void operator should always evaluate to undefined.

See ECMA 11.4.2 The void Operator


jjs> void +this
>> NaN
>> => undefined instead of NaN

jjs> (function f(){return void +(void 0)})()
>> NaN
>> => undefined instead of NaN

jjs> void function(){}
>> Exception in thread "main" java.lang.AssertionError
>> at jdk.nashorn.internal.codegen.Attr.enterFunctionBody(Attr.java:276)
>> at jdk.nashorn.internal.codegen.Attr.enterBlock(Attr.java:297)
>> ...