ScalaJS and MongoDB driver: Referring to non-existent class - mongodb

Question

Learning Scala since yesterday, I've build a project to interact with a MongoDB index (which works fine) and another one using ScalaJS to do the same thing.
Here is my build.sbt :
enablePlugins(ScalaJSPlugin)
name := "demographics"
version := "1.0"
scalaVersion := "2.12.1"
libraryDependencies += "org.mongodb.scala" %% "mongo-scala-driver" % "1.2.1"
libraryDependencies += "be.doeraene" %%% "scalajs-jquery" % "0.9.1"
libraryDependencies += "org.scala-js" %%% "scalajs-dom" % "0.9.1"
skip in packageJSDependencies := false
jsDependencies +=
"org.webjars" % "jquery" % "3.1.1" / "3.1.1/jquery.js"
and my main file:
package demographics.webapp
import org.mongodb.scala._
import scala.scalajs.js.JSApp
object DemographicsApp extends JSApp {
def main(): Unit = {
// connect to the bdd
val mongoClient: MongoClient = MongoClient()
val database: MongoDatabase = mongoClient.getDatabase("demographics")
val collection: MongoCollection[Document] = database.getCollection("demographics")
}
}
Very basic: just connect to the collection.
Sadly, when I launch a "fastOptJS" on the sbt shell, the following error appears:
[info] Compiling 1 Scala source to C:\...\target\scala-2.12\classes...
[info] Fast optimizing C:\...\target\scala-2.12\untitled1-fastopt.js
[error] Referring to non-existent class org.mongodb.scala.bson.DefaultHelper$DefaultsTo$
[error] called from demographics.webapp.DemographicsApp$.main()scala.Unit
[error] called from scala.scalajs.js.JSApp.$$js$exported$meth$main()java.lang.Object
[error] called from demographics.webapp.PrenomApp$.$$js$exported$meth$main()java.lang.Object
[error] called from demographics.webapp.DemographicsApp$.main
[error] exported to JavaScript with #JSExport
[error] involving instantiated classes:
[error] demographics.webapp.PrenomApp$
[error] Referring to non-existent class org.mongodb.scala.MongoClient$
[error] called from demographics.webapp.DemographicsApp$.main()scala.Unit
[error] called from scala.scalajs.js.JSApp.$$js$exported$meth$main()java.lang.Object
[error] called from demographics.webapp.PrenomApp$.$$js$exported$meth$main()java.lang.Object
[error] called from demographics.webapp.DemographicsApp$.main
[error] exported to JavaScript with #JSExport
[error] involving instantiated classes:
[error] demographics.webapp.DemographicsApp$
[error] Referring to non-existent class org.mongodb.scala.bson.collection.immutable.Document
[error] called from demographics.webapp.DemographicsApp$.main()scala.Unit
[error] called from scala.scalajs.js.JSApp.$$js$exported$meth$main()java.lang.Object
[error] called from demographics.webapp.PrenomApp$.$$js$exported$meth$main()java.lang.Object
[error] called from demographics.webapp.DemographicsApp$.main
[error] exported to JavaScript with #JSExport
[error] involving instantiated classes:
[error] demographics.webapp.DemographicsApp$
[error] Referring to non-existent method org.mongodb.scala.bson.DefaultHelper$DefaultsTo$.$default()org.mongodb.scala.bson.DefaultHelper$DefaultsTo
[error] called from demographics.webapp.DemographicsApp$.main()scala.Unit
[error] called from scala.scalajs.js.JSApp.$$js$exported$meth$main()java.lang.Object
[error] called from demographics.webapp.PrenomApp$.$$js$exported$meth$main()java.lang.Object
[error] called from demographics.webapp.DemographicsApp$.main
[error] exported to JavaScript with #JSExport
[error] involving instantiated classes:
[error] demographics.webapp.DemographicsApp$
[error] Referring to non-existent method org.mongodb.scala.MongoClient$.apply()org.mongodb.scala.MongoClient
[error] called from demographics.webapp.DemographicsApp$.main()scala.Unit
[error] called from scala.scalajs.js.JSApp.$$js$exported$meth$main()java.lang.Object
[error] called from demographics.webapp.PrenomApp$.$$js$exported$meth$main()java.lang.Object
[error] called from demographics.webapp.DemographicsApp$.main
[error] exported to JavaScript with #JSExport
[error] involving instantiated classes:
[error] demographics.webapp.DemographicsApp$
[trace] Stack trace suppressed: run last compile:fastOptJS for the full output.
[error] (compile:fastOptJS) There were linking errors
The same code (with adaptations in the main file) is working on the vanilla Scala project, why this error appears?

Answer 1

It appears because you use a JVM-only library. mongo-scala-driver is not published for Scala.js. It it were, you could use a %%% dependency to depend on the Scala.js version of that library, which would work with Scala.js. But you cannot use a JVM-only library in a Scala.js project.
(Conversely, you cannot use a Scala.js-only library in a Scala/JVM project, for example, scalajs-dom.)