Javascript can be used for form validation, to ensure that user entered the correct information.
Check whether user forget to enter information before submit the form:

<script language="javascript">
function checkform()
{
	var val = document.fv.n.value;
	if (val == "") {
		alert("Error, nothing entered!");
		return false;
	}
	return true;
}
</script>
<form name="fv" action="formvalidation.php"
onsubmit="return checkform()" method="post">
	<center>Form Validation Test</center><br>
	<TABLE width="100%"><TBODY><tr>
	<td width=100 style="font-size:17px" align=right>
	Input Text:</TD>
	<td width=4></TD>
	<td width=200><input name=n
	style="width:200px;font-size:22px"></TD>
	<td width=120>
	<input name=submit value="Submit" type=submit
	style="height:28px;width:120">
	</td>
	</tr></tbody></table><br>
</form>

Javascript can also be used to check the information entered by user dynamically. Following example check whether user entered a number or not, if not, delete the last input:

<script language="javascript">
function isNum(args)
{
	args = args.toString();
	if (args.length == 0)
		return false;
	for (var i = 0; i<args.length; i++){
		if ((args.substring(i,i+1) < "0"
		|| args.substring(i, i+1) > "9")){
		return false;
	}
	}
	return true;
}
function ivcheck(i)
{
	var n = document.iv.n.value;
	if (i == "1" && !isNum(n))
	{
		n = n.substring(0,n.length-1);
		document.iv.n.value = n;
		return;
	}
}
</script>
<form name="iv">
	<center>Input Box Validation Test</center><br>
	<TABLE width="100%"><TBODY><tr>
	<td width=120 style="font-size:17px" align=right>
	Input Number:</TD>
	<td width=4></TD>
	<td width=200><input name=n
	style="width:200px;font-size:22px"></TD>
	<td width=100>
	</td>
	</tr></tbody></table><br>
</form>