Emit ES modules from JavaScript backend

effekt/jvm/src/main/scala/effekt/Runner.scala

@@ -145,7 +145,7 @@ trait Runner[Executable] {
 object JSNodeRunner extends Runner[String] {
   import scala.sys.process.Process
 
-  val extension = "js"
+  val extension = "mjs"
 
   def standardLibraryPath(root: File): File = root / "libraries" / "common"
 
@@ -158,27 +158,32 @@ object JSNodeRunner extends Runner[String] {
     else Left("Cannot find nodejs. This is required to use the JavaScript backend.")
 
   /**
-   * Creates an executable `.js` file besides the given `.js` file ([[path]])
+   * Creates an executable `.mjs` file besides the given `.mjs` file ([[path]])
    * and then returns the absolute path of the created executable.
    */
   def build(path: String)(using C: Context): String =
     val out = C.config.outputPath().getAbsolutePath
-    val jsFilePath = (out / path).canonicalPath.escape
-    val jsFileName = path.unixPath.split("/").last
+    val mjsFilePath = (out / path).canonicalPath.escape
+
     // create "executable" using shebang besides the .js file
-    val jsScript = s"require('./${jsFileName}').main()"
+    val mjsFileName = path.unixPath.split("/").last
+
+    // NOTE: This is a hack since this file cannot use ES imports & exports
+    //       because it doesn't have the .mjs ending. Sigh.
+    //       Also, we add the 'file:' prefix to satisfy Windows.
+    val jsScript = s"import('file:${mjsFileName}').then(({main}) => { main(); })"
 
     os match {
       case OS.POSIX =>
         val shebang = "#!/usr/bin/env node"
-        val jsScriptFilePath = jsFilePath.stripSuffix(s".$extension")
+        val jsScriptFilePath = mjsFilePath.stripSuffix(s".$extension")
         IO.createFile(jsScriptFilePath, s"$shebang\n$jsScript", true)
         jsScriptFilePath
 
       case OS.Windows =>
-        val jsMainFilePath = jsFilePath.stripSuffix(s".$extension") + "__main.js"
-        val jsMainFileName = jsFileName.stripSuffix(s".$extension") + "__main.js"
-        val exePath = jsFilePath.stripSuffix(s".$extension")
+        val jsMainFilePath = mjsFilePath.stripSuffix(s".$extension") + "__main.mjs"
+        val jsMainFileName = mjsFileName.stripSuffix(s".$extension") + "__main.mjs"
+        val exePath = mjsFilePath.stripSuffix(s".$extension")
         IO.createFile(jsMainFilePath, jsScript)
         createScript(exePath, "node", "$SCRIPT_DIR/" + jsMainFileName)
     }

effekt/shared/src/main/scala/effekt/generator/js/JavaScript.scala

@@ -13,7 +13,7 @@ class JavaScript(additionalFeatureFlags: List[String] = Nil) extends Compiler[St
 
   // Implementation of the Compiler Interface:
   // -----------------------------------------
-  def extension = ".js"
+  def extension = ".mjs"
 
   override def supportedFeatureFlags: List[String] = additionalFeatureFlags ++ TransformerMonadicWhole.jsFeatureFlags
 
@@ -48,7 +48,7 @@ class JavaScript(additionalFeatureFlags: List[String] = Nil) extends Compiler[St
     case input @ CoreTransformed(source, tree, mod, core) =>
       val mainSymbol = Context.checkMain(mod)
       val mainFile = path(mod)
-      val doc = pretty(TransformerMonadicWhole.compile(input, mainSymbol).commonjs)
+      val doc = pretty(TransformerMonadicWhole.compile(input, mainSymbol).esModule)
       (Map(mainFile -> doc.layout), mainFile)
   }
 

effekt/shared/src/main/scala/effekt/generator/js/PrettyPrinter.scala

@@ -77,6 +77,7 @@ object PrettyPrinter extends ParenPrettyPrinter {
     case Switch(sc, branches, default) => "switch" <+> parens(toDoc(sc)) <+> jsBlock(branches.map {
       case (tag, stmts) => "case" <+> toDoc(tag) <> ":" <+> nested(stmts map toDoc)
     } ++ default.toList.map { stmts => "default:" <+> nested(stmts map toDoc) })
+    case RawExport(name, expr)         => "export const" <+> toDoc(name) <+> "=" <+> parens(toDoc(expr))
   }
 
   def jsMethod(c: js.Function): Doc = c match {

effekt/shared/src/main/scala/effekt/generator/js/Transformer.scala

@@ -104,7 +104,7 @@ trait Transformer {
 
   def jsModuleName(path: String): String = "$" + path.replace('/', '_').replace('-', '_')
 
