File System fs Module

The Node.js File System (fs) module allows you to interact with the file system on your operating system. It provides multiple API flavors: Synchronous (blocking), Callback-based (asynchronous), and Promise-based (asynchronous).

1. The Three API Flavors

Synchronous API (Blocking)

Blocks execution of the main thread until the I/O operation is complete. Avoid using this in web servers.

import fs from "fs";

// Blocks execution flow
try {
  const data = fs.readFileSync("input.txt", "utf8");
  console.log("Sync content:", data);
} catch (err) {
  console.error("Read error:", err);
}

Callback-based API (Non-Blocking)

Runs asynchronously using callback functions. This avoids blocking but can lead to deeply nested code (Callback Hell).

import fs from "fs";

fs.readFile("input.txt", "utf8", (err, data) => {
  if (err) {
    console.error("Error reading file:", err);
    return;
  }
  console.log("Async Callback content:", data);
});

Promise-based API (Modern, Non-Blocking)

Allows writing cleaner asynchronous code using async/await syntax. Highly recommended for modern Node.js applications.

import fs from "fs/promises";

async function readFileContent() {
  try {
    const data = await fs.readFile("input.txt", "utf8");
    console.log("Promise-based content:", data);
  } catch (err) {
    console.error("Async read error:", err);
  }
}

readFileContent();

2. Writing and Appending Files

You can write new data or append content to existing files using the promises API:

import fs from "fs/promises";

async function fileOperations() {
  const filePath = "output.txt";
  
  // Write fresh content (overwrites if file already exists)
  await fs.writeFile(filePath, "Hello World from Node.js\n", "utf8");
  
  // Append content to the end of the file
  await fs.appendFile(filePath, "Appending a second line\n", "utf8");
  
  console.log("Write and append operations complete");
}

fileOperations();

3. Directory Operations

The fs module also provides utilities to create, read, and delete directories, as well as fetch metadata (like file size or permissions):

import fs from "fs/promises";

async function directoryDemo() {
  const dirPath = "./sandbox";
  
  // Create a directory if it does not exist
  await fs.mkdir(dirPath, { recursive: true });
  
  // Read all files inside a directory
  const files = await fs.readdir("./");
  console.log("Files list:", files);
  
  // Fetch detailed metadata of a file or folder
  const stats = await fs.stat("output.txt");
  console.log("Is File:", stats.isFile());
  console.log("File Size in Bytes:", stats.size);
  
  // Delete a file
  await fs.unlink("output.txt");
  
  // Remove a directory
  await fs.rm(dirPath, { recursive: true, force: true });
}

directoryDemo();