Compile JavaScript and Sass in Eleventy using UglifyJS and Sass lib

.eleventy.js

const terser = require("terser");
const sass = require("sass");

module.exports = (eleventyConfig) => {
  // Compile Sass
  eleventyConfig.addTemplateFormats("scss");

  eleventyConfig.addExtension("scss", {
    outputFileExtension: "css",
    compile: function (contents, inputPath) {
      return (data) => {
        let ret = sass.compile(inputPath, { style: "compressed" });
        return ret.css;
      };
    },
  });

  // Compile Js
  eleventyConfig.addTemplateFormats("js");

  eleventyConfig.addExtension("js", {
    outputFileExtension: "js",
    compile: function (contents, inputPath) {
      if (inputPath.startsWith(`./src/_`)) return;

      return async (data) => {
        let ret = await terser.minify(contents);
        return ret.code;
      };
    },
  });

  return {
    dir: {
      input: "src",
      output: "dist",
    },
  };
};

Eleventy docs for adding custom languages https://www.11ty.dev/docs/languages/custom/