Read the You Don't Know JS series [0] - this tackles much of what people with mediocre proficiency overlook. To go beyond that you would probably want to dive deep in to the ECMAScript specs [1] to really master the language. I don't think it will be that hard, just a slog.
[0] - https://github.com/getify/You-Dont-Know-JS
[1] - https://www.ecma-international.org/publications/standards/Ec...