How to detect when the window size is resized using JavaScript ?

Last Updated : 13 Aug, 2024

Sometimes when we develop our site, we want to detect the size of the window, In this article, we are going to learn how to detect when the window size is resized using JavaScript

The window resize event occurs whenever the size of the browser window gets changed. We can listen to the resize event in two ways:

Table of Content

Using resize event
Using ResizeObserver API
Using addEventListener() Method

Method 1: Using resize event

We can add an event listener to the body element which fires whenever the window size is resized.

Example: This example shows the use of the above-explained approach.

<!DOCTYPE html>
<html>

<head>
    <title>
        How to detect and resize
        the window size
    </title>
    
    <style>
        body {
            text-align: center;
        }
        h1 {
            color:green;
        }
    </style>
</head>

<body>
    <h1>GeeksforGeeks</h1>
    
    <h3>
        Resizing the screen size and display its
        <br>corresponding width and height
    </h3>
    <div id="height">Height: <span></span></div>
    <div id="width">Width: <span></span></div>
    
    <script>
        const height = document.querySelector("#height span");
        const width = document.querySelector("#width span");
        
        // Insert values on load of page
        window.onload = function() {
            height.innerHTML = window.innerHeight;
            width.innerHTML = window.innerWidth;
        };
        
        // Change values when window is resized
        window.onresize = function() {
        
            // Setting the current height & width
            // to the elements
            height.innerHTML = window.innerHeight;
            width.innerHTML = window.innerWidth;
        };
    </script>
</body>
</html>

 
<!DOCTYPE html>
<html>​<head>    <title>        How to detect and resize        the window size    </title>        <style>        body {            text-align: center;        }        h1 {            color:green;        }    </style></head>​<body>    <h1>GeeksforGeeks</h1>        <h3>        Resizing the screen size and display its        <br>corresponding width and height    </h3>    <div id="height">Height: <span></span></div>    <div id="width">Width: <span></span></div>        <script>        const height = document.querySelector("#height span");        const width = document.querySelector("#width span");                // Insert values on load of page        window.onload = function() {            height.innerHTML = window.innerHeight;            width.innerHTML = window.innerWidth;        };                // Change values when window is resized        window.onresize = function() {                    // Setting the current height & width            // to the elements            height.innerHTML = window.innerHeight;            width.innerHTML = window.innerWidth;        };    </script></body></html>

Output:

Method 2: Using ResizeObserver API

We can use the latest ResizeObserver API to listen to the window resize event. This API is not fully supported.

Example: In this example, we are using ResizeObserver API.

<!DOCTYPE html>
<html>
<head>
    <title>
        How to detect and resize
        the window size
    </title>
    
    <style>
        body {
            text-align: center;
        }
        h1 {
            color:green;
        }
    </style>
</head>

<body>
    <h1>GeeksforGeeks</h1>
    
    <h3>
        Resizing the screen size and display its
        <br>corresponding width and height
    </h3>
    <div id="height">Height: <span></span></div>
    <div id="width">Width: <span></span></div>
    
    <script>
        const body = document.getElementsByTagName("body")[0];
        
        const height = document.querySelector("#height span");
        const width = document.querySelector("#width span");

        // Initialize resize observer object
        let resizeObserver = new ResizeObserver(() => {
            
            // Set the current height and width
            // to the element
            height.innerHTML = window.innerHeight;
            width.innerHTML = window.innerWidth;
        
        });
        
        // Add a listener to body
        resizeObserver.observe(body);
    </script>
</body>
</html>

 
<!DOCTYPE html>
<html><head>    <title>        How to detect and resize        the window size    </title>        <style>        body {            text-align: center;        }        h1 {            color:green;        }    </style></head>​<body>    <h1>GeeksforGeeks</h1>        <h3>        Resizing the screen size and display its        <br>corresponding width and height    </h3>    <div id="height">Height: <span></span></div>    <div id="width">Width: <span></span></div>        <script>        const body = document.getElementsByTagName("body")[0];                const height = document.querySelector("#height span");        const width = document.querySelector("#width span");​        // Initialize resize observer object        let resizeObserver = new ResizeObserver(() => {                        // Set the current height and width            // to the element            height.innerHTML = window.innerHeight;            width.innerHTML = window.innerWidth;                });                // Add a listener to body        resizeObserver.observe(body);    </script></body></html>

Output:

Method 3: Using `addEventListener()` Method

The addEventListener method in JavaScript is used to attach an event handler to a specified element, enabling the execution of a function or a piece of code in response to a specific event.

Example: The updateSize function is created to update the height and width based on the window size. The addEventListener method is used to attach the resize event to the window object. When the window is resized, the updateSize function is called to update the displayed height and width. The initial values are set by calling updateSize once when the page loads.