diff --git a/config/src/bloop/config/Config.scala b/config/src/bloop/config/Config.scala
index 223ac0a..9732a68 100644
--- a/config/src/bloop/config/Config.scala
+++ b/config/src/bloop/config/Config.scala
@@ -170,7 +170,13 @@ object Config {
   object ModuleSplitStyleJS {
     case object FewestModules extends ModuleSplitStyleJS("FewestModules")
     case object SmallestModules extends ModuleSplitStyleJS("SmallestModules")
-    case object SmallModulesFor extends ModuleSplitStyleJS("SmallModulesFor")
+    final case class SmallModulesFor(packages: List[String])
+        extends ModuleSplitStyleJS("SmallModulesFor")
+
+    object SmallModulesFor {
+      val id: String = SmallModulesFor(List.empty).id
+    }
+
     val All: List[String] =
       List(FewestModules.id, SmallestModules.id, SmallModulesFor.id)
   }
@@ -313,7 +319,7 @@ object Config {
     final val LatestVersion = "1.4.0"
     private[bloop] val empty = File(LatestVersion, Project.empty)
 
-    private[bloop] def dummyForTests: File = {
+    private[bloop] def dummyForTests(platformType: String): File = {
       val workingDirectory = PlatformFiles.userDir
       val sourceFile = PlatformFiles.createTempFile("Foo", ".scala")
 
@@ -331,9 +337,10 @@ object Config {
       val classpath = List(scalaLibraryJar)
       val resources = Some(List(PlatformFiles.resolve(outDir, "resource1.xml")))
 
+      val jdk8Path = PlatformFiles.getPath("/usr/lib/jvm/java-8-jdk")
+      val jdk11Path = PlatformFiles.getPath("/usr/lib/jvm/java-11-jdk")
+
       val platform = {
-        val jdk8Path = PlatformFiles.getPath("/usr/lib/jvm/java-8-jdk")
-        val jdk11Path = PlatformFiles.getPath("/usr/lib/jvm/java-11-jdk")
         Platform.Jvm(
           JvmConfig(Some(jdk8Path), Nil),
           Some("module.Main"),
@@ -343,6 +350,23 @@ object Config {
         )
       }
 
+      val platformJS = {
+        Platform.Js(
+          JsConfig(
+            "1.16.0",
+            LinkerMode.Release,
+            ModuleKindJS.ESModule,
+            false,
+            None,
+            None,
+            None,
+            List(jdk8Path),
+            Some(ModuleSplitStyleJS.SmallestModules)
+          ),
+          None
+        )
+      }
+
       val project = Project(
         "dummy-project",
         workingDirectory,
@@ -370,7 +394,11 @@ object Config {
         Some(Java(List("-version"))),
         Some(Sbt("1.1.0", Nil)),
         Some(Test(List(), TestOptions(Nil, Nil))),
-        Some(platform),
+        if (platformType == "JVM") {
+          Some(platform)
+        } else if (platformType == "JS") {
+          Some(platformJS)
+        } else None,
         Some(Resolution(Nil)),
         None,
         None
diff --git a/config/src/bloop/config/ConfigCodecs.scala b/config/src/bloop/config/ConfigCodecs.scala
index 719bbd4..c9552fc 100644
--- a/config/src/bloop/config/ConfigCodecs.scala
+++ b/config/src/bloop/config/ConfigCodecs.scala
@@ -133,43 +133,74 @@ object ConfigCodecs {
 
