Scala.js crossProject与手动设置

Question

To compile the same source code with Scala.js and Scala JVM the documentation says to use crossProject, for instance

lazy val foo = crossProject.in(file(".")).
  settings(
    name := "foo",
    version := "0.1-SNAPSHOT"
  )

lazy val fooJVM = foo.jvm
lazy val fooJS = foo.js

However it looks that the same goal can be achieved setting up the modules manually

lazy val fooSettings = Seq(
  name := "foo",
  version := "0.1-SNAPSHOT",
  scalaSource in Compile := baseDirectory.value / ".." / "shared" / "src" / "main" / "scala"
)

lazy val fooJVM = project.in(file("jvm"))
  .settings(fooSettings: _*)

lazy val fooJS = project.in(file("js"))
  .settings(fooSettings: _*)
  .enablePlugins(ScalaJSPlugin)

Does crossProject do something important or it's just more convenient way to setup stuff?

Answer 1

It looks like, in your manual setup, you have separate jvm and js subdirectories under your "shared" area -- it doesn't look like they're the same code at all.

CrossProject is about letting you have a single directory structure, with the same code, that is compiled for both sides. Possibly with some small subdirectories that are specific to one side or another, but in general the focus is that most of the code is shared -- the jvm and js subdirectories, if present (it depends on the mode), are usually just shims to adapt the common code to those sides.

All that said, it is really just to make it more convenient -- I believe you could achieve the same results without CrossProject. But in the common case where you're sharing a lot of code, it's significantly more convenient...