fix: generate modules for empty files with esModuleInterop (#992)

* fix: generate modules for empty files with esModuleInterop

* generate extension.ts

* also add empty.ts

integration/extension-import/base.proto

@@ -0,0 +1,9 @@
+syntax = "proto2";
+
+package foo;
+
+message Extendable{
+  required string field = 1;
+
+  extensions 4 to max;
+}

integration/extension-import/extension-import-test.ts

@@ -0,0 +1,13 @@
+import * as path from "node:path";
+import * as fs from "node:fs";
+import * as ts from "typescript";
+
+describe('extension-only-files', () => {
+  it("generate as external module", () => {
+    const generatedPath = path.join(__dirname, "extension.ts");
+    const generatedCode = fs.readFileSync(generatedPath, "utf8");
+    const source = ts.createSourceFile(generatedPath, generatedCode, ts.ScriptTarget.ES2018);
+
+    expect(ts.isExternalModule(source)).toBe(true);
+  })
+});

integration/extension-import/extension.proto

@@ -0,0 +1,9 @@
+syntax = "proto2";
+
+import "base.proto";
+
+package foo;
+
+extend Extendable{
+  optional int32 bar = 10;
+}

integration/extension-import/extension.ts

@@ -0,0 +1,3 @@
+/* eslint-disable */
+
+export {};

integration/extension-import/parameters.txt

@@ -0,0 +1 @@
+outputIndex=true,esModuleInterop=true

integration/simple-esmodule-interop/empty-test.ts

@@ -0,0 +1,7 @@
+import { } from './empty';
+
+describe('empty', () => {
+  it('compiles', () => {
+    // if ./empty was not a module, this would not compile.
+  });
+});

integration/simple-esmodule-interop/empty.proto

@@ -0,0 +1,2 @@
+syntax = "proto2";
+package empty;

integration/simple-esmodule-interop/empty.ts

@@ -0,0 +1,3 @@
+/* eslint-disable */
+
+export const protobufPackage = "empty";

src/main.ts

@@ -382,6 +382,15 @@ export function generateFile(ctx: Context, fileDesc: FileDescriptorProto): [stri
     chunks.push(...generateSchema(ctx, fileDesc, sourceInfo));
   }
 
+  // https://www.typescriptlang.org/docs/handbook/2/modules.html:
+  // > In TypeScript, just as in ECMAScript 2015, any file containing a top-level import or export is considered a module.
+  // > Conversely, a file without any top-level import or export declarations is treated as a script whose contents are available in the global scope (and therefore to modules as well).
+  //
+  // Thus, to mark an empty file a module, we need to add `export {}` to it.
+  if (options.esModuleInterop && chunks.length === 0) {
+    chunks.push(code`export {};`);
+  }
+
   chunks.push(
     ...Object.values(utils).map((v) => {
       if (v instanceof ConditionalOutput) {