I can't understand exactly how does SoloLearn app works

Question

I thought that SoloLearn app send program to the server that compile and execute it and then only return the output. But how does web codes work? They are executed on each device (they're not sent to server), right? If yes why can't SoloLearn's server send back compiled java file (for example) so that they are executed on the device? This would allow to see interactive java program and would allow more interesting stuff.

Answer 1

A hint for anyone considering this question:

Android mobiles run the Dalvik Virtual Machine (DVM) or ART.

What sorts of problems might arise when coding for VMs (JVM vs DVM)?
How would you compare this to coding for JS versions?

Answer 2

you (maybe) never will. its source is closed.