He's talking about recursion - a function calling itself. I find it's funny that factorials are understood easily in high school, but then recursion is mind blowing in front of a computer. Just like factorials, it can be understood best by walking through it with paper and pencil.
The examples in Eloquent Javascript need to be studied by the reader to get the most out of that book. I think people just glance at them with out typing them up and running them.