Responsive image transforms for eleventy

eleventy-image-transform.is

// This file goes in /src/transforms/

// Add this line to .eleventy.js:
// const eleventyImgTransform = require("./src/transforms/eleventy-img-transform.js");

// And add this one to module.exports in .eleventy.js:
// eleventyConfig.addTransform("eleventyImgTransform", eleventyImgTransform);

const Image = require("@11ty/eleventy-img");
const cheerio = require("cheerio");
const { resolve } = require("path");

module.exports = (value, outputPath) => {
  // Search for <img> on all pages matching this regex

  // Only run on pages in the /dist/blog folder
  if (outputPath.match(/^dist\/blog.*\.html$/)) {
    let $ = cheerio.load(value);

    // Find all the <img> within specified element, that don't have an HTML class
    $(".article-text img:not([class])")
      .not("picture > img") // Skip any <img> that's already wrapped in <picture>
      .each(function (i, elem) {
        let imgAlt = $(this).attr("alt") || "";
        let imgTitle = $(this).attr("title") || "";
        let imgSrc = $(this).attr("src");

        let imgLocation = "src" + imgSrc;
        let fullImgLocation = resolve(imgLocation);

        // TODO: Make this transform work when loading remote images
        // Only process image if it's local
        if (imgSrc.indexOf("http") == 0) {
          return;
        }

        let options = {
          // Each image size roughly 1.5X larger than last
          widths: [250, 400, 600, 900, 1300],
          formats: ["avif", "webp", "jpeg"], // In order of preference
          outputDir: "dist/11ty-img/",
          urlPath: "/11ty-img/",
        };

        Image(fullImgLocation, options); // Start the images processing

        let stats = Image.statsSync(fullImgLocation, options); // Get the locations where the processed images will appear

        let lowestSrc = stats["jpeg"][0];

        let highestSrc = stats["jpeg"][stats["jpeg"].length - 1]; // The last JPG is the largest
        let aspectRatio = highestSrc.width / highestSrc.height;

        // Set the image width attribute to 800px, unless that's too large
        let imageWidth = highestSrc.width;
        if (highestSrc.width >= 800) {
          imageWidth = 800;
        }

        // Set height to make the image the correct aspect ratio
        let imageHeight = Math.ceil(imageWidth * aspectRatio);

        // Iterate over formats and widths
        let newElement = `<picture>
      ${Object.values(stats)
        .map((imageFormat) => {
          return `  <source type="image/${
            imageFormat[0].format
          }" data-srcset="${imageFormat
            .map((entry) => entry.srcset)
            .join(", ")}">`;
        })
        .join("\n")}
      <img
          data-src="${lowestSrc.url}"
          data-sizes="auto"
          width="${imageWidth}"
          height="${imageHeight}"
          class="lazyload"
          title="${imgTitle}"
          alt="${imgAlt}">
      </picture>`;

        $(this).replaceWith(newElement);
      });

    value = $.html();
  }

  return value;
};