From 23b37235c5dad2c5b7e831e9f572a665b4e6cebb Mon Sep 17 00:00:00 2001
From: Andrew Branch <andrew@wheream.io>
Date: Thu, 7 Dec 2023 16:57:12 -0800
Subject: [PATCH] Prototype localmodule/localcommonjs

---
 src/compiler/commandLineParser.ts |  2 ++
 src/compiler/program.ts           | 15 +++++++++++++++
 src/compiler/types.ts             |  2 ++
 3 files changed, 19 insertions(+)

diff --git a/src/compiler/commandLineParser.ts b/src/compiler/commandLineParser.ts
index 4d7bceb5360..6a21c6cdf50 100644
--- a/src/compiler/commandLineParser.ts
+++ b/src/compiler/commandLineParser.ts
@@ -580,6 +580,8 @@ const moduleSubOptionDeclarations: readonly CommandLineOption[] = [
             nodenext: ModuleFormatDetectionKind.NodeNext,
             defaultmodule: ModuleFormatDetectionKind.DefaultModule,
             defaultcommonjs: ModuleFormatDetectionKind.DefaultCommonJS,
+            localmodule: ModuleFormatDetectionKind.LocalModule,
+            localcommonjs: ModuleFormatDetectionKind.LocalCommonJS,
         })),
         affectsModuleResolution: true,
         affectsSemanticDiagnostics: true,
diff --git a/src/compiler/program.ts b/src/compiler/program.ts
index 539e8feec51..ce08fb9621f 100644
--- a/src/compiler/program.ts
+++ b/src/compiler/program.ts
@@ -1341,6 +1341,21 @@ export function getImpliedNodeFormatForFileWorker(
                 fileExtensionIsOneOf(fileName, [Extension.Dcts, Extension.Cts, Extension.Cjs]) ? ModuleKind.CommonJS :
                 fileExtensionIsOneOf(fileName, [Extension.Dts, Extension.Ts, Extension.Tsx, Extension.Js, Extension.Jsx]) ? lookupFromPackageJson(defaultFormat) :
                 undefined; // other extensions, like `json` or `tsbuildinfo`, are set as `undefined` here but they should never be fed through the transformer pipeline
+        case ModuleFormatDetectionKind.LocalModule:
+        case ModuleFormatDetectionKind.LocalCommonJS:
+            if (fileExtensionIsOneOf(fileName, [Extension.Dmts, Extension.Mts, Extension.Mjs])) {
+                return ModuleKind.ESNext;
+            }
+            if (fileExtensionIsOneOf(fileName, [Extension.Dcts, Extension.Cts, Extension.Cjs])) {
+                return ModuleKind.CommonJS;
+            }
+            if (!isDeclarationFileName(fileName) && fileExtensionIsOneOf(fileName, [Extension.Ts, Extension.Tsx, Extension.Js, Extension.Jsx])) {
+                // TODO: replace with more reliable check for "is this an emittable source file of this program"
+                // TODO: what about nested package.json directories? Should special behavior be restricted to the
+                //       one in scope of the tsconfig?
+                return formatDetection === ModuleFormatDetectionKind.LocalModule ? ModuleKind.ESNext : ModuleKind.CommonJS;
+            }
+            return lookupFromPackageJson(/*defaultFormat*/ ModuleKind.CommonJS);
         default:
             return undefined;
     }
diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index a636899b09b..f9bf15d3270 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -7288,6 +7288,8 @@ export enum ModuleFormatDetectionKind {
     Bundler = 1,
     DefaultModule = 2,
     DefaultCommonJS = 3,
+    LocalModule = 4,
+    LocalCommonJS = 5,
     Node16 = 100,
     NodeNext = 199,
 }