HTML forms tutorial

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In principle, someone is working on it and there should be a better version in a not so distant future.
If you want to modify this page, please discuss it with the person working on it (see the "history")

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Introduction

Learning goals
  • Learn function an purpose of HTML forms
  • Learn how to code some kinds of forms
  • Understand how JavaScript may be used to process forms
  • Understand how forms can be processed with PHP
Prerequisites
Concurrent
Moving on
Level and target population
  • Beginners
Remarks
  • For the moment, this article is intended to be a "handout" for "lab" teaching. In other words, a teacher + hands-on activities are needed. In addition, we don't explain how to use a specific editing tool.
  • To do: develop each section and split into 4 different tutorials: forms, forms with JS, forms with PHP, and web applications and CGI principles

HTML Forms are the "basic" HTML elements for client or server-side interactivity. DHTML (i.e. the combination of HTML, DOM, JavaScript and CSS add extra possibilities.

Let's discuss a short example: 

<?xml version = "1.0" encoding="UTF-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
   "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns = "http://www.w3.org/1999/xhtml">
  <head>
    <title>HTML form</title>
    <meta http-equiv="Content-Type" content="text/html;charset=utf-8" />
  </head>

  <body>

    <h1>HTML form elements</h1>

<form method="get" action="#">

<h2>Personal data</h2>

<p>
<strong>Name</strong><br/>
<input type="text" name="name" size="25" />
</p>

<p>
<strong>Firstname</strong><br/>
<input type="text" name="firstname" size="25"/>
</p>

<p>
<strong>I'd like to have some documentation</strong>
<input type="checkbox" name="doc_wanted"/>
</p>

<p>
<strong>Countries I visited</strong>
<select name="country" size=1>
	  <option>CH:  Switzeland</option>
	  <option>D:   Germany</option>
	  <option>F:   France</option>
	  <option>A:   Austria</option>
	  <option>I:   Italy</option>
	</select>
</p>
<p>
<strong>Gender</strong>
<input type="radio" name="gender" value="1"/>Male
<input type="radio" name="gender" value="2"/>Female
</p>

<p>
<strong>Preferred main color</strong>
<input type="radio" name="colorprefs" value="y"/>Yellow
<input type="radio" name="colorprefs" value="b"/>Blue
<input type="radio" name="colorprefs" value="r"/>Red
<input type="radio" name="colorprefs" value="g"/>Green
</p>

<h2>Majoring in</h2>
Vous may choose more than one:
<p>
<input type="checkbox" name="domain1" />Education
<input type="checkbox" name="domain2" />Psychologie
<input type="checkbox" name="domain3" />Computer Science
<input type="checkbox" name="domain4" />International Relations
<input type="checkbox" name="domain5" />Business Administration
<input type="checkbox" name="domain6" />Other
</p>

<h2>Comments</h2>

Tell me anything you want to ... <p>
<textarea name="comments" rows=4 cols=60></textarea>
</p>

<h2>More</h2>
<p>
Input button for JS coding: <input type="button" value="Press me"/>
</p>
<p>
Password field: <input type="password" name="pw"/>
</p>

<p>
<input type="reset" value="Reset the form"/>
<input type="submit" value="Submit"/>
</p>
</form>
<hr/>
</body>
</html>

Live code: 1-demo-html-form-els.html

HTML form example

This example demoes the most important HTML forms elements. HTML forms all must be children of the form</form> element and which has the following syntax: 

 <form name="myform" action="#" method="get">
      
 </form>
  • The action attribute defines what resource will deal with it. "#" means "this page" and is used for JavaScript code
  • the method attribute defines which of the HTTP methods is being used, usually "GET" or "POST" (see below).
  • the name attribute is optional, but recommended for DOM processing with JavaScript.

Inside forms, a designer must enter two kinds of elements (or text)

  • various user input widgets (as shown below)
  • Text that can be embedded in normal HTML elements and that will display information like prompts that the user needs to understand what kind of input he/she should enter
  • structuring elements (e.g. paragaphs, table elements or line breaks)

Input elements

Input elements come in several forms (HTML5 includes additional ones). In this introductory tutorial, we will not introduce the hairy details (there are some), but just provide a few typical examples.

