How to change style/class of an element using JavaScript ?

Last Updated : 01 May, 2023

In this article, we will learn how we can change the style/class of an element. If you want to build a cool website or app then UI plays an important role. We can change, add or remove any CSS property from an HTML element on the occurrence of any event with the help of JavaScript. There are two common approaches that allow us to achieve this task.

style.property
Changing the class itself

Approach 1: Changing CSS with the help of the style property:

Syntax:

document.getElementById("id").style.property = new_style

Example: In this example, we have built a PAN number validator. First, we will take the input value and match it with a regex pattern. If it matches then using JavaScript add an inline style on the <p> tag. Otherwise, add a different style on the <p> tag.

HTML

   <!DOCTYPE html>
<html lang="en">
 
<body>
    <h1 style="color: green;">
        GeeksforGeeks
    </h1>
 
    <h2>
        How can the style/class of
        an element be changed?
    </h2>
 
    <b>Validate Pan Number</b>
 
    <input type="text" id="pan" />
    <p></p>
    <button id="submit">Validate</button>
 
    <script>
        const btn = document.getElementById("submit");
        btn.addEventListener("click", function () {
            const pan = document.getElementById("pan").value;
            const para = document.querySelector("p");
 
            let regex = /([A-Z]){5}([0-9]){4}([A-Z]){1}$/;
            if (regex.test(pan.toUpperCase())) {
                para.innerHTML = "Hurrey It's correct";
 
                // Inline style
                para.style.color = "green";
            } else {
                para.innerHTML = "OOps It's wrong!";
 
                // Inline style
                para.style.color = "red";
            }
        });
    </script>
</body>
 
</html>
 
  

Output:

Approach 2: Changing the class itself – We can use two properties that can be used to manipulate the classes.

The classList Property: The classList is a read-only property that returns the CSS class names of an element as a DOMTokenList object.

Syntax:

document.getElementById("id").classList

You can use the below-mentioned methods to add classes, remove classes, and toggle between different classes respectively.

The add() method: It adds one or more classes.
The remove() method: It removes one or more classes.
The toggle() method: If the class does not exist it adds it and returns true. It removes the class and returns false. The second boolean argument forces the class to be added or removed.

Example: In this example, we are using the above-explained approach.

HTML

   <!DOCTYPE html>
<html lang="en">
 
<head>
    <style>
        .hide {
            display: none;
        }
 
        .blueColor {
            color: blue;
        }
    </style>
</head>
 
<body>
    <h1 style="color: green;">
        GeeksforGeeks
    </h1>
     
    <h2>
        How can the style/class of
        an element be changed?
    </h2>
     
    <h3>Hide and Show the Para</h3>
     
    <p>
        GeeksforGeeks is a computer science portal
        for geeks. This platform has been designed
        for every geek wishing to expand their
        knowledge, share their knowledge, and is
        ready to grab their dream job. GFG have
        millions of articles, live as well
        as online courses, thousands of tutorials
        and much more just for the geek inside you.
    </p>
     
    <button id="hide">Hide</button>
    <button id="show">Show</button>
    <button id="color">Change Color</button>
 
    <script>
        const btn_hide = document.getElementById("hide");
        const btn_show = document.getElementById("show");
        const btn_color = document.getElementById("color");
 
        const para = document.querySelector("p");
        btn_hide.addEventListener("click", function () {
            para.classList.add("hide");
        });
 
        btn_show.addEventListener("click", function () {
            para.classList.remove("hide");
        });
 
        btn_color.addEventListener("click", function () {
            para.classList.toggle("blueColor");
        });
    </script>
</body>
 
</html>
 
  

Output:

In the above example, we define two manipulation classes “hide” and “toggleColor” which hide an element and change color to blue respectively. When the Hide button is clicked, the hide class is added to the “p” tag which hides it. When the Show button is clicked, it removes the present hide class from the “p” tag. When the Change Color button is clicked once it adds “toggleColor” class to the p tag(which makes the text color blue) and when clicked again it removes the toggleColor class.

The className Property: This property is used to set the current class of the element to the specified class.

Syntax:

document.getElementById("id").className = class

Example: In this example, we are using the className property to change the color of the element.

HTML

   <!DOCTYPE html>
<html lang="en">
 
<head>
    <style>
        .colorBlue {
            color: blue;
        }
 
        .colorRed {
            color: red;
        }
    </style>
</head>
 
<body>
    <h1 style="color: green;">
        GeeksforGeeks
    </h1>
     
    <h2>
        How can the style/class of 
        an element be changed?
    </h2>
     
    <h3>className Example</h3>
     
    <p class="colorBlue">
        GeeksforGeeks is a computer science portal
        for geeks.This platform has been designed
        for every geek wishing to expand their
        knowledge, share their knowledge and is
        ready to grab their dream job. GFG have
        millions of articles, live as well
        as online courses, thousands of tutorials
        and much more just for the geek inside you.
    </p>
     
    <button id="submit">Change Color</button>
 
    <script>
        const btn = document.getElementById("submit");
        const para = document.querySelector("p");
 
        btn.addEventListener("click", function () {
            para.className = "colorRed";
        });
    </script>
</body>
 
</html>
 
  

Output:

In the above example, the existing ColorBlue class is set to the colorRed class using the className property which changes font color from blue to red.

How to change font-weight of a text using JavaScript ?

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