If you are programmer in JS then this knowledge should be enough to understand Kotlin features described in the book. We've been writing this book having in mind that also JS or Swift developers who wants to switch to Android and Kotlin might be interested. But I would suggest to start from some Android tutorial, because we are assuming basic knowledge about Android ecosystem. Also most of examples are ways to improve Android development with Kotlin, so this book will be most profitable for developers who understand Android problems.