I would suggest you try to go through following:
1. Book -Javascripts: the good Parts by douglas Crockford. Make sure you understand the points he raises in the book and try to implement them accordingly.
2. Do not just try to javascript functions for website validations; as static functions in javascript is the worst thing a developer can do.
3. Try to implements OOPs concepts in Javascript and see how using functions you can model your objects and their behavior.
4. I would highly recomment small design patterns problems to be solved in javascript, as that would bring out the understanding of functions and invocation patterns in javascript.
Hope it helps.
Focus is objects oriented nature of javascript and not just the syntax
