From d0e606930acb701790c286340226b1e3e7060b4f Mon Sep 17 00:00:00 2001
From: Akshit Khokhani <67451993+akshitkhokhani@users.noreply.github.com>
Date: Fri, 9 Oct 2026 17:49:43 -0500
Subject: [PATCH] fix(javascript): honor module-sync package exports (#1361)

---
 docs/javascript-artifact-reconstruction.md    |  2 ++
 .../JavaScriptArtifactPathResolution.ts       |  4 ++-
 .../javascriptPackageExportTargets.test.ts    | 21 ++++++++++++
 .../javascriptPackagePrecedence.test.ts       | 33 ++++++++++++++++++-
 4 files changed, 58 insertions(+), 2 deletions(-)

diff --git a/docs/javascript-artifact-reconstruction.md b/docs/javascript-artifact-reconstruction.md
index 3a996e5f9..68c65ca75 100644
--- a/docs/javascript-artifact-reconstruction.md
+++ b/docs/javascript-artifact-reconstruction.md
@@ -307,6 +307,8 @@ such as `./web3:app.js` still names a local artifact.
 Package `exports` fallback arrays are supported both at the top level and under
 the root `"."` entry. The resolver selects targets in declared order using the
 same conditional and invalid-entry handling as nested exports arrays.
+For imports and requires, the active Node conditions include `module-sync`;
+earlier active conditions retain precedence over later ones.
 
 ESM relative module paths and selected package exports targets use URL suffix handling
 and one percent-decoding pass. CommonJS relative paths and legacy package main
diff --git a/src/application/javascript/JavaScriptArtifactPathResolution.ts b/src/application/javascript/JavaScriptArtifactPathResolution.ts
index c28cd45c5..f87cc2455 100644
--- a/src/application/javascript/JavaScriptArtifactPathResolution.ts
+++ b/src/application/javascript/JavaScriptArtifactPathResolution.ts
@@ -640,7 +640,8 @@ const exportTargets = (
  * Node selects an exports target by walking the declared keys in order and
  * taking the first whose condition is active for the calling resolver.
  * "default" is always active, and "node" plus "node-addons" are active for the
- * built-in resolver that owns installed-package imports and requires.
+ * built-in resolver that owns installed-package imports and requires. The
+ * supported Node runtimes also activate "module-sync" for both loaders.
  */
 const packageExportConditions = (
   moduleKind: ResolveArtifactPathInput["moduleKind"],
@@ -648,6 +649,7 @@ const packageExportConditions = (
   new Set([
     "node",
     "node-addons",
+    "module-sync",
     ...(moduleKind === undefined ? [] : [moduleKind]),
     "default",
   ]);
diff --git a/tests/boundary/filesystem/javascriptPackageExportTargets.test.ts b/tests/boundary/filesystem/javascriptPackageExportTargets.test.ts
index 4d22c8a7a..aa8596cb6 100644
--- a/tests/boundary/filesystem/javascriptPackageExportTargets.test.ts
+++ b/tests/boundary/filesystem/javascriptPackageExportTargets.test.ts
@@ -103,6 +103,27 @@ const cases: readonly {
     exports: { browser: "./../outside.cjs", default: "./actual.cjs" },
     expected: "node_modules/fixture/actual.cjs",
   },
+  {
+    name: "active module-sync rejects an invalid target instead of default",
+    exports: { "module-sync": "./../outside.cjs", default: "./actual.cjs" },
+    expected: null,
+    nodeError: "ERR_INVALID_PACKAGE_TARGET",
+    rejectedTarget: "./../outside.cjs",
+  },
+  {
+    name: "module-sync null blocks default",
+    exports: { "module-sync": null, default: "./actual.cjs" },
+    expected: null,
+    nodeError: "ERR_PACKAGE_PATH_NOT_EXPORTED",
+  },
+  {
+    name: "unmatched nested module-sync permits default",
+    exports: {
+      "module-sync": { browser: "./missing.cjs" },
+      default: "./actual.cjs",
+    },
+    expected: "node_modules/fixture/actual.cjs",
+  },
   ...["./bad%.cjs", "./bad%C3.cjs"].map((target) => ({
     name: `malformed URL ${target} is selected before URL decoding`,
     exports: [target, "./actual.cjs"],
diff --git a/tests/boundary/filesystem/javascriptPackagePrecedence.test.ts b/tests/boundary/filesystem/javascriptPackagePrecedence.test.ts
index 4c730fb73..8bc2511e1 100644
--- a/tests/boundary/filesystem/javascriptPackagePrecedence.test.ts
+++ b/tests/boundary/filesystem/javascriptPackagePrecedence.test.ts
@@ -202,6 +202,36 @@ it.each([
       exports: { import: "./exported.mjs", require: "./actual.cjs" },
     },
   },
+  {
+    name: "import selects module-sync before default",
+    kind: "import",
+    metadata: {
+      exports: { "module-sync": "./sync.mjs", default: "./actual.cjs" },
+    },
+  },
+  {
+    name: "require selects nested module-sync before default",
+    kind: "require",
+    metadata: {
+      exports: {
+        ".": { node: { "module-sync": "./sync.mjs", default: "./actual.cjs" } },
+      },
+    },
+  },
+  {
+    name: "an earlier import condition precedes module-sync",
+    kind: "import",
+    metadata: {
+      exports: { import: "./exported.mjs", "module-sync": "./sync.mjs" },
+    },
+  },
+  {
+    name: "an earlier default condition precedes module-sync",
+    kind: "require",
+    metadata: {
+      exports: { default: "./actual.cjs", "module-sync": "./sync.mjs" },
+    },
+  },
 ] as const)(
   "matches the native Node loader when $name",
   async ({ kind, metadata }) => {
@@ -210,7 +240,7 @@ it.each([
     const source =
       kind === "import"
         ? `import value from "fixture"; console.log(JSON.stringify({value, resolved: import.meta.resolve("fixture")}));`
-        : `const value = require("fixture"); console.log(JSON.stringify({value, resolved: require.resolve("fixture")}));`;
+        : `const value = require("fixture"); console.log(JSON.stringify({value: typeof value === "string" ? value : value.default, resolved: require.resolve("fixture")}));`;
     const files = {
       "package.json": JSON.stringify({
         name: "app",
@@ -226,6 +256,7 @@ it.each([
       "node_modules/fixture/index.js": "module.exports = 'index.js';",
       "node_modules/fixture/bundler.mjs": "export default 'bundler.mjs';",
       "node_modules/fixture/exported.mjs": "export default 'exported.mjs';",
+      "node_modules/fixture/sync.mjs": "export default 'sync.mjs';",
     };
     await Promise.all(
       Object.entries(files).map(async ([path, text]) => {