First part is a phone call to get a sense of who you are and what your interests are. Then you'll be asked to complete a programming challenge at home. It's an excellent challenge, it covers a lot of different aspects of javascript development and graphics programming. If you're successful with the challenge then you'll be invited to lunch followed by 2 onsite interviews, one after the other -- for a total of 4 hours.
The first onsite interview will be with 2 engineers and they will probe to get an understanding of the particulars of your challenge and why you took certain approaches. They will ask you to modify your code to achieve different results.
The second interview will be with 2 engineers who will ask high level questions about the way you solve problems. They aren't as concerned with the particulars of your work, they are more interested in patterns. You should be familiar with principles of modularity, dependency injection, unit testing and computational complexity.
Be sure to keep yourself energised through out the 4 hours because it can be exhausting. Have fun with it, and share your passions and interests.