Each button usually should have a name attribute, so that code that will process user input knows which input was used.

Text: a simple text box that allows input of a single line of text. 

Enter something: <input type="text" name="name" size="25" />

checkbox: A check box that can be either on or off. 

Yes I agree: <input type="checkbox" name="want_doc"/>

radio: Radio buttons allow a user to make a choice between one or more alternatives. Preferred main color: 

<input type="radio" name="colorprefs" value="y"/>Yellow
<input type="radio" name="colorprefs" value="b"/>Blue
<input type="radio" name="colorprefs" value="r"/>Red
<input type="radio" name="colorprefs" value="g"/>Green

file: allow the user to select a file for upload 

<input name="uploadfile" type="file">

reset: A reset button allows the user to the the browser to restore the values to their initial values. 

<input type="reset" value="Reset the form"/>

submit: This button that tells the browser to submit the form (typically to send it to a server) 

 <input type="submit" value="Submit"/>

button: This is a button that is used for JavaScript programming. 

 <input type="button" value="Press me"/>

Select element

The select element either displays as a dropt down menu or a menu window from which the user can select one or more alternatives. 

Countries I visited:
<select name="country" size=1>

<option>CH: Switzeland</option> <option>D: Germany</option> <option>F: France</option> <option>A: Austria</option> <option>I: Italy</option> </select>

Text area element

The text area allows users to enter larger text. This element is also used for programming through the web editors.

Example: 

Tell me anything you want to ... 
<textarea name="comments" rows=4 cols=60></textarea>

Server-side forms processing

HTML forms can be processed by a webserver in two ways, either trough the older, easier and still more popular HTTP requests or through the JavaScript XMLHttpRequest object (AJAX)

The principle for both is the same. The client (i.e. your web browser) sends some data to a URL. The webserver will hand over your data to the resource (program) identified by the URL, e.g. a PHP script.

HTTP

The Hypertext Transfer Protocol (HTTP) is a communications protocol used to transfer or convey information on the World Wide Web. (Wikipedia).

In the most simple case, a HTTP roundtrip from browser to server and back can be explained with the following picture:

A simple HTTP request and answer

Messages sent from browser to server and the other way round have two parts

  1. Information that the user will not see
  2. Data

Browser requests consist of the following:

A Request line, such as GET /images/logo.gif HTTP/1.1, which requests the file logo.gif from the /images directory
Headers that will give the server information about your browser, cookies etc.
An empty line
An optional message body (e.g. POST contents from a form)

Here is a sample message sent from the browser to the server and that request that the server sends the contents of the resource identified as http://www.example.com/index. 

 GET /index.html HTTP/1.1
 Host: www.example.com

Server replies include:

A first line that includes a status code (e.g. "200 OK" means "here is what you asked")
Some other information about the context, the server and the most importantly the content type.
A blank line
The data

Here is a sample message sent from the server to the browser in response: 

HTTP/1.1 200 OK
Date: Sun, 06 May 2007 19:13:32 GMT
Server: Apache/2.2.3 (Win32) PHP/5.1.5
X-Powered-By: PHP/5.1.6
Content-Length: 252
Keep-Alive: timeout=15, max=100
Connection: Keep-Alive
Content-Type: text/html

 .... Contents, e.g. HTML code ....

Request methods, server-side scripts and database

HTTP defines eight request methods that a browser may send to a web server. When dealing with forms, developers usually use either the "GET" or the "POST" request.

Forms submission with HTTP
The difference between "GET" and "POST"

GET will request a resource from the server. I.e. this is the method used when you retrieve a normal web page.

  • However, GET requests also may be used to transfer data to a resource (i.e. a script) that will deal with it.
  • In the latter case, the data are transferred in the URL which allows users to bookmark a request. Get should be used with care, since web crawlers or hackers can exploit this. Below is an example of a GET request that sends data.
http://wiki.somewhere.intheworld/mediawiki/index.php?title=Main_Page&action=history

POST is used to send data to a resource.

