HTMLImageElement.sizes

The HTMLImageElement property sizes allows you to specify the layout width of the image for each of a list of media conditions. This provides the ability to automatically select among different images—even images of different orientations or aspect ratios—as the document state changes to match different media conditions.

Each condition is specified using the same conditional format used by media queries.

A USVString containing a comma-separated list of source size descriptors followed by an optional fallback size. Each source size descriptor is comprised of a media condition, then at least one whitespace character, then the source size value to use for the image when the media condition evaluates to true.

Media conditions

Each source size descriptor consists of a media condition as defined by the media queries standard. Because a source size descriptor is used to specify the width to use for the image during layout of the page, the media condition is typically (but not necessarily) based entirely on width information. See Syntax in Using media queries for details on how to construct a media condition.

Source size values

The source size value is a CSS length. It may be specified using font-relative units (such as em or ex), absolute units (such as px or cm), or the vw unit, which lets you specify the width as a percentage of the viewport width (1vw being 1% of the viewport width).

In this example, a blog-like layout is created, displaying some text and an image which for which three size points are specified, depending on the width of the window. Three versions of the image are also available, with their widths specified. The browser takes all of this information and selects an image and width that best meets the specified values.

How exactly the images are used may depend upon the browser and what the pixel density of the user's display is.

Buttons at the bottom of the example let you actually modify the sizes property slightly, switching the largest of the three widths for the image between 40em and 50em.

The JavaScript code handles the two buttons that let you toggle the third width option between 40em and 50em; this is done by handling the click event, using the JavaScript string object method replace() to replace the relevant portion of the sizes string.

let image = document.querySelector("article img");
let break40 = document.getElementById("break40");
let break50 = document.getElementById("break50");

break40.addEventListener("click",
    event => image.sizes = image.sizes.replace(/50em,/, "40em,"));

break50.addEventListener("click",
    event => image.sizes = image.sizes.replace(/40em,/, "50em,"));

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• Media queries
• Using media queries
• Images in HTML
• Responsive images