marked-responsive-images

Marked Responsive Images

An extension for Marked (github, npm) designed to generate responsive images by parsing simple filename conventions into full <picture> elements with srcset and sizes attributes based on simple filename conventions.

Marked Responsive Images parses image filenames to detect available size and file extension variants without breaking standard markdown compatibility.

Installation

npm install marked-responsive-images

Usage

// Default factory export (recommended)
import { marked } from 'marked';
import { markedResponsiveImages } from 'marked-responsive-images';

/*
// or use UMD scripts
<script src="https://cdn.jsdelivr.net/npm/marked/lib/marked.umd.js"></script>
<script src="https://cdn.jsdelivr.net/npm/marked-responsive-images/dist/index.umd.js"></script>
*/

// Register with marked
marked.use(markedResponsiveImages());

// Render markdown
const html = marked.parse('![My Image](assets/hero__400-300_800-600.jpg)');

Naming Convention

I have written a PowerShell automation script to automatically generate image size variants, WebP alternatives, and output a fully formed Markedown image link.

Naming the Main File

The extension looks for a specific pattern at the end of your filenames to generate the <source> tags and/or srcset attribute.

Pattern: filename__width-height[-density][-extension]_[…]_currentFileWidth-currentFileHeight.png

1. Separator:
   Use two underscores (__) to separate the base name from the sizes.
2. Variants:
   Use one underscore (_) to separate different size variants.
3. Dimensions:
   Use a dash (-) to separate width and height.
4. [optional] Density:
   Use a dash (-) followed by a pixel density multiplier (e.g., 1x, 1.5x, 2x) to instruct the browser to use display density rather than viewport width.
5. [optional] Extension:
   Use a dash (-) to specify a file extension if it is different from the one used by the URL.
   • Supported formats: jpg, jpeg, png, webp, avif, gif, svg, jxl.

The "full name" image must exist on your server.
The image path you write in Markdown (e.g., hero__400-300_800-600.jpg) is used as the graceful fallback. This raw filename is assigned to the src attribute of the inner <img> tag and will be the only image loaded if the extension is disabled or if the Markdown is viewed in an environment that doesn't support responsive images.

Format Ordering:
When multiple formats of the same size are provided, the extension automatically sorts the generated <source> tags based on the formatPriority configuration array (defaulting to the most modern/efficient formats first, like JXL and AVIF). The physical order of the tokens in the filename does not matter.

This extension does not resize images.
It is your responsibility to ensure that all physical image files—both the "Full Name" fallback and the individual variants (e.g., hero__400-300.jpg)—actually exist at the destination. This extension only generates the HTML markup to point to them.

Examples

Basic Resizing:

• Markdown:

  ![Responsive image example](img/photo__400-300_800-600.jpg)

• Resulting HTML:

  <picture>
      <source
          srcset="img/photo__400-300.jpg 400w, img/photo__800-600.jpg 800w"
          type="image/jpeg"
      />
      <img
          src="img/photo__400-300_800-600.jpg"
          width="800"
          height="600"
          alt="Responsive image example"
      />
  </picture>

Format Switching:

• Markdown:

  ![Web optimized photo example](img/photo__800-600-webp_800-600.jpg)

• Resulting HTML:

  <picture>
      <source srcset="img/photo__800-600.webp 800w" type="image/webp" />
      <source srcset="img/photo__800-600.jpg 800w" type="image/jpeg" />
      <img
          src="img/photo__800-600-webp_800-600.jpg"
          width="800"
          height="600"
          alt="Web optimized photo example"
      />
  </picture>

Pixel Density (Retina Displays):

• Markdown:

  ![App screenshot](img/ui__400-300-1x_800-600-2x.png)

• Resulting HTML:

  <picture>
      <source srcset="img/ui__400-300.png 1x, img/ui__800-600.png 2x" type="image/png" />
      <img
          src="img/ui__400-300-1x_800-600-2x.png"
          width="800"
          height="600"
          alt="App screenshot"
      />
  </picture>

Configuration

You can configure global options for Marked Responsive Images using:

marked.use(
	markedResponsiveImages({
		sizes: null, // {string}
		class: '', // {string}
		pictureClass: '', // {string}
		debug: false, // {boolean}
		lazy: true, // {boolean}
		decoding: 'auto', // {'async' | 'sync' | 'auto'}
		renderSimpleImgTags: false, // {boolean}
		formatPriority: ['jxl', 'avif', 'webp', 'png', 'jpeg', 'jpg', 'gif', 'svg'], // {Array<string>}
	}),
);

Option	Type	Default	Description
sizes	string	null	The sizes attribute that should be added to <source> or <img> tags. If empty, an automatic default is set based on the largest variant width.
class	string	''	The class attribute to apply to rendered <img> tags.
pictureClass	string	''	The class attribute to apply to the <picture> wrapper tag.
lazy	boolean	true	Adds loading="lazy" to images for better page load optimization.
decoding	string	'auto'	The decoding attribute strategy to apply to the <img> tag.
debug	boolean	false	Log warnings to the console when URLs cannot be parsed or formats are malformed.
renderSimpleImgTags	boolean	false	Enable to generate a simple <img> tag with a srcset attribute instead of a full <picture> element. When enabled, format variations are automatically stripped out, as standard <img> tags do not support format negotiation.
formatPriority	Array<string>	['jxl', 'avif', 'webp', 'png', 'jpeg', 'jpg', 'gif', 'svg']	Defines the sorting priority for <source> formats. The default is ordered based on typical efficiency.