index.js

const fs = require("fs");
const path = require("path");
const postcss = require("postcss");

const REGEX = /\[.*?\]|[:*][a-z-_:]+|[^a-z-_]+/g;
const DEFAULT_OPTIONS = {
  path: "./src",
  exts: [".js", ".jsx", ".html", ".vue", ".svelte"],
  whitelist: ["body", "html"]
};

const getFiles = (pathName, exts) =>
  fs.readdirSync(pathName).flatMap(file => {
    var filename = path.join(pathName, file);
    if (
      fs.lstatSync(filename).isDirectory() &&
      !filename.includes("node_modules")
    ) {
      return getFiles(filename, exts);
    } else if (exts.includes(path.extname(filename))) {
      return [filename];
    } else {
      return [];
    }
  });

const getContent = files => files.map(f => fs.readFileSync(f, "utf8")).join();
const getWords = content => content.toLowerCase().split(REGEX).filter(Boolean);

module.exports = postcss.plugin("remove-unused", opts => css => {
  const { path, exts, whitelist } = Object.assign(DEFAULT_OPTIONS, opts);
  const content = getWords(getContent(getFiles(path, exts))).concat(whitelist);

  css.walkRules(rule => {
    rule.selectors = rule.selectors.filter(selector =>
      getWords(selector).every(word => content.includes(word))
    );
  });
});