JavaScript Form Validation

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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 was 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="Form_test.html" onsubmit="return validateForm()" method="post">
First name: <input type="text" name="fname">
<input type="submit" value="Submit">
</form>

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="Form_test.html" onsubmit="return validateForm();" method="post">
Email: <input type="text" name="email">
<input type="submit" value="Submit">
</form>

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