FULL PRODUCT VERSION :
java version "9.0.1"
Java(TM) SE Runtime Environment (build 9.0.1+11)
Java HotSpot(TM) 64-Bit Server VM (build 9.0.1+11, mixed mode)

(The same on various builds of 1.8.)

ADDITIONAL OS VERSION INFORMATION :
Mac OS High Sierra, but tried also on various versions of Windows

A DESCRIPTION OF THE PROBLEM :
When, within Nashorn, a javascript function is invoked that returns a doubly-bool-negated (e.g. !!(x) ) value, the resulting value is false when argument of double negation is an integer not equal to -1, 0, or 1. The code below ("Source code ...") demonstrares this with a value of 2.

(I need to stress that one of the installations of Java on my Mac OS machine works correctly, but I cannot reproduce that correct result anywhere else.)

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Run the main method below, or run the following javascript code within nashorn, possibly using jjs utility:

function boolNegate(x) { return !(x); }
function boolDoublyNegate(x) { return !!(x); }
print(boolNegate(-2));
print(boolDoublyNegate(-2));
print(!!(-2));

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
A printout of:
false
true
true
ACTUAL -
A printout of:
false
false
true

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
    public static void main(String[] args) throws Exception {
        new jdk.nashorn.api.scripting.NashornScriptEngineFactory().getScriptEngine().eval(
                "function boolNegate(x) { return !(x); }\n" +
                        "function boolDoublyNegate(x) { return !!(x); }\n" +
                        "\n" +
                        "print(boolNegate(2));\n" +
                        "print(boolDoublyNegate(2));\n" +
                        "print(!!(2));");
    }
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Replacing !!(...) with Boolean(...) works. That can be, however, nearly impossible when the code is third-party provided, possibly at runtime.