Purge CSS in Create React App without ejecting

package.json

{
  "name": "my-app",
  "version": "0.1.0",
  "private": true,
  "dependencies": {
    "react": "^16.8.6",
    "react-dom": "^16.8.6",
    "react-scripts": "3.0.1"
  },
  "devDependencies": {
    "purgecss": "^1.3.0"
  },
  "scripts": {
    "start": "react-scripts start",
    "build": "react-scripts build && npm run purge-css",
    "purge-css": "node ./purge-css", 
    "test": "react-scripts test",
    "eject": "react-scripts eject"
  },
  "eslintConfig": {
    "extends": "react-app"
  },
  "browserslist": {
    "production": [
      ">0.2%",
      "not dead",
      "not op_mini all"
    ],
    "development": [
      "last 1 chrome version",
      "last 1 firefox version",
      "last 1 safari version"
    ]
  }
}

purge-css.js

const Purgecss = require('purgecss');
const fs = require('fs');

const purgecss = new Purgecss({
  content: ['build/static/js/*.js'],
  css: ['build/static/css/*.css']
})
const purgecssResult = purgecss.purge();

purgecssResult.forEach((result) => {
  const file = result.file;
  const css = result.css;
  
  fs.stat(file, (error, originalFileStats) => {
    console.log(originalFileStats);
    fs.writeFileSync(file, css);
    const newFileStats = fs.statSync(file);
    console.log('\x1b[32m', `Purge CSS saved ${Math.round((originalFileStats.size - newFileStats.size) / 1000)}kb on ${file}`);
  });
});