Core Modules & Async›Extra· 30 min read

The File System (fs)

Read and write files on the server — something the browser can never do.

What you will learn

Read and write files with fs
Use asynchronous file methods
Understand why async matters

Working with files

The fs (file system) module reads and writes files on the server’s disk — something a browser is never allowed to do. Prefer the asynchronous methods so your server never freezes while waiting for the (slow) disk. Async fs methods take a callback — a function Node runs later, once the file work is done. That callback’s first argument is always an error (err), which is null if everything went fine.

The example below does two things in order — write a file, then read it back:

fs.writeFile saves the text "Hello, file!" into note.txt (creating the file if it does not exist).
When the write finishes, Node calls its callback — we check err and log a success message.
fs.readFile opens note.txt and reads its contents.
When the read finishes, its callback receives the file’s text as data, which we print.

Write then read a file (async)

const fs = require("fs");

// Write a file
fs.writeFile("note.txt", "Hello, file!", (err) => {
  if (err) throw err;
  console.log("File written.");
});

// Read a file
fs.readFile("note.txt", "utf8", (err, data) => {
  if (err) throw err;
  console.log("File says:", data);
});

Note: Output: File written. File says: Hello, file! The "utf8" in readFile tells Node to give you readable text rather than raw bytes. if (err) throw err means "if something went wrong, stop and show the error". Note both operations run asynchronously, so the exact order can vary — but here the write finishes first, then the read prints what was saved.

Note: There are also synchronous versions (readFileSync) that are simpler but block everything until done. Use them only for quick scripts, never inside a busy server.

Tip: Modern Node also offers promise-based file methods (fs.promises) so you can use async/await — cleaner than callbacks.

Q. Why prefer the asynchronous fs methods in a server?

Answer: Async file methods let the server keep handling other requests instead of blocking while the disk operation completes.

✍️ Practice

Write a script that saves some text to a file and then reads it back.
Append a new line to an existing file.

🏠 Homework

Build a tiny “notes” script that writes user-provided text to a file.