const fs = require('fs');
const postcss = require('postcss');
const postcssJs = require('postcss-js');
// e.g. body { font-size: 14px } h1 { font-size: 21px; }
const cssFile = fs.readFileSync('./path/to/file.css');

const css = postcss.parse(cssFile.toString());
const cssJsObject = postcssJs.objectify(css);

// e.g. module.exports = { body: { fontSize: "14px" }, h1: { fontSize: "21px" };
fs.writeFileSync('./path/to/file.js', `module.exports = ${JSON.stringify(cssJsObject)};`);