How can I determine which javascript engine, rhino or nashorn is running my code?

Question

There are several questions how to determine the javascript engine in browser. I have to write javascript code that has to run on rhino and nashorn.

How can I determine if my code is running on rhino or nashorn? Are there typcial functions, variables, constants where you can determine the engine?

Answer 1

Looking at the Rhino to Nashorn migration guide, I see several possible ways.

If you're not using the Rhino compatibility script, this would do it:

var usingNashorn = typeof importClass !== "function";

...since importClass is defined for Rhino but not for Nashorn (unless you include the compatibility script).

I think Java.type is Nashorn-specific, so:

var usingNashorn = typeof Java !== "undefined" && Java && typeof Java.type === "function";

You could check for wrapping of exceptions:

var usingNashorn;
try {
    // Anything that will throw an NPE from the Java layer
    java.lang.System.loadLibrary(null);
} catch (e) {
    // false!
    usingNashorn = e instanceof java.lang.NullPointerException;
}

...since the migration guide says that will be true for Nashorn but false for Rhino. It does involve throwing an exception, which is unfortunate.