JS is not any better in that regard, that simple example is ignoring nested objects, where the outer one is copied and the inner are referenced (unless you use triple dots all the way down - tedious and error prone, or even simply inefficient if done by some recursive algorithm). The syntax is cool though

I don't understand what you mean by provably. The Java example does the exact same thing as the JS version: create a shallow clone with a different age. Do you mean the builder might do something different?

    // Java
    val emily2 = emily1.toBuilder().age(30).build()

As far as adding the spread operator to Java, I think you'd have to require a Spreadable interface to use it or limit usage of the spread operators to records. It's not clear to me that the expressive power is worth it over a builder.

If a tool analyzed the JS AST, it could conclude (with fairly trivial analysis) that emily2 is a shallow clone of emily1. No such guarantee is possible with the Java version.