That is using map, not implementing map.

Implementing map:

   function map(f, ary) {
      var ret = [];
      for(var i = 0; i < ary.length; i+= 1) {
         ret.push(f(ary[i]));
      }
      return ret;
   }

Though that is not a good example by the op, because that is what you do:

https://github.com/clojure/clojure/blob/master/src/clj/cloju...

I would do it like this...

    function map(f, ary) {
        return [].map.call(ary, f);
    }

That's _using_ map. Read an implementation of map to see what I mean.