2) Statically Typed Language
In contrast to JS, Scala.js is statically typed language with type inference and due to its compiler, you can stop worrying about silly type errors. You can not mix up String with Integer.
However there are also other statically typed languages available, which get compiled into JS such as GWT, Dart, TypeScript etc . You can find list here. But beauty of Scala.js makes it preferred language over these languages, which is being described in this blog.
3) Excellent IDE support
Scala.js has excellent IDE support such as IntelliJ and Eclipse, which will help in error highlighting, catching type-error without compiling your code, refactoring field and methods.
4) Easy Refactoring
6) Shared Code
You can use same library, algorithm on both client and server side.
Below is same code written in Scala-
It is best to use === or !==
In Scala.js, you would never face these kind of warts.
There are many other reasons to use Scala.js for front-end development. I am still exploring Scala.js. If you want to dig more about Scala.js, you can have a look at www.scala-js.org.