Optional syntax

Created by: retorquere

I have a macro

macro iterate {

  // iterate (value: index from items)

  rule { ($($key:ident : $value:ident from $dict:expr)) { $body... } }   => { 
    let dict = $dict;
    let keys = Object.keys(dict);
    let length = keys.length;
    let index = 0;
    while (index < length) {
      let $key = keys[index];
      if (!dict.hasOwnProperty($key)) { continue; }
      let $value = dict[$key];
      $body...
      index++;
    }
    dict = undefined;
    keys = undefined;
  }
}

which allows for

iterate (key:value from {a: 1, b: 2}) {
  ...
}

If I use this twice in a row however, jshint complains that I'm re-declaring the variable, and Firefox (this is code for an extension, so it's chrome-javascript, not web-javascript) complains TypeError: redeclaration of variable key. Is there a way to rewrite this so I could do

iterate (var key: var value from {a: 1, b: 2})
iterate (key: let value from {a: 1, b: 2})

and follow the hints provided by the var/let to declare (or not) key and value using these?