From 06daa796270c2bea317bcbcafa28927abee662d0 Mon Sep 17 00:00:00 2001
From: Hardik Kaurani <hardikkaurani1@gmail.com>
Date: Mon, 19 Oct 2026 12:34:51 +0530
Subject: [PATCH] Fix checkJs behavior for .mjs and .cjs files next to
 declarations (#64312)

Drop legacy .js/.d.ts wildcard exception from extension priority to align behavior across .js, .mjs, and .cjs.

Fixes #64312
---
 tsc/internal/tsoptions/tsconfigparsing.go     |  6 ---
 .../compiler/checkJsExtensionPriority.symbols | 15 ++++++
 .../compiler/checkJsExtensionPriority.types   | 15 ++++++
 ...ckJsExtensionPriority_includeOrder.symbols | 14 ++++++
 ...heckJsExtensionPriority_includeOrder.types | 14 ++++++
 .../compiler/checkJsExtensionPriority.ts      | 42 +++++++++++++++++
 .../checkJsExtensionPriority_includeOrder.ts  | 46 +++++++++++++++++++
 7 files changed, 146 insertions(+), 6 deletions(-)
 create mode 100644 tsc/testdata/baselines/reference/compiler/checkJsExtensionPriority.symbols
 create mode 100644 tsc/testdata/baselines/reference/compiler/checkJsExtensionPriority.types
 create mode 100644 tsc/testdata/baselines/reference/compiler/checkJsExtensionPriority_includeOrder.symbols
 create mode 100644 tsc/testdata/baselines/reference/compiler/checkJsExtensionPriority_includeOrder.types
 create mode 100644 tsc/testdata/tests/cases/compiler/checkJsExtensionPriority.ts
 create mode 100644 tsc/testdata/tests/cases/compiler/checkJsExtensionPriority_includeOrder.ts

