Ruby (via Opal) vs ClojureScript

Ruby has a very clean syntax that makes code easier to both read and write than more traditional Object Oriented languages, such as Java. For beginning programmers, this means the focus is on the meaning of the program, where it should be, rather than trying to figure out the meaning of obscure characters.

presidents = ["Ford", "Carter", "Reagan", "Bush1", "Clinton", "Bush2"]
for ss in 0...presidents.length
    print ss, ": ", presidents[presidents.length - ss - 1], "\n";
end

Ruby is one of the most popular languages for developing web sites. As a result, there's an abundant amount of documentation, sample code, and libraries available for learning the language and getting your project up and running. The most popular features are just 'gem install' away.

Figwheel builds your ClojureScript code and hot loads it into the browser as you are coding! Every time you save your ClojureScript source file, the changes are sent to the browser so that you can see the effects of modifying your code in real time.

Lispness makes ClojureScript trivial to comprehend after an initial learning overhead.

Clojure and ClojureScript are designed to be able to interact with their host. So the language by design makes it is easy to use existing JS libraries.

Google's Closure Library converts regular JavaScript into a highly optimized form - including dead code analysis/elimination. It can even remove pieces of unused code from 3rd party libraries (eg, if you import jQuery but only use one function, Google Closure includes only that piece).

Clojure is also able to run web servers, so one can reap similar benefits to NodeJS in terms of sharing code between client and server.

Both Leiningen and Boot are great build tools that manage code dependencies and deployment.

ClojureScript has superb wrappers around React.js (see Reagent) that make building single-page apps a breeze. With figwheel, it's a web dev experience unlike any other -- hotloaded code, repl interaction, and instantly reflected changes make good development fun and fast. You can add things like Garden to make CSS-writing part of the same holistic experience and suddenly all development is a pleasant, smooth process.

From the creator of Clojure:
Spec is a new core library (Clojure 1.9 and Clojurescript) to support data and function specifications in Clojure.
Writing a spec should enable automatic: Validation, Error reporting, Destructuring, Instrumentation, Test-data generation and Generative test generation.