index.js

const minify = require("html-minifier").minify;
const path = require("path");
const fs = require("fs");

module.exports = function htmlMinifierMiddleware(options) {
  options = options || {
    collapseBooleanAttributes: true,
    collapseWhitespace: true,
    removeAttributeQuotes: true,
    removeComments: true,
    removeEmptyAttributes: true,
    removeRedundantAttributes: true
  };

  return reptar => {
    for (const relativePath in reptar.destination) {
      if (path.extname(relativePath) !== ".html") {
        continue;
      }

      const file = reptar.destination[relativePath];

      if (file.filtered) {
        continue;
      }

      const destinationPath = path.join(
        reptar.config.get("path.destination"),
        relativePath
      );

      var fileContent;
      try {
        fileContent = fs.readFileSync(destinationPath, "utf8");
      } catch (e) {
        console.log(file);
        throw e;
      }

      const newContent = minify(fileContent, options);

      fs.writeFileSync(destinationPath, newContent, "utf8");
    }
  };
};