How to change content of a span based on Drop down selection
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I have a select box and a label with a span, I want to replace span content as users change their selection on the drop down box. 

    <select class="text_select" id="field_6" name="field_6">	
    <option value="- Select -">- Select -</option>	
    <option value="Option One">Option One</option>	
    <option value="Option Two">Option Two</option>	
    <option value="Option Three">Option Three</option>	
    </select>

    <label class="form_field">Your selected <span id="aggregator_name"></span>?</label>

I'm using this script

    <script type="text/javascript">
    function notEmpty(){

    var e = document.getElementById("field_6");
    var strUser = e.options[e.selectedIndex].value;
    document.getElementById('aggregator_name').innerHTML = strUser;
		
    }
    notEmpty()
    </script>

Problem with this is, whenever I refresh the page, span is replace with "- Select -" but when I change the dropdown to another option, it doesn't change content of the span? 
Please help guys.