actionscript 3 - Using local variables in global scope after exiting function -

i understand as3 works in following way

var str1:string = "global";  function scopetest ()  {      var str1:string = "local";      trace(str1); // local  }  scopetest();  trace(str1); // global 

how can make work this? see last line

var str1:string = "global";  function scopetest ()  {      var str1:string = "local";      trace(str1); // local  }  scopetest();  trace(str1); // local 

var str1:string = "global";  function scopetest ()  {      //use global variable here     str1 = "local";      trace(str1); // local  }  scopetest();  trace(str1); // local 

or use this

var str1:string = "global";  function scopetest ()  {      //use "this" keyword     this.str1 = "local";       var str1:string = "local";     trace(str1); // local  }  scopetest();  trace(str1); // local 

full sample code:

<?xml version="1.0" encoding="utf-8"?> <s:application xmlns:fx="http://ns.adobe.com/mxml/2009"                 xmlns:s="library://ns.adobe.com/flex/spark"                 xmlns:mx="library://ns.adobe.com/flex/mx" minwidth="955" minheight="600"                creationcomplete="init(event)">     <fx:script>         <![cdata[             import mx.events.flexevent;              protected function init(event:flexevent):void             {                 scopetest();                  trace(str1); // local             }              public var str1:string = "global";              public function scopetest():void              {                  //use "this" keyword                 this.str1 = "local";                   var str1:string = "local";                 trace(str1); // local              }            ]]>     </fx:script>      <fx:declarations>         <!-- place non-visual elements (e.g., services, value objects) here -->     </fx:declarations> </s:application>