How ensure that scala.js method parameter is a literal

Question

how can I ensure that a method receive a javascript literal (js.dynamic.literal). It is not a type but it is js.Dynamic so it can be anything. Thanks

Answer 1

There is no way, the Scala.js type system is able to enforce that. It also seems a quite arbitrary restriction to me.

A couple of things come to mind:

Literally what you are asking for:

Use a macro to enforce that the argument to your method is a (JS) literal. This is what you also have to do, if you want to check if its, for example, a String literal.

Note that not every call to js.Dynamic.literal produces a JS object literal at the end. Consider:

val mySeq = Seq("a" -> 1, "b" -> 2)
js.Dynamic.literal(mySeq: _*)

What you might want to be doing:

If you are creating facade types for a library and are creating option objects, we recommend the following pattern:

def myMethod(opts: MyMethodOptions): Unit = js.native

trait MyMethodOptions extends js.Object {
  val debug: Boolean
  val fast: Boolean
}

object MyMethodOptions {
  def apply(debug: Boolean = false, fast: Boolean = false): MyMethodOptions = 
    js.Dynamic.literal(debug = debug, fast = fast).asInstanceOf[MyMethodOptions]
}

Invoke like this:

myMethod(MyMethodOptions(fast = true))

That way the cast to MyMethodOptions is contained in the companion, so the type unsafe part is minimal and contained.