-  def jsModuleFile(path: String): String = path.replace('/', '_').replace('-', '_') + ".js"
+  def jsModuleFile(path: String): String = path.replace('/', '_').replace('-', '_') + ".mjs"
 
   val `fresh` = JSName("fresh")
   val `ref`   = JSName("ref")

effekt/shared/src/main/scala/effekt/generator/js/Tree.scala

@@ -28,26 +28,26 @@ case class Export(name: JSName, expr: Expr)
 case class Module(name: JSName, imports: List[Import], exports: List[Export], stmts: List[Stmt]) {
 
   /**
-   * Generates the Javascript module skeleton for whole program compilation
+   * Generates the Javascript module skeleton for ES modules
    */
-  def commonjs: List[Stmt] = {
+  def esModule: List[Stmt] = {
     val effekt = js.Const(JSName("$effekt"), js.Object())
 
     val importStmts = imports.map {
-      // const MOD = require(PATH)
+      // import * as MOD from PATH
       case Import.All(name, file) =>
-        js.Const(name, js.Call(Variable(JSName("require")), List(JsString(s"./${file}"))))
+        js.RawStmt(s"import * as ${name.name} from '${file}';")
 
-      // const {NAMES, ...} = require(PATH)
+      // import {NAMES, ...} from PATH
       case Import.Selective(names, file) =>
-        js.Destruct(names, js.Call(Variable(JSName("require")), List(JsString(s"./${ file }"))))
+        js.RawStmt(s"import { ${names.map(_.name).mkString(", ")} } from '${file}';")
     }
 
-    val exportStatement = js.Assign(RawExpr(s"(typeof module != \"undefined\" && module !== null ? module : {}).exports = ${name.name}"),
-      js.Object(exports.map { e => e.name -> e.expr })
-    )
+    val exportStatements = exports.map { e =>
+      RawExport(e.name, e.expr)
+    }
 
-    List(effekt) ++ importStmts ++ stmts ++ List(exportStatement)
+    List(effekt) ++ importStmts ++ stmts ++ exportStatements
   }
 
   /**
@@ -189,6 +189,9 @@ enum Stmt {
 
   // e.g. <EXPR>;
   case ExprStmt(expr: Expr)
+
+  // e.g. export const <NAME> = <EXPR>;
+  case RawExport(name: JSName, expr: Expr)
 }
 export Stmt.*
 

effekt/shared/src/main/scala/effekt/util/PathUtils.scala

@@ -40,7 +40,7 @@ trait PathUtils {
    * filename of the module (like `data_option.js`).
    */
   def moduleFileName(modulePath: String) =
-    modulePath.replace('/', '_') + ".js"
+    modulePath.replace('/', '_') + ".mjs"
 
   def lastModified(src: Source): Long = src match {
     case FileSource(name, encoding) => file(name).lastModified

libraries/common/io/console.effekt

@@ -4,7 +4,7 @@ import io
 import io/time // just for testing
 
 extern js """
-  const readline = require('node:readline');
+  import readline from 'node:readline';
 """
 
 interface Console {

libraries/common/io/files.effekt

@@ -6,7 +6,7 @@ import io
 import io/error
 
 extern js """
-  const fs = require("fs");
+  import fs from 'node:fs';
 """
 
 extern type FileDescriptor

libraries/common/io/network.effekt

@@ -6,7 +6,7 @@ import io
 
 namespace js {
   extern js """
-    const net = require('node:net');
+    import net from 'node:net';
 
     function listen(server, port, host, listener) {
       server.listen(port, host);