The Scala.js jUnit plugin is a way to overcome the lack of annotation information of any test class (usually accessed through reflection). This is all the information required by the Scala.js testing framework to execute the tests.

As an example we take the following test class:

class Foo {
  @Before def before(): Unit = {
    // Initialize the instance before the tests
  }
  @Test def bar(): Unit = {
    // assert some stuff
  }
  @Ignore("baz not implemented yet") @Test def baz(): Unit = {
    // assert some other stuff
  }
}

object Foo {
  @BeforeClass def beforeClass(): Unit = {
    // Initialize some global state for the tests.
  }
}

Will generate the following bootstrapper module:

object Foo$scalajs$junit$bootstrapper extends org.scalajs.junit.JUnitTestBootstrapper {

  def metadata(): JUnitClassMetadata = {
    new JUnitClassMetadata(
      classAnnotations = List(),
      moduleAnnotations = List(),
      classMethods = List(
          new JUnitMethodMetadata(name = "before",
              annotations = List(new Before)),
          new JUnitMethodMetadata(name = "bar",
              annotations = List(new Test)),
          new JUnitMethodMetadata(name = "baz",
              annotations = List(new Test, new Ignore("baz not implemented yet")))
      ),
      moduleMethods(
          new JUnitMethodMetadata(name = "beforeClass",
              annotations = List(new BeforeClass)))
    )
  }

  def newInstance(): AnyRef = new Foo()

  def invoke(methodName: String): Unit = {
    if (methodName == "0") Foo.beforeClass()
    else throw new NoSuchMethodException(methodId)
  }

  def invoke(instance: AnyRef, methodName: String): Unit = {
    if (methodName == "before") instance.asInstanceOf[Foo].before()
    else if (methodName == "bar") instance.asInstanceOf[Foo].bar()
    else if (methodName == "baz") instance.asInstanceOf[Foo].baz()
    else throw new NoSuchMethodException(methodId)
  }
}

The test framework will identify Foo$scalajs$junit$bootstrapper as a test module because it extends JUnitTestBootstrapper. It will know which methods to run based on the info returned by Foo$scalajs$junit$bootstrapper.metadata, it will create new test instances using Foo$scalajs$junit$bootstrapper.newInstance() and it will invoke test methods using invoke on the bootstrapper.