Copy text from one textbox to another through javascript

Here In this script i'm trying to show you how to copy the text from one text box to another with out using any server event. In the image i have hilighted to things one is txtemail and txtusername .i need to copy txtemail to txtusername here i'm using  txtemail.clientid id for to get a unique id for that textbox same as txtusername and i created a function named functiononetexttoother() i'm calling that function on onkeyup event of txtemail. it's working perfectly and i'm saving someone timee too