It's java source to javascript source compilation(/translation/transpilation), so you only need to follow the semantics of the language, not the actual implementation.

As a simple example, if you create an array of numbers in Java, you don't need to worry about how the memory for that is allocated as long as you get an array of numbers in javascript.

The difficulties are more in how you get around where the two languages differ widely and making sure you match the semantics (often subtle) of the source language without tanking performance in the target language.

For that example of a numeric array, Java has very different expectations when it comes to bounds checking and exactly what numeric type is in there, for instance, and that has to be enforced by the code you generate if you are going to have any sort of sanity while authoring in the source language.