Hvorfor virker det ikke? Problem med et 'tilføj element/input script'

Kode 

<script language="javascript">
	function $(id){
		return new Blob.Element(id);
	}

	var Blob = {};
	Blob.Element = function(id){
		if(typeof id == 'string'){
			this.dom = document.getElementById('myDiv');
		} else {
			this.dom = id;
		}
	}

	Blob.Element.prototype = {
		createChild : function(config){
			var el = document.createElement(config.tag);
			delete config.tag;

			if(config.html){
				el.innerHTML = config.html;
				delete config.html;
			}

			if(config.cls){
				el.className = config.cls;
				delete config.cls;
			}

			for(var i in config){
				el[i] = config[i];
			}

			this.dom.appendChild(el);

			return $(el);
		}
	}

	function addElement(){
		var el, tr, input;

		el = $('myDiv');

		tr = el.createChild({tag:'tr'});

		input = tr.createChild({tag:'td'}).createChild({tag:'input',name:'stk[]',value:'test'});
		input = tr.createChild({tag:'td'}).createChild({tag:'input',name:'beskrivelse[]',value:'test'});

		$(document.body).createChild({tag:'div',html:'done!'});
	}
</script>
<form method="post">
	<p><a href="javascript:;" onclick="addElement();">Add Some Elements</a></p>
	<table id="myDiv">
		<tr><td><input type="text" name="stk[]"></td><td><input type="text" name="beskrivelse[]"></td></tr>
	</table>
</form>

Kode 

<script language="javascript">
function addElement() {
  var ni = document.getElementById('myDiv');
  
  var objtr = document.createElement('tr');
  ni.appendChild(objtr);
  
  var objtd1 = document.createElement('td');
  objtr.appendChild(objtd1);
  
  var obj1 = document.createElement("input");
  obj1.name = "stk[]";
  obj1.type = "text";
  objtd1.appendChild(obj1);
  
  var objtd2 = document.createElement('td');
  objtr.appendChild(objtd2);
  
  var obj2 = document.createElement("input");
  obj2.name = "beskrivelse[]";
  obj2.type = "text";
  objtd2.appendChild(obj2);

}
</script>
<form method="post">
<p><a href="javascript:;" onclick="addElement();">Add Some Elements</a></p>
<table id="myDiv">
<tr>
<td><input type="text" name="stk[]"></td>
<td><input type="text" name="beskrivelse[]"></td>
</tr>
</table>
</form>

Kode 

<table>
    <tbody id="myDiv">
        <tr><td></td></tr>
    </tbody>
</table>