   implicit val codecModuleSplitStyleJS
       : JsonValueCodec[Config.ModuleSplitStyleJS] = {
+    implicit val codecList: JsonValueCodec[List[String]] =
+      JsonCodecMaker.makeWithRequiredCollectionFields[List[String]]
     new JsonValueCodec[Config.ModuleSplitStyleJS] {
       val nullValue: Config.ModuleSplitStyleJS =
         null.asInstanceOf[Config.ModuleSplitStyleJS]
       def encodeValue(x: Config.ModuleSplitStyleJS, out: JsonWriter): Unit = {
-        val str = x match {
+        out.writeObjectStart()
+        out.writeKey("splitStyle")
+        x match {
           case Config.ModuleSplitStyleJS.FewestModules =>
-            Config.ModuleSplitStyleJS.FewestModules.id
+            out.writeVal(Config.ModuleSplitStyleJS.FewestModules.id)
           case Config.ModuleSplitStyleJS.SmallestModules =>
-            Config.ModuleSplitStyleJS.SmallestModules.id
-          case Config.ModuleSplitStyleJS.SmallModulesFor =>
-            Config.ModuleSplitStyleJS.SmallModulesFor.id
+            out.writeVal(Config.ModuleSplitStyleJS.SmallestModules.id)
+          case Config.ModuleSplitStyleJS.SmallModulesFor(packages) =>
+            val style = Config.ModuleSplitStyleJS.SmallModulesFor(packages)
+            out.writeVal(style.id)
+            out.writeKey("packages")
+            out.writeArrayStart()
+            style.packages.foreach(out.writeVal)
+            out.writeArrayEnd()
         }
-        out.writeVal(str)
+        out.writeObjectEnd()
       }
+
       def decodeValue(
           in: JsonReader,
           default: Config.ModuleSplitStyleJS
-      ): Config.ModuleSplitStyleJS =
-        if (in.isNextToken('"')) {
-          in.rollbackToken()
-          in.readString(null) match {
-            case Config.ModuleSplitStyleJS.FewestModules.id =>
-              Config.ModuleSplitStyleJS.FewestModules
-            case Config.ModuleSplitStyleJS.SmallestModules.id =>
-              Config.ModuleSplitStyleJS.SmallestModules
-            case Config.ModuleSplitStyleJS.SmallModulesFor.id =>
-              Config.ModuleSplitStyleJS.SmallModulesFor
-            case _ =>
-              in.decodeError(
-                s"Expected module split style ${Config.ModuleSplitStyleJS.All
-                    .mkString("'", "', '", "'")}"
-              )
-          }
-        } else {
-          in.rollbackToken()
-          nullValue
+      ): Config.ModuleSplitStyleJS = {
+        in.setMark()
+        in.isNextToken('{')
+        in.skipToKey("splitStyle")
+        val splitStyle = in.readString(null)
+
+        val packages =
+          if (
+            splitStyle == Config.ModuleSplitStyleJS
+              .SmallModulesFor(List.empty)
+              .id
+          ) {
+            in.nextToken()
+            if (in.skipToKey("packages")) {
+              if (in.isNextToken('[')) {
+                in.rollbackToken()
+                codecList.decodeValue(in, Nil) match {
+                  case Nil =>
+                    in.decodeError("Field packages can not contain empty array")
+                  case packages => packages
+                }
+              } else {
+                in.decodeError("missing array of packages for field package")
+              }
+            } else {
+              in.requiredFieldError("Missing required field packages ")
+            }
+          } else List.empty
+
+        in.isNextToken('}')
+
+        splitStyle match {
+          case Config.ModuleSplitStyleJS.FewestModules.id =>
+            Config.ModuleSplitStyleJS.FewestModules
+          case Config.ModuleSplitStyleJS.SmallestModules.id =>
+            Config.ModuleSplitStyleJS.SmallestModules
+          case Config.ModuleSplitStyleJS.SmallModulesFor.id =>
+            Config.ModuleSplitStyleJS.SmallModulesFor(packages)
+          case _ => in.decodeError("Invalid splitStyle field")
         }
+      }
     }
   }
 
diff --git a/config/test/src/bloop/config/ConfigCodecsSpec.scala b/config/test/src/bloop/config/ConfigCodecsSpec.scala
index 8d957ef..182e801 100644
--- a/config/test/src/bloop/config/ConfigCodecsSpec.scala
+++ b/config/test/src/bloop/config/ConfigCodecsSpec.scala
@@ -3,14 +3,19 @@ package bloop.config
 import java.nio.charset.StandardCharsets
 
