title: Specify function only for one instance
body: <p>I am a newbie in JavaScript and so don't really understand its object model, but as I understood I have to do something like. </p>
  
  <pre><code>function set_test(text) { this['test'] = text; };
  a = {};
  text = 'ok';
  a.prototype.ok = set_test(text);
  alert(a['test']); #Should be 'ok'
  text = 'fail';
  a.ok;
  alert(a['test']); #Should be 'ok'
  </code></pre>
  
  <p>Can somebody say what's wrong here?  </p>