To my understanding it’s an actual subset of Java, a very limited subset, but a subset of the real Java nonetheless. Java Card applets are compiled using a real Java compiler, and then the bytecode is translated into a simplified format for running on the card.

This video from defcon gives a good overview of what Java Card applets is, the limitations and what it can do: https://www.youtube.com/watch?v=31D94QOo2gY

It doesn't resemble JS. It's a JVM/Java subset. You can even write apps using the Java Servlet API with JC 3.x

That's only on JavaCard 3 connected, though – which is dead: https://stackoverflow.com/questions/9546812/javacard-3-in-re...

But I agree – Java to JavaScript is not the best comparison. Maybe a better one would be C programs written for a Unix system vs. C on a microcontroller: Same language; vastly different instruction set, OS and hardware capabilities and APIs available.

Unlike C on microcontrollers, Java Card applications are hardware and OS agnostic, though!

That's what I said, it's so limited compared to Java that saying SIM cards run "a thing like Java" is massively overstating it. It doesn't even have floats.

It's a UICC not a SIM card anyway.