 import bloop.config.Config.File
+import com.github.plokhotnyuk.jsoniter_scala.{core => jsoniter}
 
 class ConfigCodecsSpec extends munit.FunSuite {
   import ConfigCodecsSpec._
 
   override def afterAll() = {
     val filesToDelete =
-      dummyFile.project.classpath ++ dummyFile.project.sources :+ dummyFile.project.classesDir :+ dummyFile.project.out
+      dummyFileJVM.project.classpath ++ dummyFileJVM.project.sources :+ dummyFileJVM.project.classesDir :+ dummyFileJVM.project.out
+
+    val jsFilesToDelete =
+      dummyFileJS.project.classpath ++ dummyFileJS.project.sources :+ dummyFileJS.project.classesDir :+ dummyFileJS.project.out
     filesToDelete.foreach(PlatformFiles.deleteTempFile)
+    jsFilesToDelete.foreach(PlatformFiles.deleteTempFile)
 
   }
 
@@ -37,8 +42,62 @@ class ConfigCodecsSpec extends munit.FunSuite {
 
   }
 
-  test("simple-config-jsons") {
-    parseFile(dummyFile)
+  test("simple-config-jsons-jvm") {
+    parseFile(dummyFileJVM)
+  }
+
+  test("simple-config-jsonsjs") {
+    parseFile(dummyFileJS)
+  }
+
+  test("write-module-split-style-smallestmodules") {
+
+    val config: Config.ModuleSplitStyleJS =
+      Config.ModuleSplitStyleJS.SmallestModules
+    val written = jsoniter.writeToString(config)(
+      implicitly(bloop.config.ConfigCodecs.codecModuleSplitStyleJS)
+    )
+    assertEquals(written, """{"splitStyle":"SmallestModules"}""")
+  }
+
+  test("read-module-split-style-smallestmodules") {
+
+    val config: Config.ModuleSplitStyleJS =
+      Config.ModuleSplitStyleJS.SmallestModules
+    val read = jsoniter.readFromString[Config.ModuleSplitStyleJS](
+      """{"splitStyle":"SmallestModules"}"""
+    )(
+      implicitly(bloop.config.ConfigCodecs.codecModuleSplitStyleJS)
+    )
+    assertEquals(read, config)
+  }
+
+  test("write-module-split-style-smallmodulesfor") {
+
+    val config: Config.ModuleSplitStyleJS =
+      Config.ModuleSplitStyleJS.SmallModulesFor(List("one", "two"))
+    val written = jsoniter.writeToString(config)(
+      implicitly(bloop.config.ConfigCodecs.codecModuleSplitStyleJS)
+    )
+    assertEquals(
+      written,
+      """{"splitStyle":"SmallModulesFor","packages":["one","two"]}"""
+    )
+  }
+
+  test("read-module-split-style-smallmodulesfor") {
+
+    val config: Config.ModuleSplitStyleJS =
+      Config.ModuleSplitStyleJS.SmallModulesFor(List("one", "two"))
+    val read = jsoniter.readFromString[Config.ModuleSplitStyleJS](
+      """{"splitStyle":"SmallModulesFor","packages":["one","two"]}"""
+    )(
+      implicitly(bloop.config.ConfigCodecs.codecModuleSplitStyleJS)
+    )
+    assertEquals(
+      read,
+      config
+    )
   }
 
   test("test-idea") {
@@ -77,5 +136,6 @@ class ConfigCodecsSpec extends munit.FunSuite {
 }
 
 object ConfigCodecsSpec {
-  val dummyFile = File.dummyForTests
+  val dummyFileJVM = File.dummyForTests("JVM")
+  val dummyFileJS = File.dummyForTests("JS")
 }