onChange vaciar campo

[goyo_]

Esto me funciono:

Código HTML:

<input type="text"  name="fr1" id="fr1">
<input type="text"  name="fr2" id="fr2">

<script language="javascript" type="text/javascript">
function validaNum(that, mini, maxi) {
	n = parseInt(that.value, 10);
	
	if (n < mini || n > maxi) {
		that.value = '';
		alert("dato incorrecto");
	}
}

document.getElementById('fr1').onchange = function() {validaNum(this, '1', '21')};
document.getElementById('fr2').onchange = function() {validaNum(this, '1', '99999')};
</script>