My Data-Only Object Notation Converter

Here's my home-rolled method for serializing a data-only object to PON (Perl Object Notation) or JSON. "Data-Only" means that the object only contains scalars, hashes, and arrays -- no classes.

   // to output JSON, pass in a pair of ":" instead of "=>".
   private function encodePON( obj:Object, pair:String="=>" ):String
   {
    if ( obj is String ) return "'" + obj + "'";
    if ( obj is Number ) return obj.toString();
    
    var out:String = "";
    var ary:Array;
    
    if ( obj is Array )
    {
     out += "[";
     ary = new Array();
     for each (var item:Object in obj as Array)
      ary.push( encodePON( item ) );
     out += ary.join( ", " );
     out += "]";
    }  
    else if ( obj is Object )
    {
     out += "{";
     ary = new Array();
     for (var key:String in obj)
      ary.push( "'" + key + "' " + pair + " " + encodePON( obj[key] ) );
     out += ary.join( ", " );
     out += "}";
    }
    
    return out;
   }