import javafx.scene.Scene;
import javafx.scene.layout.StackPane;
import javafx.scene.web.WebEngine;
import javafx.scene.web.WebView;
import javafx.stage.Stage;
import netscape.javascript.JSObject;

public class CharBindingFailure extends Application {
    public char c = 'm';
    public char doChar(char c) {
        return c;
    }
    @Override
    public void start(Stage stage) throws Exception {
        WebView view = new WebView();
        WebEngine engine = view.getEngine();
        stage.setScene(new Scene(new StackPane(view), 320, 240));
        stage.setOnShown((e) -> {
            JSObject window = (JSObject) engine.executeScript("window;");
            window.setMember("testObject", this);
            window.setMember("testValue", 'a');
            Object resultObject = engine.executeScript("testValue");
            System.out.println(resultObject instanceof Character);
            resultObject = engine.executeScript("testObject.doChar(testValue)");
            System.out.println(resultObject instanceof Character);
            resultObject = engine.executeScript("testObject.doChar('n')");
            System.out.println(resultObject instanceof Character);
            resultObject = engine.executeScript("testObject.c = 'hello world';");
            System.out.println(resultObject instanceof String);
            resultObject = engine.executeScript("testObject.c;");
            System.out.println(resultObject instanceof Character);
        });
        stage.show();
    }
    public static void main(String[] args) {
        launch(args);
    }
}

Above code should print "true" all cases.