Is there any way to get kotlin property's default value?

I'm not sure if Kotlin does defaults.

i mean

data class Test(val x:String="I'm default")

Ah, this is reflection territory. I'm not sure how but you can get it via reflection.

I need it in JS to handle undefined states...

Reflection is JS is not possible generally

The problem is that default values are actually full pieces of code, for example:

fun foo(x: List<Int> = listOf(1, 2, 3)) {}

You can't really get a default value from that, it's an expression that needs to be evaluated every time.

Even worse, you can have this:

fun foo(x: Int, y: Int = x+1)

That's good to know actually, and yes that's a lot worse!