In this tutorial you can learn how to check and validate radio, and checkbox buttons in JavaScript.
To verify if a radio, or checkbox button is checked, use the checked property.
If the button is checked, the JavaScript checked property returns true, otherwise returns false.

To check /validate checkbox button it's easy because each checkbox input must have an unique name.
Syntax:
if(form_name.checkbox_name.checked == false){
 alert('Checkbox button not checked');
}

It becomes a bit more complicated when it must check radio buttons, because multiple radio buttons has the same name, and JavaScript stores them into an array. So, the syntax:
form_name.radio_name
- Returns an array with all the radio buttons with name='radio_name' in the 'form_name'.
To validate the radio buttons, you must traverse that array, and check each element (each button), like in the code bellow.
var radio_buttons = form_name.radio_name;
var re = false;

// traverse the array with radio buttons, if one is checked, make 're' true and stops
for(var i=0; i<radio_buttons.length; i++){
 if(radio_buttons[i] == true){
 re = true;
 }
}

// if 're' false, means no one radio button checked
if(re == false) alert('Radio button not checked');

Example validating radio and checkbox buttons

Below it's a practical example with a form that contains two radio buttons and three checkboxes. The user must check an radio button (vegetables), and at least one checkbox (fruits), otherwise, when it submits the form, the function checkButons() alerts an error message, and returns False.
<h4>Check a vegetable, and at least one fruit</h4>
<p>- If you try to submit the form without checking the checkbox and radio button, the form is not submitted.</p>
<form action='#' method='post' name='f1' onsubmit='return checkButons(this);'>
Vegetables: <input type='radio' name='vegetable' value='cabbage' />Cabbage
 <input type='radio' name='vegetable' value='carrot' />Carrot<br>
Fruits:<br>
 <input type='checkbox' name='fruit1' value='apple' />Apple<br>
 <input type='checkbox' name='fruit2' value='pear' />Pear<br>
 <input type='checkbox' name='fruit3' value='banana' />Banana<br>
 <input type='submit' value='Submit' />
</form>

<script>
// checks buttons (radio, checkbox) - coursesweb.net
function checkButons(frm) {
 var re = false; // used to determine when a button is checked
 var err = ''; // to store the errors

 var vegetables = frm.vegetable; // contains an array with all radio buttons 'vegetable'

 // create an Array in JSonn format with checkbox buttons
 var fruits = [frm.fruit1, frm.fruit2, frm.fruit3];

 // traverse the radio buttons
 // if one is checked sets re to true, and stops the iteration with 'break'
 for(var i=0; i<vegetables.length; i++) {
 if(vegetables[i].checked == true) {
 re = true;
 break;
 }
 }

 // if 're' is false means no radio button checked, add error in 'err' 
 if(re == false) err += '- You must check at least one vegetable';

 // make 're' again False, and traverse the checkbox buttons
 // if one is checked sets re to true, and stops the iteration with 'break'
 re = false
 for(var i=0; i<fruits.length; i++) {
 if(fruits[i].checked == true) {
 re = true;
 break;
 }
 }

 // if 're' is false means no checkbox button checked, add error in 'err' 
 if (re == false) err += '\n - You must check at least one fruit';

 // if 'err' not empty, alert the error(s) and returns False to stop submitting form
 if(err != '') {
 alert(err);
 return false;
 }
 else return re;
}
</script>

Daily Test with Code Example

HTML
CSS
JavaScript
PHP-MySQL
Which tag defines the clickable areas inside the image map?
<map> <img> <area>
<img src="image.jpg" usemap="#map1">
<map name="map1">
  <area shape="rect" coords="9, 120, 56, 149" href="#">
  <area shape="rect" coords="100, 200, 156, 249" href="#">
</map>
Which CSS property defines what is done if the content in a box is too big for its defined space?
display overflow position
#id {
  overflow: auto;
}
Click on the event which is triggered when the mouse is positioned over an object.
onclick onmouseover onmouseout
document.getElementById("id").onmouseover = function(){
  document.write("Have Good Life");
}
Indicate the PHP variable that contains data added in URL address after the "?" character.
$_SESSION $_GET $_POST
if(isset($_GET["id"])) {
  echo $_GET["id"];
}
Validate radio and checkbox buttons

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