"They must also retain the original source as well as the parsed AST representations due to other semantics (toString()), which wastes gobs of memory on memory

"They must also retain the original source as well as the parsed AST representations due to other semantics (toString()), which wastes gobs of memory on memory constrained devices." Wrong. SpiderMonkey doesn't. It retains the bytecode only, throws away the original source, and uses a decompiler when toString() is used. V8 saves the original source, but that's its problem :)