From 7ae853cc231bb48b6af5d197362f04c1f513bb02 Mon Sep 17 00:00:00 2001
From: Knut Wannheden <knut@moderne.io>
Date: Thu, 27 Aug 2026 16:37:18 +0200
Subject: [PATCH] JS: a test spec can opt into the style its own source is
 written in

A spec that formats its output is measured against the built-in defaults,
so a fixture indented with tabs comes back indented with four spaces and
the expectation cannot be written as the file reads. Declaring a style
meant hand-building a NamedStyles marker and overriding the spec's parser.

withDetectedStyle samples the file and attaches what it finds, wired per
spec via beforeRecipe. It stays opt-in to match Java, where RewriteTest
never autodetects and a test asks for a style with
spec.parser(JavaParser.fromJavaVersion().styles(...)). Applying it to
every spec instead would diverge from that and change what 9 existing
tests are measured against, 8 of them in format.test.ts, which feed
deliberately messy input and assert canonical output.
---
 .../rewrite/src/javascript/assertions.ts          | 15 +++++++++++++++
 .../rewrite/test/javascript/format/format.test.ts | 11 ++++++++++-
 2 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/rewrite-javascript/rewrite/src/javascript/assertions.ts b/rewrite-javascript/rewrite/src/javascript/assertions.ts
index 75ff1f97903..6ed1a349c9b 100644
--- a/rewrite-javascript/rewrite/src/javascript/assertions.ts
+++ b/rewrite-javascript/rewrite/src/javascript/assertions.ts
@@ -256,6 +256,21 @@ export function packageLockJson(before: string, after?: AfterRecipeText): Source
     };
 }
 
+/**
+ * Gives a source file the style its own text is written in. A spec that formats its output is
+ * measured against the built-in defaults unless it opts in with
+ * `{...typescript(src), beforeRecipe: withDetectedStyle}`; a file parsed through {@link npm} is
+ * sampled alongside its siblings and needs no opt-in.
+ */
+export async function withDetectedStyle(sourceFile: JS.CompilationUnit): Promise<JS.CompilationUnit> {
+    const detector = Autodetect.detector();
+    await detector.sample(sourceFile);
+    const detected = detector.build();
+    return produce(sourceFile, draft => {
+        draft.markers = replaceMarkerByKind(draft.markers, detected);
+    });
+}
+
 export function javascript(before: string | null, after?: AfterRecipeText): SourceSpec<JS.CompilationUnit> {
     return {
         kind: JS.Kind.CompilationUnit,
diff --git a/rewrite-javascript/rewrite/test/javascript/format/format.test.ts b/rewrite-javascript/rewrite/test/javascript/format/format.test.ts
index 4ac75e93999..f8d7673947d 100644
--- a/rewrite-javascript/rewrite/test/javascript/format/format.test.ts
+++ b/rewrite-javascript/rewrite/test/javascript/format/format.test.ts
@@ -40,7 +40,8 @@ import {
     autoFormat,
     AutoformatVisitor,
     JavaScriptVisitor,
-    typescript
+    typescript,
+    withDetectedStyle
 } from "../../../src/javascript";
 
 
@@ -702,5 +703,13 @@ const x = 1;`
         )
     });
 
+    test('a spec opting into its own style keeps the tabs it is written with', () => {
+        return spec.rewriteRun({
+            //language=typescript
+            ...typescript("function f() {\n\tconst x = 1;\n\treturn x;\n}\n"),
+            beforeRecipe: withDetectedStyle
+        })
+    });
+
 });