Flash cards
Review the key moves
What is the main idea behind Node.js File System Module?
Lesson checks
Practice each idea before moving on
Short Mimo-style checks built from this lesson's code, terms, and sequence.
Which statement best captures the main point of this lesson?
Complete the missing token from the example code.
___ fs = require('fs');Put the learning moves in the order that makes the concept easiest to apply.
Introduction to Node.js File System
The Node.js File System module (fs) provides a comprehensive set of methods for working with the file system on your computer.
It allows you to perform file I/O operations in both synchronous and asynchronous ways.
Note
The File System module is a core Node.js module, so no installation is required.
Importing the File System Module
You can import the File System module using CommonJS require() or ES modules import syntax:
const fs = require('fs');import fs from 'fs';
// Or for specific methods:
// import { readFile, writeFile } from 'fs/promises';Promise-based API
Node.js provides promise-based versions of the File System API in the fs/promises namespace, which is recommended for modern applications:
// Using promises (Node.js 10.0.0+)
const fs = require('fs').promises;
// Or with destructuring
const { readFile, writeFile } = require('fs').promises;
// Or with ES modules
// import { readFile, writeFile } from 'fs/promises';Common Use Cases
- Read and write files
- Create and delete files
- Append to files
- Rename and move files
- Change file permissions
- Create and remove directories
- List directory contents
- Watch for file changes
- Get file/directory stats
- Check file existence
- File streams
- File descriptors
- Symbolic links
- File watching
- Working with file permissions
Performance Tip: For large files, consider using streams ( fs.createReadStream and fs.createWriteStream ) to avoid high memory usage.
Reading Files
Node.js provides several methods to read files, including both callback-based and promise-based approaches.
The most common method is fs.readFile() .
Note
Always handle errors when working with file operations to prevent your application from crashing.
Reading Files with Callbacks
Here's how to read a file using the traditional callback pattern:
myfile.txt
This is the content of myfile.txtCreate a Node.js file that reads the text file, and return the content:
const fs = require('fs');
// Read file asynchronously with callback
fs.readFile('myfile.txt', 'utf8', (err, data) => {
if (err) {
console.error('Error reading file:', err);
return;
}
console.log('File content:', data);
});
// For binary data (like images), omit the encoding
fs.readFile('image.png', (err, data) => {
if (err) throw err;
// data is a Buffer containing the file content
console.log('Image size:', data.length, 'bytes');
});Reading Files with Promises (Modern Approach)
Using fs.promises or util.promisify for cleaner async/await syntax:
// Using fs.promises (Node.js 10.0.0+)
const fs = require('fs').promises;
async function readFileExample() {
try {
const data = await fs.readFile('myfile.txt', 'utf8');
console.log('File content:', data);
} catch (err) {
console.error('Error reading file:', err);
}
}
readFileExample();
// Or with util.promisify (Node.js 8.0.0+)
const { promisify } = require('util');
const readFileAsync = promisify(require('fs').readFile);
async function readWithPromisify() {
try {
const data = await readFileAsync('myfile.txt', 'utf8');
console.log(data);
} catch (err) {
console.error(err);
}
}
readWithPromisify();Reading Files Synchronously
For simple scripts, you can use synchronous methods, but avoid them in production servers as they block the event loop:
const fs = require('fs');
try {
// Read file synchronously
const data = fs.readFileSync('myfile.txt', 'utf8');
console.log('File content:', data);
} catch (err) {
console.error('Error reading file:', err);
}Best Practice: Always specify the character encoding (like 'utf8') when reading text files to get a string instead of a Buffer.
Creating and Writing Files
Node.js provides several methods for creating and writing to files.
