Will heavily depend on what you are trying to achieve. If it is to develop a web front-end then JS would be a better option since it is more established on the web front-end side, and is quicker/easier to bootstrap projects. On the other hand if it is to develop a full stack web app then Kotlin is a better option due to its explicit nature (eg more errors caught during compile time) unlike JS where magical development is the norm, and the fact that it is easier to develop APIs using a statically typed language (very important for the back-end). Also Kotlin makes it easier to develop modules, and share common code (eg domain models, business logic) via MPP ( Multi Platform Projects -
https://kotlinlang.org/docs/reference/multiplatform.html ) which is a good system to use with a full stack project.