given a type union of either an object or an array, how can I check (in Kotlin) whether I can cast a...

# javascript

[Adam S]

given a type union of either an object or an array, how can I check (in Kotlin) whether I can cast a property to an array or object?

export type Extension = {extension: Extension} | readonly Extension[]

interface EditorStateConfig {
    extensions?: Extension;
}

[jw]

is Array<*> should work

[Adam S]

thanks! I’ll give it a go.

[Adam S]

I wasn’t sure about the best way because all the JavaScript guides say ‘use `Array.isArray()`’ https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/isArray, but I couldn’t find any usage of that with Kotlin/JS

[Adam S]

@turansky hi 👋 Perhaps Array.isArray() should be added to JsArray, or would that be redundant and equivalent to is Array<*>?

[turansky]

is Array<*> is preferable, equivalent isn’t required

[turansky]

Current plan for JsArray - use it internally only for public extensions like this:

// not implemented now
fun JsIterator<T>.toTypedArray(): ReadonlyArray<T> = 
    JsArray.from(this)

[turansky]

About initial question

export type Extension = {extension: Extension} | readonly Extension[]

{extension: Extension} - looks like fine TS/JS sugar
readonly Extension[]   - looks like what you will use in Kotlin

In Kotlin Wrappers we leave Kotlin signatures only if they used as input values (your case?)

[Adam S]

cool, thanks for the answer. If I think a bit more about it, it makes sense, because JsArray.isArray() isn’t easily able to infer types:

fun foo(arg: Any) {
  if (JsArray.isArray(arg)) {
    arg.size // ERROR: Unresolved reference: size
  }
  if (arg is Array<*>) {
    arg.size // ✓ type is inferred correctly
  }
}

[Adam S]

but maybe adding JsArray.isArray(), but marking it as @Deprecated & with ReplaceWith() would help converting JS/TS code to Kotlin, and help developers to learn Kotlin?

@Deprecated("Using an `is Array<*>` typecheck is preferred", ReplaceWith("value is Array<*>"))
inline fun JsArray.Companion.isArray(value: dynamic): Boolean = value is Array<*>

[Adam S]

In Kotlin Wrappers we leave Kotlin signatures only if they used as input values (your case?)

Hmm I’m not sure I follow. Do you mean that I should only write Kotlin wrappers to handle the readonly Extension[] part of the union, because it’s more general, and the other part {extension: Extension} most likely won’t be used directly?

Ignoring part of the type union certainly would make the code simpler, and help with writing wrappers.

[turansky]

To support {extension: Extension} you will need marker interface, which you probably won’t do 🙂

[Sergei Grishchenko]

is Array<*> is preferable, equivalent isn’t required

I totally disagree with it, JsArray.isArray() is essential check, we just need to support Kotlin contract for it to provide smart cast.

[Sergei Grishchenko]

I checked it Kotlin playground, it is already converts is Array<*> to Array.isArray()