area for clarification - Destructuring

[robin-raymond]

https://github.com/jchadwick/EssentialTypeScriptBook/blob/master/manuscript/02.ECMAScript2015.md

function countdown({ initial, final: final = 0, interval: interval = 1, initial: current }) {

    while(current > final) {
        console.log(current);
        current -= interval;
    }

}

I totally understand that this is a low priority feature but as a long time programmer but newbie to ts, this confused me. The only thing I've seen thus far in the book is <varname> : <typename>, e.g. foo : string so in this syntax it looks like final is type final and initial is type current, but clearly that's not what's going on here. Everything else in the book has been clear but this point lost me. I would recommend tweaking this part of the book.

BTW, I'm happily enjoying the book thus far. I'll rate it on Amazon once I get further along as I'm only in the ts baby steps right now :)