modifyFileContents.js

const fs = require("fs");

const args = process.argv.slice(2);

if (args.length > 1) {
  console.error("Invalid number of arguments. Please only pass one!");
}

const fileName = args[0];

if (!fileName) {
  console.error("Invalid file name passed!");
}

const FILE_PATH = __dirname + "/" + fileName;

function readFromFile() {
  return new Promise(resolve => {
    fs.readFile(FILE_PATH, "utf8", (err, data) => {
      if (err) {
        console.error(`Error reading file: ${FILE_PATH}`, err);
      } else if (data) {
        resolve(data);
      }
    });
  });
}

function modifyFileContents(fileContents) {
  return fileContents
    .split("")
    .reduce((acc, next) => {
      if (next === '"') {
        acc.push("\\");
      }
      acc.push(next);
      return acc;
    }, [])
    .join("");
}

function writeToFile(contents) {
  return new Promise(resolve => {
    fs.writeFile(FILE_PATH, contents, err => {
      if (err) {
        console.error(`Error writing to file: ${FILE_PATH}`, err);
      } else {
        resolve();
      }
    });
  });
}

readFromFile()
  .then(fileContents => modifyFileContents(fileContents))
  .then(modifiedFileContents => writeToFile(modifiedFileContents));