in the expression

Cat && Dog

// => Dog

Because you're using &&, JavaScript is promising you that it will verify that both sides of the expression are true. In this case, Dog is the just the last evaluated thing.

To be more explicit, you could do something like

var c = Cat != null && Dog != null

It's a little bit more wordy, but this boils down to

var c = true && true

console.log(c)

// => true

If you want a simple shortcut for the boolean, use the Boolean constructor -

var c = Boolean(Cat && Dog)

console.log(c)

// => true

If you just use a simple REPL or JavaScript console, you'd be able to see this output very easily.

Per one of the comments below

Using ||, JavaScript is promising you that at least one of the sides is true. Since Cat is true, it stops there and returns Cat. This is known as Short-circuit evaluation