Is this javascript only at this moment? Looking for something that can handle rails
No, it's any code and even things that aren't code. It will write and revise documentation for you, create jquery, nosql databases, and more. Basically anything a human can do with text files.
If that's not enough for you, projects like Auto-GPT give GPT-4 full autonomy to figure out how to do complex multi-step tasks beyond modifying text all on its own with only a vague goal provided. https://github.com/Torantulino/Auto-GPT