onload

The window’s load event

For the window object, the load event is fired when the whole webpage (HTML) has loaded fully, including all dependent resources, including JavaScript files, CSS files, and images.

To handle the load event, you register an event listener using the addEventListener() method:

window.addEventListener('load', (event) => {
    console.log('The page has fully loaded');
});Code language: JavaScript (javascript)

Or use the onload property of the window object:

window.onload = (event) => {
    console.log('The page has fully loaded');
};Code language: JavaScript (javascript)

If you maintain a legacy system, you may find that the load event handler is registered in of the body element of the HTML document, like this:

<!DOCTYPE html>
<html>
<head>
    <title>JS load Event Demo</title>
</head>
<body onload="console.log('Loaded!')">
</body>
</html>Code language: HTML, XML (xml)

It’s a good practice to use the addEventListener() method to assign the onload event handler whenever possible.

The image’s load event

The load event also fires on images. To handle the load event on images, you use the addEventListener() method of the image elements.

The following example uses the load event handler to determine if an image, which exists in the DOM tree, has been completely loaded:

<!DOCTYPE html>
<html>
<head>
    <title>Image load Event Demo</title>
</head>
<body>
    <img id="logo">
    <script>
        let logo = document.querySelector('#logo');

        logo.addEventListener('load', (event) => {
            console.log('Logo has been loaded!');
        });

        logo.src = "logo.png";
    </script>
</body>
</html>
Code language: HTML, XML (xml)

You can assign an onload event handler directly using the onload attribute of the <img> element, like this:

<img id="logo" 
     src="logo.png" 
     onload="console.log('Logo loaded!')">
Code language: HTML, XML (xml)

If you create an image element dynamically, you can assign an onload event handler before setting the src property as follows:

window.addEventListener('load' () => {
    let logo = document.createElement('img');
    // assign and onload event handler
    logo.addEventListener('load', (event) => {
        console.log('The logo has been loaded');
    });
    // add logo to the document
    document.body.appendChild(logo);
    logo.src = 'logo.png';
});
Code language: JavaScript (javascript)

How it works:

  • First, create an image element after the document has been fully loaded by placing the code inside the event handler of the window’s load event.

  • Second, assign the onload event handler to the image.

  • Third, add the image to the document.

  • Finally, assign an image URL to the src attribute. The image will be downloaded to the element as soon as the src property is set.

The script’s load event

The <script> element also supports the load event slightly different from the standard ways. The script’s load event allows you to check if a JavaScript file has been completely loaded.

Unlike images, the web browser starts downloading JavaScript files only after the src property has been assigned and the <script> element has been added to the document.

The following code loads the app.js file after the page has been completely loaded. It assigns an onload event handler to check if the app.js has been fully loaded.

window.addEventListener('load', checkJSLoaded)

function checkJSLoaded() {
    // create the script element
    let script = document.createElement('script');
    
    // assign an onload event handler
    script.addEventListener('load', (event) => {
        console.log('app.js file has been loaded');
    });

    // load the script file
    script.src = 'app.js';
    document.body.appendChild(script);
}Code language: JavaScript (javascript)

Summary

  • The load event occurs when the document has been completely loaded, including dependent resources like JavaScript files, CSS files, and images.

  • The <img> and <script> elements also support the load event.

  • Use the addEventListener() method to register an onload event handler.

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