Java (via GWT) vs Scala.js

That's Java, so it is strongly typed, and we can use fully the IDE: error detection, refactoring, auto-completion, etc.

Perfect if you have a strong background in Java programming, and need to code for the client side.

Because Java is statically typed, integrated development environments (IDEs) for Java can provide a lot more feedback on errors you will encounter. Java IDEs can give you specific errors in the location where they occur without having to run the code every time. This makes is faster to debug and learn from your mistakes.

IDEs also have extensive auto complete capabilities that can help you learn the programming libraries you are using faster and tell you what functions are available.

Java is one of the most popular languages in industry, consistently ranking either first, or occasionally second (behind C or Javascript) in terms of usage. Polls (see sources below) show it to be consistently in high demand, particularly as measured by job board postings. This makes Java a great time investment, as you will be easily able to get a job utilizing your skills, particularly as those Java applications in production now will continue to need maintenance in the future. It also results in great support for tools and plenty of computer science books, example projects and online tutorials.

Most Java code follows very standardized coding styles. This means that when you're starting out, there are fewer questions about how you should implement something as the programming styles and patterns are well established and consistent. This consistent style means that it's often easier to follow others' example code, and that it's more likely to meet at least a certain minimum standard of quality. This discipline with consistent stylistic standards also becomes useful later, when collaborating on projects with larger teams.

Produced JS is highly optimized, can be separated in modules with lazy loading, can be internationalized while loading only the chosen language.

Scala offers type inference, which, while giving the same safety as Java's type system, allows programmers to focus on the code itself, rather than on updating type annotations.

In contrast to other options, Scala.js is a compiler plugin for an already existing and mature language: Scala.
As such, it benefits automatically from the existing compiler, from the language design choices made for Scala, which exists and is established in the industry since years.

The JavaScript-interop of Scala.js is very direct and lightweight.

The strength of Scala (JVM) on the server can not be underestimated, and is probably superior to any other choice listed here (where applicable; many have no server-side equivalent).

Being able to use such a powerful language (and ecosystem) on the server AND on the client, and sharing code between the two, is a big advantage.

Scala.js implements most of the Scala standard library and many parts of the Java one. Among others, it supports Scala's rich collection library.

Scala.js performs dead-code elimination out of the box (when running in the "fullOpts" mode).

Scala supports both Functional and Object Oriented styles of programming. Beginners can learn both paradigms without having to learn a new language, and experts can switch between the two according to what best suits their needs at the time.

Keeps your client and server sources in sync.

Through SBT, Scala.js supports incremental compilation out of the box. That is: SBT automatically picks-up the changes (think "watch" in other tools) and only recompiles what is needed.

This makes the development cycle fast and very pleasant to work with.

The same good and mature tooling that can be used for Scala can be used for Scala.js out of the box (code-completion, refactoring, immediate feedback, etc.).

The immutable values make it perfect for working with concurrency.

Because Scala.js is a plugin to the Scala compiler, the whole power of the Scala language is available at compile-time. Which includes macros. Very expressive things can be done, in a type-safe way, which are difficult or impossible in other languages.

While SBT is arguably not as simple as Maven, it beats the chaos of the JavaScript build ecosystem.