How to validate an HTML form?

Question

I have an HTML form and a button which when the user clicks will check whether or not the form is filled. The details for billing address is input from database using php

<button id="btnConfirm" type="submit" onclick="validate()"style="float: right" value="Confirm Payment" >Confirm Payment</button>
<form id="au" name="paymentform" method="post" action="" >
<fieldset class="billing">
<legend>Billing Details</legend>
<br />
<label class="reglabel" for="fname">First Name:</label>
<input id="fname" name="billingfname" required="" type="text" value="<?php echo $fname ?>" /><br />
<label class="reglabel" for="lname">Last Name:</label>
<input id="lname" name="billinglname" required="" type="text" value="<?php echo $lname ?>" /><br />
<label class="reglabel" for="address">Address:</label>
<input id="address" name="billingaddress" required="" type="text" value="<?php echo $address ?>" /><br />
<label class="reglabel" for="town">Town:</label>
<input id="town" name="billingtown" required="" type="text" value="<?php echo $town ?>" /><br />
<label class="reglabel" for="postcode">Post Code:</label>
<input id="postcode" name="billingpostcode" required="" type="text" value="<?php echo $postcode ?>" /><br />
<label class="reglabel" for="phone">Phone:</label>
<input id="phone" name="billingphone" required="" type="text" value="<?php echo $phone ?>" /><br />
<label id="EmailLabel" class="reglabel" for="email">E-mail:</label>
<input id="email" name="billingemail" required="" type="email" value="<?php echo $email ?>" /><br />
</fieldset> 
<fieldset class="shipping">
<legend>Shipping Details</legend>
<br />
<input name="shippingsame" onclick="fillShipping(this.form)" type="checkbox">Check 
    this box if billing address and shipping address are the same <br />
<label class="reglabel" for="fname">First Name:</label>
<input id="fname" name="shippingfname" placeholder="Required" required="" type="text" /><br />
<label class="reglabel" for="lname">Last Name:</label>
<input id="lname" name="shippinglname" placeholder="Required" required="" type="text" /><br />
<label class="reglabel" for="address">Address:</label>
<input id="address" name="shippingaddress" placeholder="Required" required="" type="text" /><br />
<label class="reglabel" for="town">Town:</label>
<input id="town" name="shippingtown" placeholder="Required" required="" type="text" /><br />
<label class="reglabel" for="postcode">Post Code:</label>
<input id="postcode" name="shippingpostcode" placeholder="Required" required="" type="text" /><br />
<label class="reglabel" for="phone">Phone:</label>
<input id="phone" name="shippingphone" placeholder="Required" required="" type="text" /><br />
<label id="EmailLabel" class="reglabel" for="email">E-mail:</label>
<input id="email" name="shippingemail" placeholder="Required" required="" type="email" /><br />
</fieldset> 
   </form>

This is the code which Im trying to check whether or not he field is empty, but it doesnt seem to work. It alerts undefined

function validate()
{
var firstname = document.getElementsByName("shippingfname").value;
if (firstname == "")
{
    alert('empty'); 
}   
else
{
    alert(firstname);
}
}

Answer 1

the function document.getElementsByName returns an array, so you should try this:

function validate()
{
var firstname = document.getElementsByName("shippingfname")[0].value;
if (firstname == "")
{
    alert('empty'); 
}   
else
{
    alert(firstname);
}
}

Answer 2

Or you can use jquery validation plugin easily. ıt is great for client side validations http://docs.jquery.com/Plugins/validation

Answer 3

getElementsByName() returns an array. Not an individual element. In your example you will want to use the ID attribute to access individual elements.

<input name="shippingsame" id="shippingsame" onclick="fillShipping(this.form)" type="checkbox">

function validate()
{
  var firstname = document.getElementByID("shippingfname").value;
  if (firstname == "")
  {
      alert('empty'); 
  }   
  else
  {
      alert(firstname);
  }
}