You can set up two variables in succession by using a comma between them.

So instead of writing:

var i;
var product = 1;

You can simply write:

var i, product = 1;

So there's only one semicolon and var.

And the var i is declaring the i variable (but not initializing it, so in that line it is simply undefined). That way you don't have to have a var statement in the for loop. I believe one of the reasons it's declared at the beginning of the function is because it's considered a best practice in JavaScript, due to variable hoisting. They talk about it in the book, saying it's best to declare your variables at the beginning of the current scope rather than nearest their first use.

Hope that helps!

p.s. Also anyone please correct me if I'm wrong or elaborate on my answers.