Using fs.writeFile()
Creates a new file or overwrites an existing file with the specified content:
const fs = require('fs').promises;
async function writeFileExample() {
try {
// Write text to a file
await fs.writeFile('myfile.txt', 'Hello, World!', 'utf8');
// Write JSON data
const data = { name: 'John', age: 30, city: 'New York' };
await fs.writeFile('data.json', JSON.stringify(data, null, 2), 'utf8');
console.log('Files created successfully');
} catch (err) {
console.error('Error writing files:', err);
}
}
writeFileExample();Using fs.appendFile()
Appends content to a file, creating the file if it doesn't exist:
const fs = require('fs').promises;
async function appendToFile() {
try {
// Append a timestamped log entry
const logEntry = `${new Date().toISOString()}: Application started\n`;
await fs.appendFile('app.log', logEntry, 'utf8');
console.log('Log entry added');
} catch (err) {
console.error('Error appending to file:', err);
}
}
appendToFile();Using File Handles
For more control over file operations, you can use file handles:
const fs = require('fs').promises;
async function writeWithFileHandle() {
let fileHandle;
try {
// Open a file for writing (creates if doesn't exist)
fileHandle = await fs.open('output.txt', 'w');
// Write content to the file
await fileHandle.write('First line\n');
await fileHandle.write('Second line\n');
await fileHandle.write('Third line\n');
console.log('Content written successfully');
} catch (err) {
console.error('Error writing to file:', err);
} finally {
// Always close the file handle
if (fileHandle) {
await fileHandle.close();
}
}
}
writeWithFileHandle();Using Streams for Large Files
For writing large amounts of data, use streams to avoid high memory usage:
const fs = require('fs');
const { pipeline } = require('stream/promises');
const { Readable } = require('stream');
async function writeLargeFile() {
// Create a readable stream (could be from HTTP request, etc.)
const data = Array(1000).fill().map((_, i) => `Line ${i + 1}: ${'x'.repeat(100)}\n`);
const readable = Readable.from(data);
// Create a writable stream to a file
const writable = fs.createWriteStream('large-file.txt');
try {
// Pipe the data from readable to writable
await pipeline(readable, writable);
console.log('Large file written successfully');
} catch (err) {
console.error('Error writing file:', err);
}
}
writeLargeFile();File Flags: When opening files, you can specify different modes:
- 'w' - Open for writing (file is created or truncated)
- 'wx' - Like 'w' but fails if the path exists
- 'w+' - Open for reading and writing (file is created or truncated)
- 'a' - Open for appending (file is created if it doesn't exist)
- 'ax' - Like 'a' but fails if the path exists
- 'r+' - Open for reading and writing (file must exist)
Deleting Files and Directories
Node.js provides several methods to delete files and directories.
Here's how to handle different deletion scenarios:
Deleting a Single File
Use fs.unlink() to delete a file:
const fs = require('fs').promises;
async function deleteFile() {
const filePath = 'file-to-delete.txt';
try {
// Check if file exists before deleting
await fs.access(filePath);
// Delete the file
await fs.unlink(filePath);
console.log('File deleted successfully');
} catch (err) {
if (err.code === 'ENOENT') {
console.log('File does not exist');
} else {
console.error('Error deleting file:', err);
}
}
}
deleteFile();Deleting Multiple Files
To delete multiple files, you can use Promise.all() with fs.unlink():
const fs = require('fs').promises;
const path = require('path');
async function deleteFiles() {
const filesToDelete = [
'temp1.txt',
'temp2.txt',
'temp3.txt'
];
try {
// Delete all files in parallel
await Promise.all(
filesToDelete.map(file =>
fs.unlink(file).catch(err => {
if (err.code !== 'ENOENT') {
console.error(`Error deleting ${file}:`, err);
}
})
)
);
console.log('Files deleted successfully');
} catch (err) {
console.error('Error during file deletion:', err);
}
}
deleteFiles();Deleting Directories
To delete directories, you have several options depending on your needs:
const fs = require('fs').promises;
const path = require('path');
async function deleteDirectory(dirPath) {
try {
// Check if the directory exists
const stats = await fs.stat(dirPath);
if (!stats.isDirectory()) {
console.log('Path is not a directory');
return;
}
// For Node.js 14.14.0+ (recommended)
await fs.rm(dirPath, { recursive: true, force: true });
// For older Node.js versions (deprecated but still works)
// await fs.rmdir(dirPath, { recursive: true });
console.log('Directory deleted successfully');
} catch (err) {
if (err.code === 'ENOENT') {
console.log('Directory does not exist');
} else {
console.error('Error deleting directory:', err);
}
}
}
// Usage
deleteDirectory('directory-to-delete');Emptying a Directory Without Deleting It
To remove all files and subdirectories within a directory but keep the directory itself:
const fs = require('fs').promises;
const path = require('path');
async function emptyDirectory(dirPath) {
try {
// Read the directory
const files = await fs.readdir(dirPath, { withFileTypes: true });
// Delete all files and directories in parallel
await Promise.all(
files.map(file => {
const fullPath = path.join(dirPath, file.name);
return file.isDirectory()
? fs.rm(fullPath, { recursive: true, force: true })
: fs.unlink(fullPath);
})
);
console.log('Directory emptied successfully');
} catch (err) {
console.error('Error emptying directory:', err);
}
}
// Usage
emptyDirectory('directory-to-empty');Security Note: Be extremely careful with file deletion, especially when using recursive options or wildcards. Always validate and sanitize file paths to prevent directory traversal attacks.
