Node.js File System (fs Module)

After completing this topic

You will be able to write code to read, write, and delete files in Node.js.

Things you can't do in a browser

JavaScript in the browser cannot access files on your computer for security reasons. However, since Node.js is a server-side environment, you can freely read and write files. The fs (File System) module provides this functionality.

// Import the fs module
const fs = require("fs");

// Read a file (synchronous method — pauses until reading is complete)
const data = fs.readFileSync("memo.txt", "utf8");
console.log(data);

require("fs") is the way to import built-in modules in Node.js. No separate installation is required. If you omit "utf8", a Buffer (binary data) will be returned, and you will see numbers instead of characters.

Writing and Appending to Files

const fs = require("fs");

// Write to a file — creates the file if it doesn't exist, overwrites it if it does
fs.writeFileSync("output.txt", "First line\n");

// Append content to a file (does not overwrite)
fs.appendFileSync("output.txt", "Second line\n");

// Check
const result = fs.readFileSync("output.txt", "utf8");
console.log(result);
// First line
// Second line

writeFileSync completely overwrites the existing content. To preserve the existing content while adding new content, use appendFileSync.

Checking for and Deleting Files

const fs = require("fs");

// Check if a file exists
if (fs.existsSync("output.txt")) {
  console.log("The file exists");

  // Delete the file
  fs.unlinkSync("output.txt");
  console.log("Deleted");
}

// Create a directory (folder)
if (!fs.existsSync("logs")) {
  fs.mkdirSync("logs");
}

// List files in a directory
const files = fs.readdirSync(".");
console.log(files); // ["index.js", "logs", ...]

All the methods used here have Sync (synchronous) attached. The code execution will pause until the file operation is complete. In a real server environment, the asynchronous versions (readFile, writeFile — no Sync) are used, which is covered in Synchronous vs. Asynchronous.