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JavaScript can be used to validate input data
in HTML forms before sending off the content to a server.
JavaScript Form Validation
JavaScript can be used to validate input 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 required field has been left
empty. If the required field is blank, an alert box alerts a
message and the function returns false. If a value is entered,
the function returns true (means that data is OK):
function validate_required(field,alerttxt)
{
with (field)
{
if (value==null||value=="")
{alert(alerttxt);return false}
else {return true}
}
}
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The entire script, with the HTML form could look something
like this:
<html>
<head>
<script type="text/javascript">
function validate_required(field,alerttxt)
{
with (field)
{
if (value==null||value=="")
{alert(alerttxt);return false}
else {return true}
}
}
function validate_form(thisform)
{
with (thisform)
{
if (validate_required(email,"Email must be filled out!")==false)
{email.focus();return false}
}
}
</script>
</head>
<body>
<form action="submitpage.htm"
onsubmit="return validate_form(this)"
method="post">
Email: <input type="text" name="email" size="30">
<input type="submit" value="Submit">
</form>
</body>
</html>
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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 at least an @
sign and a dot (.). Also, the @ must not be the first character
of the email address, and the last dot must at least be one
character after the @ sign:
function validate_email(field,alerttxt)
{
with (field)
{
apos=value.indexOf("@")
dotpos=value.lastIndexOf(".")
if (apos<1||dotpos-apos<2)
{alert(alerttxt);return false}
else {return true}
}
}
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The entire script, with the HTML form could look something
like this:
<html>
<head>
<script type="text/javascript">
function validate_email(field,alerttxt)
{
with (field)
{
apos=value.indexOf("@")
dotpos=value.lastIndexOf(".")
if (apos<1||dotpos-apos<2)
{alert(alerttxt);return false}
else {return true}
}
}
function validate_form(thisform)
{
with (thisform)
{
if (validate_email(email,"Not a valid e-mail address!")==false)
{email.focus();return false}
}
}
</script>
</head>
<body>
<form action="submitpage.htm"
onsubmit="return validate_form(this);"
method="post">
Email: <input type="text" name="email" size="30">
<input type="submit" value="Submit">
</form>
</body>
</html>
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