<input type=text name=txt>
<SCRIPT LANGUAGE="JavaScript">
var ivalue=prompt("Please input a number","")
if(isNaN(ivalue)){
alert("Request a number!");
}else{txt.value=ivalue}
</SCRIPT>
<SCRIPT LANGUAGE="JavaScript">
var ivalue=prompt("Please input a number","")
if(isNaN(ivalue)){
alert("Request a number!");
}else{txt.value=ivalue}
</SCRIPT>
<input type=text name=txt2>
<input type=button value=selectvalue onclick="selValue()">
</form>
<script>
function selValue()
{
var a=prompt("please input a number","");
if(isNaN(a))
{selValue();}
else
{document.form1.txt2.value=a;}
}
</script>