diff --git a/tsc/internal/tsoptions/tsconfigparsing.go b/tsc/internal/tsoptions/tsconfigparsing.go
index 16b231c42e126..cd2254f498e55 100644
--- a/tsc/internal/tsoptions/tsconfigparsing.go
+++ b/tsc/internal/tsoptions/tsconfigparsing.go
@@ -1807,12 +1807,6 @@ func hasFileWithHigherPriorityExtension(file tspath.RootedFilePath, extensions [
 			return false
 		}
 		if hasFile(file.ChangeExtension(ext)) {
-			if ext == tspath.ExtensionDts && (file.ExtensionIs(tspath.ExtensionJs) || file.ExtensionIs(tspath.ExtensionJsx)) {
-				// LEGACY BEHAVIOR: An off-by-one bug somewhere in the extension priority system for wildcard module loading allowed declaration
-				// files to be loaded alongside their js(x) counterparts. We regard this as generally undesirable, but retain the behavior to
-				// prevent breakage.
-				continue
-			}
 			return true
 		}
 	}
diff --git a/tsc/testdata/baselines/reference/compiler/checkJsExtensionPriority.symbols b/tsc/testdata/baselines/reference/compiler/checkJsExtensionPriority.symbols
new file mode 100644
index 0000000000000..7b93d669d151a
--- /dev/null
+++ b/tsc/testdata/baselines/reference/compiler/checkJsExtensionPriority.symbols
@@ -0,0 +1,15 @@
+//// [tests/cases/compiler/checkJsExtensionPriority.ts] ////
+
+=== /a.d.ts ===
+export declare const n: number;
+>n : Symbol(n, Decl(a.d.ts, 0, 20))
+
+=== /b.d.mts ===
+export declare const n: number;
+>n : Symbol(n, Decl(b.d.mts, 0, 20))
+
+=== /c.d.cts ===
+export declare const n: number;
+>n : Symbol(n, Decl(c.d.cts, 0, 20))
+
+
diff --git a/tsc/testdata/baselines/reference/compiler/checkJsExtensionPriority.types b/tsc/testdata/baselines/reference/compiler/checkJsExtensionPriority.types
new file mode 100644
index 0000000000000..e2d4db7f44368
--- /dev/null
+++ b/tsc/testdata/baselines/reference/compiler/checkJsExtensionPriority.types
@@ -0,0 +1,15 @@
+//// [tests/cases/compiler/checkJsExtensionPriority.ts] ////
+
+=== /a.d.ts ===
+export declare const n: number;
+>n : number
+
+=== /b.d.mts ===
+export declare const n: number;
+>n : number
+
+=== /c.d.cts ===
+export declare const n: number;
+>n : number
+
+
diff --git a/tsc/testdata/baselines/reference/compiler/checkJsExtensionPriority_includeOrder.symbols b/tsc/testdata/baselines/reference/compiler/checkJsExtensionPriority_includeOrder.symbols
new file mode 100644
index 0000000000000..a18c297699957
--- /dev/null
+++ b/tsc/testdata/baselines/reference/compiler/checkJsExtensionPriority_includeOrder.symbols
@@ -0,0 +1,14 @@
+//// [tests/cases/compiler/checkJsExtensionPriority_includeOrder.ts] ////
+
+=== /a.d.ts ===
+export declare const n: number;
+>n : Symbol(n, Decl(a.d.ts, 0, 20))
+
+=== /b.d.mts ===
+export declare const n: number;
+>n : Symbol(n, Decl(b.d.mts, 0, 20))
+
+=== /c.d.cts ===
+export declare const n: number;
+>n : Symbol(n, Decl(c.d.cts, 0, 20))
+
diff --git a/tsc/testdata/baselines/reference/compiler/checkJsExtensionPriority_includeOrder.types b/tsc/testdata/baselines/reference/compiler/checkJsExtensionPriority_includeOrder.types
new file mode 100644
index 0000000000000..9e63783c747bb
--- /dev/null
+++ b/tsc/testdata/baselines/reference/compiler/checkJsExtensionPriority_includeOrder.types
@@ -0,0 +1,14 @@
+//// [tests/cases/compiler/checkJsExtensionPriority_includeOrder.ts] ////
+
+=== /a.d.ts ===
+export declare const n: number;
+>n : number
+
+=== /b.d.mts ===
+export declare const n: number;
+>n : number
+
+=== /c.d.cts ===
+export declare const n: number;
+>n : number
+
diff --git a/tsc/testdata/tests/cases/compiler/checkJsExtensionPriority.ts b/tsc/testdata/tests/cases/compiler/checkJsExtensionPriority.ts
new file mode 100644
index 0000000000000..b93888dac4e54
--- /dev/null
+++ b/tsc/testdata/tests/cases/compiler/checkJsExtensionPriority.ts
@@ -0,0 +1,42 @@
+// @Filename: /tsconfig.json
+{
+    "compilerOptions": {
+        "target": "esnext",
+        "module": "nodenext",
+        "moduleResolution": "nodenext",
+        "allowJs": true,
+        "checkJs": true,
+        "strict": true,
+        "noEmit": true
+    }
+}
+
+// @Filename: /package.json
+{
+    "type": "module"
+}
+
+// @Filename: /a.js
+export const n = 1;
+const bad = null;
+bad.a.b.c();
+
+// @Filename: /a.d.ts
+export declare const n: number;
+
+// @Filename: /b.mjs
+export const n = 1;
+const bad = null;
+bad.a.b.c();
+
+// @Filename: /b.d.mts
+export declare const n: number;
+
+// @Filename: /c.cjs
+export const n = 1;
+const bad = null;
+bad.a.b.c();
+
+// @Filename: /c.d.cts
+export declare const n: number;
+
diff --git a/tsc/testdata/tests/cases/compiler/checkJsExtensionPriority_includeOrder.ts b/tsc/testdata/tests/cases/compiler/checkJsExtensionPriority_includeOrder.ts
new file mode 100644
index 0000000000000..88151cbbbebc9
--- /dev/null
+++ b/tsc/testdata/tests/cases/compiler/checkJsExtensionPriority_includeOrder.ts
@@ -0,0 +1,46 @@
+// @Filename: /tsconfig.json
+{
+    "compilerOptions": {
+        "target": "esnext",
+        "module": "nodenext",
+        "moduleResolution": "nodenext",
+        "allowJs": true,
+        "checkJs": true,
+        "strict": true,
+        "noEmit": true
+    },
+    "include": [
+        "**/*.js",
+        "**/*.d.ts",
+        "**/*.mjs",
+        "**/*.d.mts",
+        "**/*.cjs",
+        "**/*.d.cts"
+    ]
+}
+
+// @Filename: /package.json
+{
+    "type": "module"
+}
+
+// @Filename: /a.js
+const bad = null;
+bad.a();
+
+// @Filename: /a.d.ts
+export declare const n: number;
+
+// @Filename: /b.mjs
+const bad = null;
+bad.b();
+
+// @Filename: /b.d.mts
+export declare const n: number;
+
+// @Filename: /c.cjs
+const bad = null;
+bad.c();
+
+// @Filename: /c.d.cts
+export declare const n: number;