Emitting JavaScript modules

Basic Module Setup

By default, the -fastopt.js and -fullopt.js files produced by Scala.js are top-level scripts, and their @JSExported stuff are sent to the global scope. With modern JavaScript toolchains, we typically write modules instead, which import and export things from other modules. You can configure Scala.js to emit a JavaScript module instead of a top-level script.

Two kinds of modules are supported: CommonJS modules (traditional module system of Node.js) and ECMAScript modules. They are enabled with the following sbt settings:

// ECMAScript
scalaJSLinkerConfig ~= { _.withModuleKind(ModuleKind.ESModule) }
// CommonJS
scalaJSLinkerConfig ~= { _.withModuleKind(ModuleKind.CommonJSModule) }

Note: when using ECMAScript modules, the optimizations performed in fullLinkJS are limited, because the Google Closure Compiler cannot be used with them.

When emitting a module, @JSExportTopLevels are really exported from the Scala.js module. Moreover, you can use top-level @JSImport to import native JavaScript stuff from other JavaScript module.

For example, consider the following definitions:

import scala.scalajs.js
import scala.scalajs.js.annotation._

@JSImport("bar.js", "Foo")
class JSFoo(val x: Int) extends js.Object

class Foobaz(x: String) extends js.Object {
  val inner = new JSFoo(x.length)

  def method(y: String): Int = x + y

Once compiled under ModuleKind.ESModule, the resulting module would be equivalent to the following JavaScript module:

import { Foo as JSFoo } from "bar.js";

class Foobaz {
  constructor(x) {
    this.x = x;
    this.inner = new JSFoo(x.length);

  method(y) {
    return this.x + y;

export { Foobaz as Babar };

With ModuleKind.CommonJSModule, it would instead be equivalent to:

var bar = require("bar.js");

class Foobaz {
  constructor(x) {
    this.x = x;
    this.inner = new bar.Foo(x.length);

  method(y) {
    return this.x + y;

exports.Babar = Foobaz;

Module Splitting

When emitting modules, the Scala.js linker is able to split its output into multiple JavaScript modules (i.e. files).

There are several reasons to split the JavaScript output into multiple files:

  • Share code between different parts of an application (e.g. user/admin interfaces).
  • Load parts of a large app progressively
  • Create smaller files to minimize changes for incremental downstream tooling.

The Scala.js linker can split a full Scala.js application automatically based on:

  • Entry points (top-level exports and module initializers)
  • Dynamic import boundaries (calls to js.dynamicImport)
  • The split style (fewest modules, smallest modules, or a combination thereof)

Entry Points

Scala.js-generated code has two different kinds of entry points:

The Scala.js linker determines how to group entry points into different (public) modules by using their assigned moduleID. The default moduleID is "main".

The moduleID of a top-level export can be specified using the moduleID parameter. The moduleID of a ModuleInitializer can be specified by the withModuleID method.


Say you have the following App.scala and build.sbt:

package my.app

import scala.collection.mutable
import scala.scalajs.js.annotation._

// Separate objects to allow for splitting.

object AppA {
  @JSExportTopLevel(name = "start", moduleID = "a")
  def a(): Unit = println("hello from a")

object AppB {
  private val x = mutable.Set.empty[String]

  @JSExportTopLevel(name = "start", moduleID = "b")
  def b(): Unit = {
    println("hello from b")

  def main(): Unit = x.add("something")
import org.scalajs.linker.interface.ModuleInitializer

scalaJSModuleInitializers in Compile += {
  ModuleInitializer.mainMethod("my.app.AppB", "main").withModuleID("b")

This would generate two public modules a.js / b.js. a.js will export a method named start that calls AppA.a. b.js will export a method named start that calls AppB.b. Further, importing b.js will call AppB.main.

Note that there is no public module main.js, because there is no entry point using the default moduleID.

Dynamic Imports

Warning: Dynamic imports in Scala.js 1.4.0 are affected by #4386, see the issue for a workaround.

Dynamic imports allow a Scala.js application to be loaded in multiple steps to reduce initial loading time. To defer loading of a part of your Scala.js application to a later point in time, use js.dynamicImport:


import scala.scalajs.js
import scala.scalajs.js.annotation._

import scala.concurrent.ExecutionContext.Implicits.global

class HeavyFeature {
  def doHeavyFeature(x: Int): Int =
    x * 2

object MyApp {
  @JSExportTopLevel(name = "onClick")
  def onClick(input: Int): Unit = {
    val resultPromise: js.Promise[Int] = js.dynamicImport {
      new HeavyFeature().doHeavyFeature(input)
    for (result <- resultPromise.toFuture)

  private def updateUIWithOutput(i: Int): Unit = ???

The js.dynamicImport method has the following signature:

def dynamicImport[A](body: => A): js.Promise[A]

Semantically, it will evaluate body asynchronously and return a Promise of the result. More importantly, it acts as a border for the Scala.js linker to split out a module that will be dynamically loaded. The above program would generate

  • a public module main.js containing onClick and its direct dependencies
  • an internal module MyApp$$anon$1.js containing HeavyFeature
  • an internal module main-MyApp$$anon$1.js containing common dependencies of main.js and MyApp$$anon$1.js.