Renaming and Moving Files
The fs.rename() method can be used for both renaming and moving files.
It's a versatile method for file system operations that involve changing file paths.
Basic File Renaming
To rename a file in the same directory:
const fs = require('fs').promises;
async function renameFile() {
const oldPath = 'old-name.txt';
const newPath = 'new-name.txt';
try {
// Check if source file exists
await fs.access(oldPath);
// Check if destination file already exists
try {
await fs.access(newPath);
console.log('Destination file already exists');
return;
} catch (err) {
// Destination doesn't exist, safe to proceed
}
// Perform the rename
await fs.rename(oldPath, newPath);
console.log('File renamed successfully');
} catch (err) {
if (err.code === 'ENOENT') {
console.log('Source file does not exist');
} else {
console.error('Error renaming file:', err);
}
}
}
// Usage
renameFile();Moving Files Between Directories
You can use fs.rename() to move files between directories:
const fs = require('fs').promises;
const path = require('path');
async function moveFile() {
const sourceFile = 'source/file.txt';
const targetDir = 'destination';
const targetFile = path.join(targetDir, 'file.txt');
try {
// Ensure source file exists
await fs.access(sourceFile);
// Create target directory if it doesn't exist
await fs.mkdir(targetDir, { recursive: true });
// Move the file
await fs.rename(sourceFile, targetFile);
console.log('File moved successfully');
} catch (err) {
if (err.code === 'ENOENT') {
console.log('Source file does not exist');
} else if (err.code === 'EXDEV') {
console.log('Cross-device move detected, using copy+delete fallback');
await moveAcrossDevices(sourceFile, targetFile);
} else {
console.error('Error moving file:', err);
}
}
}
// Helper function for cross-device moves
async function moveAcrossDevices(source, target) {
try {
// Copy the file
await fs.copyFile(source, target);
// Delete the original
await fs.unlink(source);
console.log('File moved across devices successfully');
} catch (err) {
// Clean up if something went wrong
try { await fs.unlink(target); } catch (e) {}
throw err;
}
}
// Usage
moveFile();Batch Renaming Files
To rename multiple files matching a pattern:
const fs = require('fs').promises;
const path = require('path');
async function batchRename() {
const directory = 'images';
const pattern = /^image(\d+)\.jpg$/;
try {
// Read directory contents
const files = await fs.readdir(directory);
// Process each file
for (const file of files) {
const match = file.match(pattern);
if (match) {
const [_, number] = match;
const newName = `photo-${number.padStart(3, '0')}.jpg`;
const oldPath = path.join(directory, file);
const newPath = path.join(directory, newName);
// Skip if the new name is the same as the old name
if (oldPath !== newPath) {
await fs.rename(oldPath, newPath);
console.log(`Renamed: ${file} - ${newName}`);
}
}
}
console.log('Batch rename completed');
} catch (err) {
console.error('Error during batch rename:', err);
}
}
batchRename();Atomic Rename Operations
For critical operations, use a temporary file to ensure atomicity:
const fs = require('fs').promises;
const path = require('path');
const os = require('os');
async function updateFileAtomic(filePath, newContent) {
const tempPath = path.join(
os.tmpdir(),
`temp-${Date.now()}-${Math.random().toString(36).substr(2, 9)}`
);
try {
// 1. Write to temp file
await fs.writeFile(tempPath, newContent, 'utf8');
// 2. Verify the temp file was written correctly
const stats = await fs.stat(tempPath);
if (stats.size === 0) {
throw new Error('Temporary file is empty');
}
// 3. Rename (atomic on most systems)
await fs.rename(tempPath, filePath);
console.log('File updated atomically');
} catch (err) {
// Clean up temp file if it exists
try { await fs.unlink(tempPath); } catch (e) {}
console.error('Atomic update failed:', err);
throw err;
}
}
// Usage
updateFileAtomic('important-config.json', JSON.stringify({ key: 'value' }, null, 2));Cross-Platform Note: The fs.rename() operation is atomic on Unix-like systems but may not be on Windows.
For cross-platform atomic operations, consider using a temporary file approach as shown in the example above.