This code converts any px value in the js of given directory and it's sub-directories file to rem value.

pxToRemConversion.js

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

const directoryPath = "src";
const rootFontSize = 16;

function convertFilesInDirectory(directoryPath) {
  const files = fs.readdirSync(directoryPath);

  for (const file of files) {
    const filePath = path.join(directoryPath, file);

    if (fs.statSync(filePath).isDirectory()) {
      convertFilesInDirectory(filePath); // recursively process subdirectories
    } else if (path.extname(file) === ".js") {
      // only process javascript files
      const data = fs.readFileSync(filePath, "utf8");
      const newData = data.replace(/(\d*\.?\d+)px/g, (match, value) => {
        const remValue = (parseFloat(value) / rootFontSize).toFixed(4);

        // 1000 means we want 3 digit after decimal point
        return `${Math.round(remValue * 1000) / 1000}rem`;
      });

      //   Write into the file
      fs.writeFileSync(filePath, newData, "utf8");
      console.log(`Converted ${file} to use rem units`);
    }
  }
}

convertFilesInDirectory(directoryPath);