title: Get the identifier
body: <p>At the starting I must tell that I am a newbie and sorry for a such a silly question
  Second that <code>.extract</code> is just an example.</p>
  
  <p>Here is the code</p>
  
  <pre><code>String.prototype.extract=function(start,end){
       var value=this;
       var rval="";
       for(var i=start; i&lt;=end; i++){
            rval+=value.charAt(i)
       }
       ????? = rval;
  }
  
  var str="abcdef";
  str.extract(1,4)
  alert(str) //must be bcde
  </code></pre>
  
  <p>Now I want the caller of method extract (str) and the value of rval must be saved to that only.</p>
  
  <p>I could do it by: 
  <code>str = str.extract;</code>
  and in the function <code>return rval</code></p>
  
  <p>but I want that like we do it as <code>something.toUpperCase</code> instead of <code>something=something.toUpperCase</code></p>
  
  <p>????? must be replaced by the caller of method extract (str)</p>
  
  <p>any help would be useful</p>