WartRemover warts for Scala.js.

1. Setup WartRemover.

2. Add the following to your plugins.sbt:

   addSbtPlugin("org.danielnixon" % "sbt-scalajswarts" % "0.4.0")

3. Add the following to your build.sbt:

   wartremoverWarnings ++= Seq(
     ScalaJSWart.ArrayPartial,
     ScalaJSWart.UndefOrOpsPartial
   )

scala.scalajs.js.Array[T] has apply, pop and shift methods, all of which can return undefined (even though their return type is T, not UndefOr[T]). This can lead to UndefinedBehaviorErrors.

You can wrap these methods in an implicit that might look something like this:

  @SuppressWarnings(Array("org.danielnixon.scalajswarts.ArrayPartial"))
  implicit class SaferArray[A](val value: scala.scalajs.js.Array[A]) extends AnyVal {
    def applyOpt(index: Int): Option[A] = liftUndefined(value.apply(index))

    def popOpt(): Option[A] = liftUndefined(value.pop())

    def shiftOpt(): Option[A] = liftUndefined(value.shift())

    private def liftUndefined[T <: scala.Any](v: T): Option[T] = {
      if (scalajs.js.isUndefined(v)) None else Some(v)
    }
  }

scala.scalajs.js.UndefOrOps has a get method which will throw if the value is undefined. The program should be refactored to use UndefOrOps#getOrElse or UndefOrOps#fold to explicitly handle both the defined and undefined cases.

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