The File System Module

One of Node.js's most powerful features is direct access to the file system. The built-in fs module provides functions for reading, writing, updating, deleting, and watching files and directories.

Requiring the fs Module

const fs = require("fs");

Reading Files

Node.js provides both asynchronous and synchronous versions of most fs operations. The async version is preferred in production because it does not block the event loop.

// Asynchronous read (non-blocking)
fs.readFile("notes.txt", "utf8", function (err, data) {
  if (err) {
    console.error("Error reading file:", err.message);
    return;
  }
  console.log(data);
});

// Synchronous read (blocking — avoid in servers)
const data = fs.readFileSync("notes.txt", "utf8");
console.log(data);

Always pass "utf8" (or another encoding) as the second argument when you want a string. Without it you get a raw Buffer object.

Writing Files

// Overwrite or create the file
fs.writeFile("output.txt", "Hello, world!", "utf8", function (err) {
  if (err) throw err;
  console.log("File written successfully");
});

// Append to an existing file
fs.appendFile("log.txt", "New log entry\n", "utf8", function (err) {
  if (err) throw err;
});

Checking Whether a File Exists

fs.access("config.json", fs.constants.F_OK, function (err) {
  if (err) {
    console.log("File does not exist");
  } else {
    console.log("File exists");
  }
});

Working with Directories

// Create a directory
fs.mkdir("uploads", { recursive: true }, function (err) {
  if (err) throw err;
  console.log("Directory created");
});

// List directory contents
fs.readdir(".", function (err, files) {
  if (err) throw err;
  files.forEach(function (file) {
    console.log(file);
  });
});

Promises API

Since Node.js 10 the fs/promises module provides a Promise-based API that works cleanly with async/await:

const fs = require("fs/promises");

async function readConfig() {
  try {
    const data = await fs.readFile("config.json", "utf8");
    return JSON.parse(data);
  } catch (err) {
    console.error("Could not read config:", err.message);
    return {};
  }
}

readConfig().then(function (config) {
  console.log(config);
});

Deleting Files

fs.unlink("temp.txt", function (err) {
  if (err) throw err;
  console.log("File deleted");
});

The fs module is essential for any Node.js application that handles configuration files, user uploads, logs, or any persistent data stored on disk. Always prefer asynchronous variants inside HTTP servers to keep the event loop free.