Andreas Rieber reported this:

------------------------
var guiPkgs = { JFrame: function() { print("created"); } };

with (guiPkgs) {
     function main() {
        var frame; // <---- this local variable causes scope to be not set properly

        function createFrame() {
            frame = new JFrame();
        }

        createFrame();
    }
}

main();

>>
scriptpad.js:9 ReferenceError: "JFrame" is not defined

See also JDK-8006755