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Preload and Prefetch for Images: How do they differ?
These are the best techniques used to improve the performance of web pages by loading resources in advance.
You will learn everything you need to know about preloading images and how they differ from prefetch tags in this guide.
Below are the points at which a Shopify store may implement preloading and prefetching.
- What are preloading images?
- Preload images: how do they work?
- Prefetch Images: How Do They Work?
- Difference between preload and prefetch
- What assets should be preloaded?
- What Assets Should be Preloaded?
Preload and prefetch are terms used in the context of optimizing images, particularly for site speed. Although they are related concepts, they refer to slightly different strategies.
What are Preloading Images?
Preloading images using the HTML tag. These two value options of the rel attribute (relationship between the current document and the linked document) are most relevant to the issue: prefetch: load the given resource while the page is benignly rendered.
Ex:-
Preload might involve loading certain web pages or resources in the background so that when a user clicks a link, the page can be displayed more quickly because of some of the necessary data.
<link rel="preload"as="image"href="important.png">Product Image Example:-
<link rel="preload"as="image"href="{{image_url | replace: '_1x1', '' }}"imagesrcset="{{preloadSrcset}}"/>Preload Images: How Do They Work?
This technique is used to improve the performance of web pages by loading images in advance before the user needs them. This can make the page seem to load faster and improve the overall user experience.
When a browser encounters a <img> tag in HTML, it typically starts downloading the image file as soon as it encounters the tag.
Preloading works by telling the browser to start downloading an image file in the background, even before it is needed by the user. This can be done using a variety of techniques, such as:
- The
<link>tag with therel="preload"attribute: This tag can be used to specify the URL of the image file, as well as its size and media type. - The
Image()constructor in JavaScript: This constructor can be used to create an Image object, which can then be assigned the URL of the image file. - The
fetch()function in JavaScript: This function can be used to download an image file and then store it in a variable.
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Prefetch Images: How Do They Work?
Prefetching images is a technique used to improve web page performance by proactively downloading images, especially for those on slower connections.
How Prefetching Works
When a user visits a web page, the browser starts parsing the HTML code and downloading the necessary resources, including images. Prefetching works by providing hints to the browser about which images are likely to be needed.
There are two primary methods for prefetching images.
- Link Preloading: This method uses the tag with the rel=”prefetch” attribute to specify the URLs of the images to be prefetched. The browser will download these images in the background, giving them a higher priority than other resources.
- Resource Hints in JavaScript: JavaScript can also be used to provide prefetch hints to the browser. The fetch() API can be used to fetch images and store them in memory, effectively prefetching them. Additionally, the Cache API can be used to cache prefetched images, making them available for future use
Difference Between Preload and Prefetch
The difference between “preload” and “prefetch” remains consistent with the general web development principles.
| Feature | Preload | Prefetch |
| Purpose | Load critical resources for the current page | Load resources for future navigation or interactions |
| Priority | High | Low |
| When to use | Essential resources for the user experience | Resources that are likely to be used soon. |
What Assets Should be Preloaded?
Assets that should be preloaded are critical for the initial rendering of the page. These assets typically include:
- Images that are above the fold, meaning they are visible to the user without scrolling.
- Fonts that are used in the main content of the page.
- Stylesheets that are needed to style the main content of the page.
- JavaScript is needed for the initial functionality of the page.
Read this Blog and Find:- Preloading Images and Other Assets for Shopify
What Assets Should be Prefetch?
In addition to preloading critical assets, you can also prefetch assets that are likely to be needed soon but are not essential for the initial rendering of the page, These assets might include:
- Images that are below the fold, but that the user is likely to scroll to.
- JavaScript is needed for the secondary functionality of the page or app.
Prefetching these assets can help to reduce the amount of time it takes for the user to interact with the page, as the assets will already be loaded by the time they are needed.
In conclusion, By carefully considering these factors, you can optimize your preload and prefetch strategies to improve the performance of your page or app.
