Is Kotlin's singleton thread-safe?

Someone correct me if I'm wrong. On the Native and JS backend it doesn't matter. On the JVM backend, it is the same as Java.

Java does not provide singleton on language level. what do you saying ?

According documentation, it is. https://kotlinlang.org/docs/reference/object-declarations.html#object-declarations For JVM (according decompiled version of simple object), it is initialized in static block and static class init is guaranteed to be thread-safe AFAIK

By "it doesn't matter", I meant you can't have shared mutable state. So you do not need to think about Thread-safety.