Both Scala and JavaScript feature exception handling, but Scala only allowsinstances of Throwable to be thrown/caught (which is enforced by the typechecker). JavaScript can throw/catch any type of value.

To reconcile the two worlds, Scala.js lifts all exceptions that are notinstances of Throwable inside js.JavaScriptException. This lifting works bothways.

Throwing an exception from Scala.js:

• Scala.js throws an instance of js.JavaScriptException -> JS catches the wrapped value
• Scala.js throws an instance of Throwable -> JS catches it as is

Throwing an exception from JS:

• JS throws an instance of Scala.js’ Throwable -> Scala.js catches it as is
• JS throws something else -> Scala.js catches it wrapped in a js.JavaScriptException