Why is it defined that $(-1)!!=1$?

Question

Why is it defined that $(-1)!!$ equal to $1$, where $!!$ is the double factorial?

I've only seen it defined that $(-1)!!=1$, but I don't see why it should be so.

Answer 1

First, the post is low quality. Please write it in a better style.

Also, by definition, $(-1)!!=0!!=1$

This is by definition, as $0!=1$ by definition.

Answer 2

Note that $(n+2)!!=(n+2)\,n!!$. Since $1!!=1$, the logical extension would be $(-1)!!=1$.