  • POST will include the form data to be processed in the body of the request, e.g. you can not see it in your browser.
  • POST also allows to send large information (unless GET which is limited)

Modern web servers now usually include a direct method that allows the server to provide the resource with data that comes from the user. The common gateway interface (CGI) which is still used for certain applications, defined how webservers can talk to external programs.

URL encoding

Data that is sent from the browser to the web server is spcially encoded and must be decoded by the server. This has historical reasons, i.e. the Internet only could deal with a limited character set.

According to Wikipedia, the list of reserved characters that must be specially encoded is:

Reserved characters after percent-encoding
! * " ' ( ) ; : @ & = + $ , / ? % # [ ]
%21 %2A %22 %27 %28 %29 %3B %3A %40 %26 %3D %2B %24 %2C %2F %3F %25 %23 %5B %5D

In addition, all non-ASCII characters (e.g. accents) are also encoded.

Use of databases

A request processing script often will "talk" to a database as the following picture shows:

Forms submission with HTTP and databases

Web development

In order to promote modularity and reuse of code, modern web development is ususally based on a three tiers model that we show below.

Three tier web application architecture

In larger web development shops, one usually separates as best three types of work:

  • People that deal with style (CSS)
  • People that deal with contents (usually through a CMS
  • People that deal with client coding (either HTML, HTML forms plus JavaScript or portalware configuration plus templating)
  • People that deal with server-side coding
  • People that deal with database issues

An other trend is represented by the so-called AJAX technique which is a combination of:

XHTML (or HTML) and CSS, for marking up and styling information.
The DOM accessed with a client-side scripting language, usually ECMAScript (JavaScript)
The XMLHttpRequest object is used to exchange data asynchronously with the web server.
XML is sometimes used as the format for transferring data between the server and client.

The principle is the same, but instead of sending a new page to the navigator, it will send data that then is incorporated with "DOM surgery" in the page from which the request originated.

Processing forms with PHP

PHP scripts are put on a web server (like HTML files). However, the server deals with PHP in a very different way:

  • HTML files are just sent to the browser
  • PHP files are executed by the web server and only the result (which usually is some HTML code) is then sent to the browsers.

PHP code can be be small, e.g. sit in a single file. Or it may consists of hundreds of files. Many (or most) popular portalware is coded with PHP, including this wiki.

PHP code is inserted between: <?php ..... ?> 

<?php echo("I am a php instruction\n"); ?>

Example

calculate.html and calculate.php (but these files need to be put on a server)

The HTML form: 

 <form '''action="calculate.php"''' '''method="post"'''>

 What do you know about HTML ?
 <input type="radio" name="choice" value="1" checked>little
 <input type="radio" name="choice" value="2">some
 <input type="radio" name="choice" value="3">everything

 <br>
 What is your programming experience ?
 <input type="radio" name="choice2" value="1" checked>none
 <input type="radio" name="choice2" value="2">some
 <input type="radio" name="choice2" value="3">good
 <P>
 <input type="submit" value="See result!">

 </form>

Contents of the Php file 

  <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
  <html><head><title>Simple test with</title></head><body>
      <h1>Simple test with PHP</h1><hr>
  <?php

  // Get variables from the form
  $choice = $_POST['choice'];
  $choice2 = $_POST['choice2'];

  // Compute the score
  $score = $choice + $choice2;
  // Display the score
  echo "<h3>Your score is " . $score . "</h3>";
  if ($score < 3) {
    echo "<p>You are a beginner</p>";
  } elseif ($score < 5) {
    echo "<p>You have some knowledge</p>";
  } else {
    echo "<p>You are an expert !</p>";
  }
  ?>
  <hr>DKS - 2007
  </body></html>

Playing at home

To play with a webserver on your personal computer, install a WAMP package for Windows or a LAMP package for Unix, or a MAMP package for the Mac.

MAMP means
Windows, the operating system;
Apache, the Web server;
MySQL, the database management system
PHP

Forms processing with JavaScript

Forms processing with JavaScript in your browser is somewhat easier, since you don't need to have access to the webserver) and somewhat more difficult (since JavaScript/DOM programming is a bit more difficult than PHP programming).

Let's examine an simple HTML example that displays two series of radio buttons. Each series is identified by a name attribute. Series one by "question1" and series two by "question2". 