Internal modules allow the Scala.js linker to split code internally. Unlike public modules, internal modules may not be imported by user code. Doing so is undefined behavior and subject to change at any time.

In the example above, the js.dynamicImport is replaced by import("./MyApp$$anon$1.js"), followed by an invocation of the main entry point in MyApp$$anon$1.js (the body passed to js.dynamicImport). Therefore, when main.js is loaded, we do not need to load, nor download MyApp$$anon$1.js. It will only be loaded the first time onClick is actually called. This reduces the initial download time for users.

Dynamic imports and entry points can be arbitrarily combined.

Module Split Styles

So far, we have seen how public modules and dynamic import boundaries can be defined.

Based on these, the Scala.js linker automatically uses the dependency graph of the code to generate appropriate internal modules.

However, there are still choices involved. They can be configured with the moduleSplitStyle:

import org.scalajs.linker.interface.ModuleSplitStyle
scalaJSLinkerConfig ~= (_.withModuleSplitStyle(ModuleSplitStyle.SmallestModules))

There are currently three module split styles: FewestModules, SmallestModules and SmallModulesFor(packages).


Create as few modules as possible

  • while respecting dynamic import boundaries and
  • without including unnecessary code.

This is the default.

In the entry points example above, this would generate:

  • a.js: public module, containing AppA and the export of start.
  • b.js: public module, containing AppB, mutable.Set, the export of start and the call to AppB.main
  • a-b.js: internal module, Scala.js core and the implementation of println.

This also works for more than two public modules, creating intermediate shared (internal) modules as necessary.

The dynamic import example above already assumes this module split style so a module listing is omitted.


Create modules that are as small as possible. The smallest unit of splitting is a Scala class (see Splitting Granularity below for more).

Using this mode typically results in an internal module per class with the exception of classes that have circular dependencies: these are put into the same module to avoid a circular module dependency graph.

In the entry points example above, this would generate:

  • a.js: public module, containing the export of start.
  • b.js: public module, containing the export of start and the call to AppB.main
  • many internal small modules (~50 for this example), approximately one per class.

In the dynamic import example, this would generate:

  • main.js: public module, containing the export of onClick.
  • many internal small modules (~150 for this example), approximately one per class.

Generating many small modules can be useful if the output of Scala.js is further processed by downstream JavaScript bundling tools. In incremental builds, they will not need to reprocess the entire Scala.js-generated .js file, but instead only the small modules that have changed.

SmallModulesFor(packages: List[String])

Create modules that are as small as possible for the classes in the specified packages (and their subpackages). For all other classes, create as few modules as possible. This is a combination of the two other split styles.

The typical usage pattern is to list the application’s packages as argument. This way, often-changing classes receive independent, small modules, while the stable classes coming from libraries are bundled together as much as possible. For example, if your application code lives in my.app, you could configure your module split style as:

import org.scalajs.linker.interface.ModuleSplitStyle
scalaJSLinkerConfig ~= (_.withModuleSplitStyle(ModuleSplitStyle.SmallModulesFor(List("my.app"))))

Splitting Granularity

Scala.js only splits modules along class boundaries. It is important to be aware of this when structuring your application to avoid unnecessary grouping.

For example, the following structure likely leads to poor splitting (if FeatureNs are not always used together):

object UI {
  def renderFeature1(): Unit = ???
  def renderFeature2(): Unit = ???
  def renderFeature3(): Unit = ???

object Calc {
  def calcFeature1(): Unit = ???
  def calcFeature2(): Unit = ???
  def calcFeature3(): Unit = ???

For better splitting, group code that belongs to the same feature:

object Feature1 {
  def render(): Unit = ???
  def calc(): Unit = ???

object Feature2 {
  def render(): Unit = ???
  def calc(): Unit = ???

object Feature3 {
  def render(): Unit = ???
  def calc(): Unit = ???

Linker Output

With module splitting, the set of files created by the linker is not known at invocation time. To support this new requirement, the linker output is configured as follows:

  • A directory where all files go: scalaJSLinkerOutputDirectory
  • Patterns for output file names: outputPatterns on scalaJSLinkerConfig.

Both of these have reasonable defaults and usually do not need to be changed. The exception is file extensions for Node.js, for that, see the next section.

In order to make sense of the files in the directory, the linking tasks (fastLinkJS/fullLinkJS) return a Report listing the public modules and their file names.

ES modules and Node.js

Node.js needs explicit signaling that a module is an ECMAScript module (the default is CommonJS).

There are two ways to achieve this:

  • Use the file extension .mjs.
  • Configure it in package.json.

For details, see the Node.js packages documentation.

To set the extension used by Scala.js to .mjs use the following setting:

import org.scalajs.linker.interface.OutputPatterns

scalaJSLinkerConfig ~= {
  // Enable ECMAScript module output.
  // Use .mjs extension.

Note for Scala.js 1.2.x and earlier:

OutputPatterns was introduced in Scala.js 1.3.0. In earlier versions, the following settings were necessary:

artifactPath in (proj, Compile, fastOptJS) :=
  (crossTarget in (proj, Compile)).value / "myproject.mjs"

artifactPath in (proj, Test, fastOptJS) :=
  (crossTarget in (proj, Test)).value / "myproject-test.mjs"