Code runs with fastOptJS but fails with fullOptJS in Scala.js - scala.js

Question

I'm catching an UndefinedBehaviorError thrown by the bad asInstanceOf in my code like this:
$("button").on(
"click",
".showDialog",
(event: JQueryEventObject) => {
event.preventDefault()
val helpText = "Enter any number"
js.Dynamic.global.swal(
JC(
title = helpText,
text = helpText,
`type` = "input",
showCancelButton = true,
closeOnConfirm = false,
inputPlaceholder = helpText
),
(input: Any) => {
try {
input.asInstanceOf[Boolean]
} catch {
case _: Throwable =>
try {
val correctInput = input.asInstanceOf[String].toDouble
global.console.log("success")
} catch {
case _: Throwable =>
js.Dynamic.global.swal.showInputError("Incorrect data!")
}
}
}
)
}
)
It works fine with fastOptJS but fails with fullOptJS. How to correctly rewrite this code to run it with fullOptJS or what are the other options?

Answer 1

In Scala.js, a bad cast does not reliably throw a ClassCastException, unlike in Scala/JVM. Instead, a bad cast is an undefined behavior, as specified here. This means that in fullOpt, the program can behave arbitrarily, and in particular is allowed not in throw. In fastOpt, however, the compiler is kind and reliably throws, but not a ClassCastException (as it would give a false sense of security), rather an UndefinedBehaviorError.
UndefinedBehaviorErrors must never be caught, as catching one hides a bug which will surface in fullOpt. This is your mistake here: you are catching that UndefinedBehaviorError under case _: Throwable =>. Note that catching Throwable is bad practice in Scala. Instead, you should be using case scala.util.control.NonFatal(_) =>. A NonFatal handler does not catch UndefinedBehaviorError, and would therefore not hide the bug.
Now to solve your issue, you simply not use asInstanceOf as a means to type-test. Even on Scala/JVM, that code would be super bad practice. Use pattern matching instead:
(input: Any) => {
def reportIncorrectData(): Unit =
js.Dynamic.global.swal.showInputError("Incorrect data!")
input match {
case input: Boolean =>
// Here, input is a Boolean
input
case input: String =>
// Here, it is a String; let's try to convert it to Double
try {
val correctInput = input.toDouble
} catch {
case _: NumberFormatException =>
reportIncorrectData()
}
case _ =>
// It is something else, always incorrect data
reportIncorrectData()
}
}