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
          "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" lang="en">
  <head>
    <title>XHTML form example</title>
    <meta http-equiv="Content-Type" content="text/html;charset=iso8859-1" />
    <script type="text/javascript" language="Javascript" src="2-xhtml-radio-buttons.js">
    </script>
  </head>
  
  <body>
    <h1>XHTML form example</h1>

    <p>Below is a simple quiz, please fill it in.</p>

    <form name="quiz" action="#" method="get" onsubmit="give_feedback()">  <br/>
        <p>
        Among those three choices, what do you prefer to do to relax ? <br/>
	Watching TV <input type="radio" name="question1" value="1"/> <br/>

	Playing computer games <input type="radio" name="question1" value="2"/> <br/>
	Reading books <input type="radio" name="question1" value="3"/> <br/>
        </p>
        <p>
	What do you prefer to do on Saturday nights ?  <br/>
	Staying at home <input type="radio" name="question2" value="1"/> <br/>

	Going out alone <input type="radio" name="question2" value="2"/> <br/>
	Inviting friends <input type="radio" name="question2" value="3"/> <br/>
	Going out with friends <input type="radio" name="question2" value="4"/> <br/>
        </p>
	<input type="submit" value="Hit me"/>
    </form>     
    
    <hr/>
  </body>
</html>

Live file: 2-xhtml-radio-buttons.html

  • In the head element, we load JavaScript code that defines a "give_feedback" function.
  • This give_feedback function is triggered when the user presses the submit button because in the form tag we defined the attribute onsubmit="give_feedback().

There a several ways of dealing with HTML forms. Code presented below can be called somewhat old style JavaScript programming, since it includes JavaScript code within the HTML form code. 

 <form name="quiz" action="#" method="get" onsubmit="give_feedback()"> .... </form>

Below is the contents of the 2-xhtml-radio-buttons.js JavaScript file: 

// Made by Daniel K. Schneider, TECFA, March 2008. This is freeware.
// Needs file 2-xhtml-radio-buttons.html

function give_feedback() {
  
  // f is variable that points to the form, just a shortcut
  var f = document.forms["quiz"];
    
  // ---------------- debugging --------------------
  // Let's provide some debugging information to the designer. 
  // Remove this section at some point or add other debugging information to print out
  // Information about the form and its current state can be retrieved by various properties
  info = "This alert provides some debugging info to remove at later stage:\n"
    + "f.elements: " + f.elements + "\n"
    + "f.elements[0]: " + f.elements[0] + "\n"
    + "f.elements[7]: " + f.elements[8] + "\n"
    + "f.length: " + f.length + "\n"
    + "f.onsubmit: " + f.onsubmit + "\n"
    + "f.name: " + f.elements + "\n"
    + "f.acceptCharset: " + f.acceptCharset + "\n"
    + "f.action: " + f.action + "\n"
    + "f.enctype: " + f.enctype + "\n"
    + "f.encoding: " + f.encoding + "\n"
    + "f.method: " + f.method + "\n"
    + "f.target: " + f.target + "\n"
    + "f.elements['question1'][0].checked:" + f.elements['question1'][0].checked + "\n"
    + "f.elements['question1'][1].checked:" + f.elements['question1'][1].checked + "\n"
    + "f.elements['question1'][2].checked:" + f.elements['question1'][2].checked + "\n"
    + "f.elements['question1'][0].value:" + f.elements['question1'][0].value + "\n"
    + "f.elements['question1'][1].value:" + f.elements['question1'][1].value + "\n"
    + "f.elements['question1'][2].value:" + f.elements['question1'][2].value + "\n";
  // show this information
  alert (info);				      
  
  // ---------------- Compute a score --------------------
  // Initially, score is 0
  var score = 0; 

  for (var i=0; i < (f.length); i++)  {
    // if the user selected a radio button it is checked
    // In this case we add its value to the score (see the HTML form)
    if (f.elements[i].checked)  {
      score = score + parseInt(f.elements[i].value);
    }
  }

  // ---------------- Provide feedback --------------------

  // Let's check if the user answered at least one question
  if (!score) {
    alert ("Hey you have to fill in at least something");
    return "lazy user";
  }

