Node.js Read and Write File

In the world of Node.js, interacting with the file system is a core task. Whether you are building a web server that needs to read configuration files, a logging system that records events, or a tool that processes CSV data, you will rely on the built-in fs (File System) module. This module provides everything you need to manage files and directories efficiently.

Key Features of File Read/Write

1. Asynchronous and Synchronous Operations: Node.js offers a choice. Asynchronous methods use callbacks (or Promises) to ensure your application remains responsive, while synchronous methods block execution until the task is finished.
2. Error Handling: File operations are prone to external issues, such as missing files or restricted permissions. Using try...catch or checking the err argument in callbacks is essential.
3. Text and Binary Data: Node.js can handle plain text (like .txt or .json) as well as binary data (like images or PDFs) using Buffers.

Reading Files

1. fs.readFile()

The fs.readFile() method is the standard way to read file contents without stopping the rest of your code. It reads the entire file into memory and then triggers a callback function.

const fs = require('fs');

// Reading a file asynchronously
fs.readFile('example.txt', 'utf8', (err, data) => {
  if (err) {
    console.error("Failed to read file:", err.message);
    return;
  }
  console.log("File content received:");
  console.log(data);
});

Output:

Content of the example.txt file

• Encoding: Adding 'utf8' as the second argument tells Node.js to return the data as a readable string. If you omit this, Node.js will return a Buffer object (raw binary data).

2. fs.readFileSync()

The fs.readFileSync() method is the "blocking" version. It is simpler to write because it returns the data directly, but it stops all other code execution until the file is read.

const fs = require('fs');

try {
  const data = fs.readFileSync('example.txt', 'utf8');
  console.log(data);
} catch (err) {
  console.error("Error reading file synchronously:", err);
}

Output:

Content of the example.txt file

Writing Files

1. fs.writeFile()

The fs.writeFile() method saves data to a file. If the file already exists, it is completely replaced. If it doesn't exist, Node.js creates it for you.

const fs = require('fs');

const content = 'Hello, Node.js! This is a fresh write.';

fs.writeFile('output.txt', content, (err) => {
  if (err) throw err;
  console.log('File written successfully');
});

Output:

File written successfully

2. fs.writeFileSync()

Similar to reading, this is the synchronous version of writing. It is useful when you need to ensure a file is written before moving on to the next logic step in a script.

const fs = require('fs');

const userData = JSON.stringify({ name: "John", age: 30 });

try {
  fs.writeFileSync('user.json', userData);
  console.log('User data saved successfully');
} catch (err) {
  console.error('Save failed:', err);
}

Output:

User data saved successfully

3. fs.appendFile()

If you want to add data to an existing file without deleting what is already there—like adding a new line to a log file—use fs.appendFile().

const fs = require('fs');

const logEntry = `\nLog Entry at ${new Date().toISOString()}`;

fs.appendFile('server.log', logEntry, (err) => {
  if (err) throw err;
  console.log('New log entry added!');
});

Output:

New log entry added!

Summary

Mastering the fs module is a milestone for any Node.js developer. You have two main paths: the Asynchronous path (readFile, writeFile), which is better for performance-critical applications like web servers, and the Synchronous path (readFileSync, writeFileSync), which is simpler for quick scripts and one-off tasks. By combining these methods with proper error handling and encoding, you can manage any data persistence needs your application requires.