I have not tried JavaScript, but I have developed my own Java classes that I use for complex response checks.
You do not need to change the scripting language in SoapUI. To call the Java class, I have a groovy step in my test that creates an instance of an object from my java class, and then I call the key method for the object. You can pass the objects that SoapUI passes to the groovy script, so that you can process the response.
The java scripts themselves are located in the bin / scripts folder in SoapUI.
When working on a java class, I use an external editor, for example, brackets. When I save the changes, SoapUI detects this change and recompiles the java class, so yup does not need to restart SoapUI after every setting in your class.
The smart bear site and other places have tutorials to help you get started.
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