  // Now provide feedback
  // Of course, this is a rather dumb test ... as example code tends to be
  // If you plan to reuse this code, you certainly MUST change this
 
  if (score > 6) feedback = "Wow, you are a really active person"
  else if (score > 4) feedback = "You are quite an active person"
  else if (score > 3) feedback = "You are an active person"
  else feedback = "You are not too active";

  alert ("Here are some test results: " + feedback);
}
  • Understanding how to deal with HTML forms is not trivial, i.e. at some point you may have to learn about DOM and be able to understand texts like the Gecko HTML Form Interface documentation.
  • In our code we included a popup window that includes DOM information, i.e. it will tell how what DOM "sees" in the form elements. You may remove the following line or even the whole debugging section.
  alert (info);
  • However, for starters, you just may try to adapt code to your need.

Let's now explain a bit what the give_feedback() function does:

The following line will retrieve a handle on the form from the DOM 

 // f is variable that points to the form, just a shortcut
 var f = document.forms["quiz"];

In other words the f variable now includes an object that represents the current state of the form and allows to retrieve various information

The following code will loop through all form elements, i.e. all radio buttons. For each one it will first check if the the button is "check" (i.e. if the user clicked on it). If so it adds the value to the score variable which is initially set to zero. 

  // ---------------- Compute a score --------------------
  // Initially, score is 0
  var score = 0; 

  for (var i=0; i < (f.length); i++)  {
    // if the user selected a radio button it is checked
    // In this case we add its value to the score (see the HTML form)
    if (f.elements[i].checked)  {
      score = score + parseInt(f.elements[i].value);
    }
  }

Once we computed the total socre, we then first checked if the user answered at least one question. If he didn't then we complain and stop execution. 

  // Let's check if the user answered at least one question
  if (!score) {
    alert ("Hey you have to fill in at least something");
    return "lazy user";
  }

The following code will provide feedback. The variable score holds the total score of the user. We then use a so-called if statement to provide differentiate feedback. You should notice that we start from the highest possible score if (score>6) and then provide feedback for intermediate scores e.g. else if (score > 3), before finally providing a feedback for all other cases. 

  // Now provide feedback
  // Of course, this is a rather dumb test ... as example code tends to be
  // If you plan to reuse this code, you certainly MUST change this
 
  if (score > 6) feedback = "Wow, you are a really active person"
  else if (score > 4) feedback = "You are quite an active person"
  else if (score > 3) feedback = "You are an active person"
  else feedback = "You are not too active";

The syntax of an if statements goes like this: 

if (condition) ..... 
 else if (condition2) .....
 else if (condition3) .....
 // etc.
 else .....

The feedback variable will hold the feedback string, that then is displayed with an alert widget. 

 alert ("Here are some test results: " + feedback);

JavaScript new style

The 3-xhtml-radio-buttons.html example is nearly identical. The only difference is in the forms element. 

<form id="quiz" action="#" method="get">

The JavaScript code available in 3-xhtml-radio-buttons.js is also very similar. The big difference is that we will use some "init" code to link a JavaScript function with a user event. 

// When the page loads the init function will be called
window.onload = init;

// Init code executed when the page loads
function init () {
  document.getElementById("quiz").onsubmit = give_feedback;
}

Dealing with checkboxes

Dealing with checkboxes is a bit more difficult. In the following example we introduce a combined HTML/JS page that shows how to deal with these. 

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" lang="en">
  <head>
    <title>XHTML form example</title>
    <meta http-equiv="Content-Type" content="text/html;charset=iso8859-1" />
    <script type="text/javascript" language="javascript">

