[K/JS] Add .mjs extension to main file in package.json for ES modules compilation

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/kotlin/org/jetbrains/kotlin/gradle/Kotlin2JsGradlePluginIT.kt

@@ -624,6 +624,61 @@ class Kotlin2JsIrGradlePluginIT : KGPBaseTest() {
         }
     }
 
+
+    @DisplayName("package json contains correct extension for ES-modules")
+    @GradleTest
+    fun testPackageJsonWithEsModules(gradleVersion: GradleVersion) {
+        project("kotlin-js-browser-project", gradleVersion) {
+            buildGradleKts.modify(::transformBuildScriptWithPluginsDsl)
+            subProject("app").buildGradleKts.modify {
+                it + """
+                    |
+                    |kotlin.target.useEsModules()
+                    |
+                """.trimMargin()
+            }
+
+            build(":app:packageJson") {
+                val packageJson = projectPath
+                    .resolve("build/js/packages/kotlin-js-browser-app")
+                    .resolve(NpmProject.PACKAGE_JSON)
+                    .let {
+                        Gson().fromJson(it.readText(), PackageJson::class.java)
+                    }
+
+                assertEquals(packageJson.main, "kotlin/kotlin-js-browser-app.mjs")
+
+            }
+        }
+    }
+
+    @DisplayName("public package json contains correct extension for ES-modules")
+    @GradleTest
+    fun testPublicPackageJsonWithEsModules(gradleVersion: GradleVersion) {
+        project("kotlin-js-browser-project", gradleVersion) {
+            buildGradleKts.modify(::transformBuildScriptWithPluginsDsl)
+            subProject("app").buildGradleKts.modify {
+                it + """
+                    |
+                    |kotlin.target.useEsModules()
+                    |
+                """.trimMargin()
+            }
+
+            build(":app:publicPackageJson") {
+                val packageJson = subProject("app").projectPath
+                    .resolve("build/tmp/publicPackageJson")
+                    .resolve(NpmProject.PACKAGE_JSON)
+                    .let {
+                        Gson().fromJson(it.readText(), PackageJson::class.java)
+                    }
+
+                assertEquals(packageJson.main, "kotlin-js-browser-app.mjs")
+
+            }
+        }
+    }
+
     @DisplayName("Multiple targets works without clash")
     @GradleTest
     fun testMultipleJsTargets(gradleVersion: GradleVersion) {

libraries/tools/kotlin-gradle-plugin/src/common/kotlin/org/jetbrains/kotlin/gradle/targets/js/npm/NpmProject.kt

@@ -7,6 +7,7 @@ package org.jetbrains.kotlin.gradle.targets.js.npm
 
 import org.gradle.api.Project
 import org.gradle.process.ExecSpec
+import org.jetbrains.kotlin.gradle.dsl.JsModuleKind
 import org.jetbrains.kotlin.gradle.plugin.KotlinCompilation
 import org.jetbrains.kotlin.gradle.plugin.KotlinPlatformType
 import org.jetbrains.kotlin.gradle.plugin.mpp.KotlinJsCompilation
@@ -31,7 +32,12 @@ val KotlinJsCompilation.npmProject: NpmProject
 open class NpmProject(@Transient val compilation: KotlinJsCompilation) : Serializable {
     val compilationName = compilation.disambiguatedName
 
-    private val extension = if (compilation.platformType == KotlinPlatformType.wasm) ".mjs" else ".js"
+    private val extension by lazy {
+        if (
+            compilation.platformType == KotlinPlatformType.wasm ||
+            compilation.kotlinOptions.moduleKind == JsModuleKind.MODULE_ES.kind
+        ) ".mjs" else ".js"
+    }
 
     val name: String by lazy {
         buildNpmProjectName()

libraries/tools/kotlin-gradle-plugin/src/common/kotlin/org/jetbrains/kotlin/gradle/targets/js/npm/PublicPackageJsonTask.kt

@@ -36,6 +36,9 @@ abstract class PublicPackageJsonTask :
     @get:Input
     abstract val jsIrCompilation: Property<Boolean>
 
+    @get:Input
+    abstract val esModules: Property<Boolean>
+
     @get:Input
     abstract val npmProjectName: Property<String>
 
@@ -85,7 +88,8 @@ abstract class PublicPackageJsonTask :
             externalDependencies,
             packageJsonHandlers.get()
         ).let { packageJson ->
-            packageJson.main = "${npmProjectName.get()}.js"
+            val extension = if (esModules.get()) "mjs" else "js"
+            packageJson.main = "${npmProjectName.get()}.$extension"
 
             if (jsIrCompilation.get()) {
                 packageJson.types = "${npmProjectName.get()}.d.ts"

libraries/tools/kotlin-gradle-plugin/src/common/kotlin/org/jetbrains/kotlin/gradle/targets/js/npm/resolver/KotlinCompilationNpmResolver.kt

@@ -16,6 +16,7 @@ import org.gradle.api.attributes.Usage
 import org.gradle.api.internal.artifacts.DefaultProjectComponentIdentifier
 import org.gradle.api.tasks.TaskProvider
 import org.gradle.api.tasks.bundling.Zip
+import org.jetbrains.kotlin.gradle.dsl.JsModuleKind
 import org.jetbrains.kotlin.gradle.plugin.KotlinCompilation
 import org.jetbrains.kotlin.gradle.plugin.categoryByName
 import org.jetbrains.kotlin.gradle.plugin.mpp.KotlinJsCompilation
@@ -80,6 +81,7 @@ class KotlinCompilationNpmResolver(
             it.jsIrCompilation.set(compilation is KotlinJsIrCompilation)
             it.npmProjectName.set(npmProject.name)
             it.npmProjectMain.set(npmProject.main)
+            it.esModules.set(compilation.compilerOptions.options.moduleKind.get() == JsModuleKind.MODULE_ES)
         }.also { packageJsonTask ->
             project.dependencies.attributesSchema {
                 it.attribute(publicPackageJsonAttribute)