innerHTML

Many HTML elements have an innerHTML property, and this property allows you to access the text related to an element and modify it.

Demo :

Click here for my email address!

HTML

<p id="email" onclick="showemail()" style="cursor:hand">Click here for my email address!</p>

Javascript

<script language="javascript">
function showemail(){
document.getElementById('email').innerHTML="<a href='mailto:[email protected]'>[email protected]</a>";
}
</script>