    function give_feedback() {
      var f = document.forms["quiz"];
      info = "This alert shows what we can get from the form:\n"
      info += "Let's first look at the " + f.length + " elements of this form.\n";
      for (var i=0; i < (f.length); i++) {
        info += "Object " + i + ": " + f.elements[i] + " ";
        info += " [Name="    + f.elements[i].name  + "] ";
        info += " [Value="   + f.elements[i].value  + "] ";
        info += " [Checked=" + f.elements[i].checked  + "]\n";
      }
      info += "\nLet's see what we get when we get elements by name \n";
      info += "f.elements['color'] returns a list since we got several: "
           + f.elements['color'] + "\n";
      info += "f.elements['color'][0] returns the first element (red): " 
           + f.elements['color'][0] + "\n";
      info += "f.elements['color'][0].value gives the first object's value: "
           + f.elements['color'][0].value + "\n";
      info += "f.elements['color'][0].checked gives the first object's checked: " 
           + f.elements['color'][0].checked + "\n";
      info += "f.elements['red'] gives a single object 3 element : " 
           + f.elements['red'] + "\n";
      info += "f.elements['red'].value gives the object 3 element's value: " 
           + f.elements['red'].value + "\n";
      info += "f.elements['red'].checked gives the object 3 element's checked: " 
           + f.elements['red'].checked + "\n";
     alert (info);
     return false;
     }

    </script>
  </head>
  
  <body>
    <h1>XHTML form example</h1>

    <p>Below is a simple form with checkboxes, please fill it in.</p>

    <form name="quiz" action="#" method="get" onsubmit="give_feedback()"> 
        <p>
      Tick colors you like (take 1)
       <input type="checkbox" name="color" value="red"/>Red
       <input type="checkbox" name="color" value="blue"/>Blue
       <input type="checkbox" name="color" value="green"/>Green
         </p>
        <p>
      Tick colors you like (take 2):
       <input type="checkbox" name="red" />RED
       <input type="checkbox" name="blue" />BLUE
       <input type="checkbox" name="green" />GREEN

        </p>

	<input type="submit" value="Hit me"/>
     Repeat this with different answers and study the alert box. Also watch the URL on top.
    </form>     
    
    <hr/>
  </body>
</html>

Live exemple: 5-xhtml-checkboxes.html

  • In the head, some JavaScript code that defines a "give_feedback" function is loaded
  • Checkboxes are a bit more difficult to handle than radio buttons. Also, unlike radio buttons you can either choose to differentiate between check boxes by either giving them the same name but different value combination or alternatively by giving each checkbox its own name.
  • In the first case you have to retrieve the element by an index, in the second you don't need to.
  • To decide if a user checked a checkbox, you need to look at the the "checked" property.

To understand this code, you have to study both the HTML markup of the input elements and then study the results of the debugging code shown.

Form validation

JavaScript is often used to validate forms, i.e. make sure that user fill in all the elements. You may find many form validation scripts on the Internet, some of which are listed in the JavaScript links entry in this wiki.

Below we present a simple example to validate radio and checkboxes input: 

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" lang="en">
  <head>
    <title>Validation of radio and check boxes input</title>
    <meta http-equiv="Content-Type" content="text/html;charset=iso8859-1" />
    <script type="text/javascript" language="javascript">

// will return false if nothing is checked or the selected value

function get_selected_radio_value (list) {
  var selection = false;
  
  for (i=0;i<list.length;i++) {
    //alert (list[i].checked + "-" + list[i].value);
    if (list[i].checked) {
      selection = list[i].value;
    }
  }
  return selection;
}

function give_feedback() {
  // f is a shortcut to the form
  var f = document.forms["quiz"];
  
  info = "This alert shows what we can get from the form:\n"
    
  info += "Let's first look at the " + f.length + " elements of this form.\n";
  
  // This loop will list all the form elements and add some information about each.
  for (var i=0; i < (f.length); i++) {
    info += "Object " + i + ": " + f.elements[i] + " ";
    info += " [Name="    + f.elements[i].name  + "] ";
    info += " [Value="   + f.elements[i].value  + "] ";
    info += " [Checked=" + f.elements[i].checked  + "]\n";
  }
  info += "\nLet's see what we get when we get elements by name \n";
  info += "f.elements['color'] returns a list of checkbox objects since we got several: " 
    + f.elements['color'] + "\n";
  info += "f.elements['color'].checked returns: " 
    + f.elements['color'].checked + "\n";
  info += "f.elements['color'][0] returns the first element (red): "
    + f.elements['color'][0] + "\n";
  info += "f.elements['color'][0].value gives the first object's value: "
    + f.elements['color'][0].value + "\n";
  info += "f.elements['color'][0].checked gives the first object's checked: " 
    + f.elements['color'][0].checked + "\n\n";
  
