jsweet | A Java to JavaScript transpiler | Runtime Evironment library

I think you could argue that this is a bug in TypeScript. Not a high-priority one, probably, but... :-)

It's because Object ignores the this it's called with, and returns a new, blank object. The way TypeScript compiles that code (when targeting ES5 environments), it ends up calling Object like this in Example:

Regarding the constructor

You guessed right, there are no multiple constructors like you are used to have in Java (aka. constructor overloading). You only have one constructor in TypeScript and you need to have optional parameters.

In your case, semesterNumber and label could be either undefined or number/string, no need for a type check.

The way to check for "what got passed in" you don't need so many checks in one if-statement. Have a look on the following answer here: https://stackoverflow.com/a/44017547/8745384

Regarding the map

The equivalent of "putIfAbsent" is a simple "set(key, value)"

Let me start with the easy question:

Is there a trick to equate js.Array and Array?

No, there is no such "trick". Those data structures are fundamentally different. js.Array's length is variable, and can be patched with additional properties. Array is Java's fixed-length array. If there was any way to equate them, Scala.js would have done that for you, like it does for Double and number.

One relatively easy way to unify the APIs would be to rebuild the API of JAMA in Scala.js code, where every class is a wrapper for the facaded JS class coming from the JSweet-compiled library. This allows the Scala-level API of JAMA to be exactly equivalent between Scala/JVM and Scala.js. It does require some amount of code writing to replicate the APIs, but at least the body of the methods need not be rewritten.

A completely different approach, very radical and requiring an insane amount of man·hours would be fork the JSweet compiler to emit Scala.js IR (.sjsir) files instead of .js files. That way, you could link the JSweet-generated .sjsir files for JAMA together with the Scala.js-generated .sjsir files for your application. This would give the maximum performance, since the Scala.js optimizer would be able to optimize across the app/library border.

Normally, JSweet just erases unknown annotations, so your code should transpile fine.

First thing to check: do you have a JPA jar in your classpath or in your Maven dependencies? JSweet uses javac, which requires all types to be in the classpath. I guess that the @Column annotation should be in there: https://mvnrepository.com/artifact/javax.persistence/persistence-api/1.0.2

As for the second part of your question, JSweet v2 provides an API to tune the generation of the code. See the specs. In the PrinterAdapter API, you can override the needsImport method to return null when the import is not needed. However I believe that you don't need this for your case since annotations are erased automatically.