Possible java script question

<div class="body"> Hi I have a C# web application and need to cocantenate a string and display it in a text box.  The contents of the string are the selected values from two drop down boxes, not the text in the drop down box in addition to a string variable.  I am currently doing this with a button click event at the server but was looking for an example (java script?) if possible. Here is what I have for the button click event
string stemp = "Hi " + dropdown1.SelectedValue + dropdown2.SelectedValue + "Bye";
textbox.text = stemp;
so would like to do this without posting back to the server, thanks.
</div> ...