Adding CSS class dynamically by using .className property in JavaScript

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In this tutorial, I am going to explain the procedure of adding the CSS class using JavaScript. The need for adding CSS by JavaScript is that sometimes, we need to add style in a dynamic manner, which means that styling should kick in upon the user interaction. Inline style rules of CSS do not fulfill this requirement of dynamic styling.

I will be using the .className property to deploy the dynamic interaction on the web-page. The .className property can be used to retrieve or set a class to the HTML element.

For retrieving the class we can use:

HTMLElementObject.className

While for setting a new class we have to use:

HTMLElementObject.className = class

Code:

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>setProperty() Method using JavaScript </title>
<style>
.main-div
{
 width: 50%;
 margin: 0px auto;
}

 .addCSS {
          color: blue;
          font-size: 3.0rem;
        }

   </style>
 </head>
 <body>
   <div class="main-div">
   <center>
     <h1>
      Setting className property using JavaScript
     </h1>

     <p id = "text-data">
      Hello World!
    </p>

   <button onclick = "addClass()">
      AddClass
   </button>
</center>
</div>
  <script>
     function addClass() {
       var addelement = document.getElementById("text-data");
       addelement.className += "addCSS";
      }

   </script>
 </body>
</html>

Output:

Before Clicking the Button