The website for Stanford University's Computer Science 101 contains lecture notes and interactive JavaScript exercises, and is available for free to the public. The exercises can be completed within the browser.
The technology used to create the in-browser exercises is discussed here.
The course material includes lessons on:
MIT also has open courseware for its Python-based Introduction to Computer Science and Programming available here.
The HTML version of the book Eloquent JavaScript also provides an interactive, Web-based introduction to computer science and JavaScript. You can find more resources for learning JavaScript here.