How can Any script have to be compiled into intermediate code

import javax.script.Invocable;
import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;

public class InvocableDemo {

public static void main(String[] args) throws Exception {
ScriptEngineManager manager = new ScriptEngineManager();

ScriptEngine engine = manager.getEngineByName("js");

engine.eval("function add (a, b) {c = a + b; return c; }");
Invocable jsInvoke = (Invocable) engine;

Object result1 = jsInvoke.invokeFunction("add", new Object[] { 10, 5 });
System.out.println(result1);

Adder adder = jsInvoke.getInterface(Adder.class);
int result2 = adder.add(10, 5);
System.out.println(result2);
}
}

interface Adder {
int add(int a, int b);
}