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Java 9 now includes ECMAScript 6 support, as claimed by this article . However, it doesn't explain how to run it from Java with ScriptEngine . The linked Java magazine also doesn't explain it. The article says the following:

To activate ES6 support, use --language=es6 on the command line.

This does work with jjs , but I can't find a way how to enable this from Java code. To test it, I used the following code:

import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;
import javax.script.ScriptException;
ScriptEngine engine = new ScriptEngineManager().getEngineByMimeType("application/javascript");
try {
    engine.eval("const a = 20;");
} catch (ScriptException e) {
    e.printStackTrace();
It fails with the following exception:
javax.script.ScriptException: <eval>:1:0 Expected an operand but found const
const a = 20;
^ in <eval> at line number 1 at column number 0
    [STACK TRACE OMITTED]
I've tried to list all available ScriptEngineFactories with this code:
import java.util.List;
import javax.script.ScriptEngineFactory;
import javax.script.ScriptEngineManager;
List<ScriptEngineFactory> factories = new ScriptEngineManager().getEngineFactories();
for (ScriptEngineFactory factory : factories) {
    System.out.println("-----------------------");
    System.out.println(factory.getLanguageName());
    System.out.println(factory.getLanguageVersion());
This outputs just the following:
-----------------------
ECMAScript
ECMA - 262 Edition 5.1
Does this mean I can't run ECMAScript 6 from Java and only using jjs? Or is there something I have missed?
Thanks beforehand.
While browsing through Nashorn questions here, I stumbled upon this question. Its answers describe two ways how to pass Nashorn engine commandline arguments. This answer suggests to use NashornScriptEngineFactory directly, code following:
import javax.script.ScriptEngine;
import javax.script.ScriptException;
import jdk.nashorn.api.scripting.NashornScriptEngineFactory;
ScriptEngine engine = new NashornScriptEngineFactory().getScriptEngine("--language=es6");
try {
    engine.eval("const a = 20;\n"
            + "print(a);");
} catch (ScriptException e) {
    e.printStackTrace();
Even though this works, it's not a nice solution because it makes you use jdk package which is not an officially supported package. Another answer says you can set the arguments with system property nashorn.args. Code:
import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;
import javax.script.ScriptException;
System.setProperty("nashorn.args", "--language=es6");
ScriptEngine engine = new ScriptEngineManager().getEngineByName("Nashorn");
try {
    engine.eval("const a = 20;\n"
            + "print(a);");
} catch (ScriptException e) {
    e.printStackTrace();
This also isn't a nice way of doing it, because it relies on:
Nashorn engine existing which may not be the case on other Java distributions, and
Nashorn engine being up-to-date to support the language command line parameter.
I, personally, prefer the first version, because it will throw a ClassNotFoundException on Java 1.8 since NashornScriptEngineFactory doesn't exist there, while the second version will just silently ignore the setting of property. 
                I still am open to other solutions to this problem, since this is flawed and I can't accept my own answer in 48 hours.
– TheZeus121
                Feb 22, 2018 at 0:08
        
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