This code prints "0" on JVM and "undefined" on js. In this example it's obvious that I'm forcing something the compiler doesn't like (without the lambda, it will correctly complain that foo is not initialized) but in my actual use case it was not obvious and took a lot of time to track down why js behavior was different than jvm. Is there anything the compiler can do to help with this, the way it does with lateinit?

import kotlin.test.Test

class Undefined {
    init {
        println({ -> foo }.invoke())
    }

    val foo = 0
}

class Foo {
    @Test
    fun testUndefined() {
        Undefined()
    }
}

it's basically the same as https://youtrack.jetbrains.com/issue/KT-10573

you're right, my example didn't make that clear.

if you explicitly wrote

val foo: Any = 0

then you'd see

null

on JVM