How to update an input value with the value from a selected option using Prototype

Today, I needed to set the value of an HTML input element based on the value of a option in a select element. This is fairly easy to do with Prototype's writeAttribute. Here's an example:

A simple javascript function to do the work:

function populate_input(){  
    var field = $('tf\_select').getValue();   
    $('tf').writeAttribute('value', field);  
}  

A simple HTML snippet to see it in action:

<div>  
<p><select id="tf\_select" name="tf\_select" onchange="populate\_input();">  
<option value="">- choose one -</option>  
<option value="v1">value 1</option>  
<option value="v2">value 2</option>  
</select><br/><input type="test" id="tf" name="tf" value=""/></p>  
</div>  

And yes... I know the title of this post is almost longer than the post itself!