JavaScript Form Validation:
JavaScript
can be used to validate data in HTML forms before sending off the content to a
server.
Form data
that typically are checked by a JavaScript could be:
has the user
left required fields empty?
has the user
entered a valid e-mail address?
has the user
entered a valid date?
has the user
entered text in a numeric field?
Required Fields
The function
below checks if a field has been left empty. If the field is blank, an alert
box alerts a message, the function returns false, and the form will not be
submitted:
function
validateForm()
{
var
x=document.forms["myForm"]["fname"].value;
if (x==null
|| x=="")
{
alert("First name must be filled
out");
return false;
}
}
The function
above could be called when a form is submitted:
Example
<form
name="myForm" action="demo_form.asp" onsubmit="return
validateForm()" method="post">
First name:
<input type="text" name="fname">
<input
type="submit" value="Submit">
</form>
Output :
First
name:
E-mail Validation:
The function
below checks if the content has the general syntax of an email.
This means
that the input data must contain an @ sign and at least one dot (.). Also, the
@ must not be the first character of the email address, and the last dot must
be present after the @ sign, and minimum 2 characters before the end:
function
validateForm()
{
var
x=document.forms["myForm"]["email"].value;
var
atpos=x.indexOf("@");
var
dotpos=x.lastIndexOf(".");
if
(atpos<1 || dotpos<atpos+2 || dotpos+2>=x.length)
{
alert("Not a valid e-mail
address");
return false;
}
}
The function
above could be called when a form is submitted:
Example
<form
name="myForm" action="demo_form.asp" onsubmit="return
validateForm();" method="post">
Email:
<input type="text" name="email">
<input
type="submit" value="Submit">
</form>
Output :
Email:
