Scala.js Warts

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WartRemover warts for Scala.js.


Scala.js Warts version WartRemover version Scala.js version Scala version
0.4.0 2.0.1 0.6.14 2.11.8, 2.12.1


  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(



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:

  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.

See also