index.js

const sharp = require('sharp')
    , p = require('path')
;



module.exports = ({presets = [], ...options}) => (plugin) => {

  plugin.registerTemplateHelper('srcset', (metadata, src, preset, attributes = {}) => {

    // either get the sizes and srcset from the preset or extract them from the attributes
    let sizes, srcset;
    if (preset) {
      if( !presets[preset]) throw new Error('Unknown srcset preset');
      ({sizes, srcset} = presets[preset]);
    } else {
      ({sizes, srcset} = attributes);
    }

    const path = p.isAbsolute(src)
      ? src
      : p.join('/', metadata.dirname, src)
    ;

    const resizedFiles = srcset.map((width) => {
      // add size to file name https://stackoverflow.com/a/10802339
      const outputPath = path.substring(0, path.lastIndexOf('.')) + '-' + width + path.substring(path.lastIndexOf('.'));
      plugin.enqueueFile(outputPath, async () => {
        return plugin.getFile(path).then((file) => sharp(file, options).resize({width}).toBuffer());
      });
      return `${outputPath} ${width}w`;
    });

    attributes.sizes = sizes;
    attributes.srcset = resizedFiles.join(',');

    const output = ['<img'];
    for (attribute in attributes) {
      const value = attributes[attribute];
      if (value !== false) output.push(` ${attribute}="${value}"`);
    }
    output.push('>');
    return output.join('');

  });
}