When do javascript images load?

When do JavaScript images load?

In a website or application, images are important resources. By adding pictures, we can make our pages more vivid and eye-catching. But how is image loading handled when loading a page? Here, we will introduce the process and best practices of JavaScript image loading.

Image loading order

When the browser loads a page, it processes images in the following order:

1. Images in HTML files

HTML tags usually include the tag, which contains a src attribute, which contains the URL address of the image. If the HTML file contains multiple image elements, the browser will download them one by one in the order in which they appear. When the browser starts downloading the image, it creates a placeholder in the page.

2. CSS background image

The background image in the CSS file can be set using the background-image attribute. The browser will download all background images when downloading the CSS file.

3. JavaScript loads images

JavaScript can use the Image object to load images. The Image object implements the W3C's HTMLImageElement interface, which loads images by specifying an image path through a URL. Using this method, you can dynamically load and display images when needed, without having to preload all images when the page loads.

Image Download Process

When the browser downloads the first image element, it will not start downloading the second image element until the download is complete. The browser handles HTTP requests and responds in a single thread, and it will only start downloading the next image after receiving a response from the previous image.

The time for image downloading depends on the file size, network speed and server response time. If the image is very large, or the server response time is slow, the user will wait a while to see the full content of the page. To mitigate this situation, we can use the following best practices.

Best Practices

1. Compress Image Files

Compressing image files can reduce file size and download time. There are many tools available to compress and optimize images, such as TinyPNG and ImageOptim.

2. Use appropriate image formats

Choose appropriate image formats appropriately, such as JPEG, PNG and GIF. JPEG is suitable for photos and images, PNG is suitable for transparent and high-quality images, and GIF is suitable for animations and simple images.

3. Lazy loading of images

In order to avoid page response time being too long, you can delay loading of some images. This means that when the page first loads, not all images are loaded, only the visible portion of the image. Using lazy loading technology, we can load other images as the page scrolls. Lazy loading of images can improve website performance and user experience.

4. Use preloading technology

Preloading technology can preload images during page loading instead of waiting until the user visits the page. This speeds up the user experience because when they visit the page, all images are loaded and cached and can be viewed immediately.

Summary

When writing web applications, it is crucial to understand the process of JavaScript image loading. By implementing best practices, we can improve application performance, speed up page load times, and improve user experience. Best practices are important for applications that load large numbers of images because the speed at which images load directly affects user satisfaction.

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