPath;
@@ -164,7 +157,7 @@ for (const [sidecarPath, content] of allSidecarFiles) {
   if (only) {
     try {
       const existing = await readFile(sidecarPath, "utf-8");
-      merged = mergeMarkerContent(content, existing);
+      merged = mergeMarkerDeclarations(content, existing);
     } catch {
       // File doesn't exist yet, write fresh content
     }

src/run.ts

@@ -607,52 +607,42 @@ function createAutoPrepassFailureError(
 }
 
 /**
- * Merge new sidecar marker content into an existing .stylex.ts file, preserving
- * user-owned exports (e.g. defineVars). If the file doesn't exist, returns content as-is.
- *
- * New marker declarations (`export const XMarker = stylex.defineMarker()`) are
- * appended only if they don't already exist in the file. The stylex import is
- * ensured at the top.
+ * Merge marker declarations from `incoming` into `base`, appending only new
+ * `export const XMarker = stylex.defineMarker()` lines. Returns `base` unchanged
+ * if all markers already exist.
  */
-export function mergeSidecarContent(sidecarPath: string, newContent: string): string {
-  let existing: string;
-  try {
-    existing = readFileSync(sidecarPath, "utf-8");
-  } catch {
-    // File doesn't exist yet — use new content as-is
-    return newContent;
-  }
-
-  // Extract marker export lines from the new content
+export function mergeMarkerDeclarations(base: string, incoming: string): string {
   const markerLineRe = /^export const \w+ = stylex\.defineMarker\(\);$/gm;
-  const newMarkers: string[] = [];
-  for (const m of newContent.matchAll(markerLineRe)) {
-    newMarkers.push(m[0]);
-  }
-
+  const newMarkers = [...incoming.matchAll(markerLineRe)].map((m) => m[0]);
   if (newMarkers.length === 0) {
-    return newContent;
+    return base;
   }
-
-  // Filter out markers already present in the existing file
-  const markersToAdd = newMarkers.filter((line) => !existing.includes(line));
-
+  const markersToAdd = newMarkers.filter((line) => !base.includes(line));
   if (markersToAdd.length === 0) {
-    // All markers already exist — no changes needed
-    return existing;
+    return base;
   }
-
   // Ensure the stylex import exists
-  let merged = existing;
+  let merged = base;
   if (!merged.includes("@stylexjs/stylex")) {
     merged = `import * as stylex from "@stylexjs/stylex";\n\n${merged}`;
   }
-
-  // Append new marker declarations at end
   const trailingNewline = merged.endsWith("\n") ? "" : "\n";
-  merged = merged + trailingNewline + markersToAdd.join("\n") + "\n";
+  return merged + trailingNewline + markersToAdd.join("\n") + "\n";
+}
 
-  return merged;
+/**
+ * Merge new sidecar marker content into an existing .stylex.ts file, preserving
+ * user-owned exports (e.g. defineVars). If the file doesn't exist, returns content as-is.
+ */
+export function mergeSidecarContent(sidecarPath: string, newContent: string): string {
+  let existing: string;
+  try {
+    existing = readFileSync(sidecarPath, "utf-8");
+  } catch {
+    // File doesn't exist yet — use new content as-is
+    return newContent;
+  }
+  return mergeMarkerDeclarations(existing, newContent);
 }
 
 /** Run formatter commands on a list of files, logging warnings on failure. */

src/transform.ts

@@ -6,6 +6,7 @@ import type { API, FileInfo, Options } from "jscodeshift";
 import { basename, dirname, join, resolve as pathResolve } from "node:path";
 import { realpathSync } from "node:fs";
 
+import { mergeMarkerDeclarations } from "./run.js";
 import { Logger } from "./internal/logger.js";
 import { TransformContext } from "./internal/transform-context.js";
 import type {
@@ -72,20 +73,12 @@ export default function transform(file: FileInfo, api: API, options: Options): s
           join(dirname(file.path), `${basename(file.path).replace(/\.\w+$/, "")}.stylex.ts`);
         // Merge with existing content when multiple files write to the same sidecar path
         const existing = sidecarFiles.get(sidecarPath);
-        if (existing) {
-          const markerLineRe = /^export const \w+ = stylex\.defineMarker\(\);$/gm;
-          const newMarkers = [...result.sidecarContent.matchAll(markerLineRe)].map((m) => m[0]);
-          const markersToAdd = newMarkers.filter((line) => !existing.includes(line));
-          if (markersToAdd.length > 0) {
-            const trailingNewline = existing.endsWith("\n") ? "" : "\n";
-            sidecarFiles.set(
-              sidecarPath,
-              existing + trailingNewline + markersToAdd.join("\n") + "\n",
-            );
-          }
-        } else {
-          sidecarFiles.set(sidecarPath, result.sidecarContent);
-        }
+        sidecarFiles.set(
+          sidecarPath,
+          existing
+            ? mergeMarkerDeclarations(existing, result.sidecarContent)
+            : result.sidecarContent,
+        );
       }
     }