  info += "f.elements['metallicred'] returns a check box object: "  
    + f.elements['metallicred'] + "\n";
  info += "f.elements['metallicred'].value gives the object's value: "
    + f.elements['metallicred'].value + "\n";
  info += "f.elements['metallicred'].checked returns the object's checked: "
    + f.elements['metallicred'].checked + "\n\n";
  
  info += "get_selected_radio_value (f.elements['colorpref']) will return: "
    + get_selected_radio_value(f.elements['colorpref'])+ "\n\n"; ;
  
  info += "f.elements['colorselect'].value returns the select box checked value: "
    + f.elements['colorselect'].value + "\n\n";
  alert (info);
  return false;
}

function validate(f) {
  var feedback = "Please fill in all required elements !\n";
  var ok = true;

  if (!(f.elements['color'][0].checked||f.elements['color'][1].checked
        ||f.elements['color'][2].checked)) {
    ok = false;
    feedback+="Please select at least a color from the first color list! \n";
  }
  if (!(f.elements['metallicred'].checked||f.elements['metallicblue'].checked
        ||f.elements['metallicgreen'].checked)) {
    ok = false;
    feedback+="Please select at least a color for the metallic list! \n";
  }
  if (get_selected_radio_value(f.elements['colorpref'])==false){
    ok = false;
    feedback+="Please select a color from the radio buttons !\n";
  }
  if (f.elements['colorselect'].value=="noselect") {
    ok = false;
    feedback+="Please select a color from the pulldown menu !\n";
  }

  if (!ok) alert (feedback);
  // page should not reload
  return false;
}


    </script>
  </head>
  
  <body>
    <h1>Validation of radio and check boxes input</h1>

    <p>Below is a simple form with checkboxes and radio buttons, please fill it in.</p>

    <form name="quiz" action="#" method="get" 
          onsubmit="give_feedback();validate(this);"> 
        <p>
      Tick colors you like (each checkbox has same name)
       <input type="checkbox" name="color" value="red"/>Red
       <input type="checkbox" name="color" value="blue"/>Blue
       <input type="checkbox" name="color" value="green"/>Green
         </p>
        <p>
      Tick again colors you like (each checkbox as a different name):
       <input type="checkbox" name="metallicred" />Metallic red
       <input type="checkbox" name="metallicblue" />Metallic blue
       <input type="checkbox" name="metallicgreen" />Metallic green
        </p>

        <p>
      Select once more a color you like (radio buttons):
       <input type="radio" name="colorpref" value="red"/>Pastel red
       <input type="radio" name="colorpref" value="blue"/>Pastel blue
       <input type="radio" name="colorpref" value="green"/>Pastel green
        </p>
        <p>
      Select a color you like (select menu):
	<select name="colorselect">
	  <option value="noselect">--select--</option>
	  <option value="red">RED</option>

	  <option value="blue">BLUE</option>
	  <option value="green">GREEN</option>
	  <option value="yellow">YELLOW</option>
	</select>
        </p>
      <input type="submit" value="Hit me"/>
    </form>     
  </body>

</html>
Live code: [http://tecfa.unige.ch/guides/js/ex/coap/week-3/6-form-validation.html 6-form-validation.html]

* In the head, some JavaScript code that defines a "give_feedback" function is loaded
* To check if the user selected a radio-button to retrieve a selection value we wrote a little function. <code>function get_selected_radio_value (list)</code> will take a DOM node list representing a series of checkboxes.
<source lang="javascript">
function get_selected_radio_value (list) {
  var selection = false;
  
  for (i=0;i<list.length;i++) {
    //alert (list[i].checked + "-" + list[i].value);
    if (list[i].checked) {
      selection = list[i].value;
    }
  }
  return selection;
}

To test if someone picked a value from a color group we use code like: 

 if (!(f.elements['color'][0].checked||f.elements['color'][1].checked
       ||f.elements['color'][2].checked)) { ....}
or
 (!(f.elements['metallicred'].checked||f.elements['metallicblue'].checked
    ||f.elements['metallicgreen'].checked)) { .... }

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