Congratulations! From this point, you should start to feel more comfortable with JavaScript, and should soon be solving problems with the language.
If this is the first time you've read through a programming book, you've probably forgotten most of the material. Don't fret: that's normal. Learning to program, like learning any skill, requires repetition and practice; over time, you begin to absorb the material, and that's when the real learning starts.
Don't take a sequential approach when learning to program. You won't master all the material in this book on the first try. Take a circular approach to learning, and you'll have "ah ha" moments the second, third or even fourth time you approach a topic. Use other resources, review another introductory level book, and the second time around, you'll surprise yourself with how much better you understand and remember the material.
Most importantly, don't stop coding and practicing.