JavaScript Tutorial/Document/getSelection
document.getSelection()
The getSelection() method returns the text that is selected within the HTML document.
<html>
<form name="Form1">
<input type="text"
name="TextArea">
</form>
<script language="JavaScript">
<!--
document.captureEvents(Event.MOUSEUP);
document.onmouseup = function(event){document.Form1.TextArea.value = document.getSelection()};
-->
</script>
</html>