Node.js File System Tutorial - Read, Write Files

Node.js File System

In Node.js, file I/O operations are built upon simple wrappers around standard POSIX functions. The File System (fs) module in Node.js enables developers to work with files easily — from reading and writing to updating and deleting files.

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To use the File System module, you can import it using the following syntax:

var fs = require("fs");

Node.js FS – Reading a File

Every method in the fs module has both synchronous and asynchronous versions.

Asynchronous methods take a callback function as the last parameter and do not block the event loop, allowing other code to execute while the file operation is in progress.
Synchronous methods, on the other hand, block program execution until the operation completes.

The asynchronous approach is generally preferred for better performance and scalability.

Let’s look at an example.

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Create a text file named input.txt with the following content:

File: input.txt

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Now, create a JavaScript file named main.js with the following code:

File: main.js

var fs = require("fs");  

// Asynchronous read  
fs.readFile('input.txt', function (err, data) {  
   if (err) {  
       return console.error(err);  
   }  
   console.log("Asynchronous read: " + data.toString());  
});  

// Synchronous read  
var data = fs.readFileSync('input.txt');  
console.log("Synchronous read: " + data.toString());  

console.log("Program Ended");

Run the code in the Node.js terminal using the command:

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node main.js

Node.js – Opening a File

You can open a file in asynchronous mode using the following syntax:

fs.open(path, flags[, mode], callback)

Parameters:

path: The name or path of the file to be opened.
flags: Specifies how the file should be opened (read, write, append, etc.).
mode: Defines the file permissions (default is 0666, which allows read and write).
callback: A function that receives two arguments — err and fd (file descriptor).

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Common File Open Flags

Flag	Description
`r`	Open file for reading. Throws an error if the file doesn’t exist.
`r+`	Open file for reading and writing. Throws an error if the file doesn’t exist.
`rs`	Open file for reading in synchronous mode.
`rs+`	Open file for reading and writing in synchronous mode.
`w`	Open file for writing. Creates the file if it doesn’t exist, or truncates it if it does.
`wx`	Like `w` but fails if the file already exists.
`w+`	Open file for reading and writing. Creates or truncates as needed.
`wx+`	Like `w+` but fails if the file exists.
`a`	Open file for appending. Creates the file if it doesn’t exist.
`ax`	Like `a` but fails if the file exists.
`a+`	Open file for reading and appending. Creates the file if it doesn’t exist.
`ax+`	Like `a+` but fails if the file exists.

Here’s an example of opening a file asynchronously:

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File: main.js

var fs = require("fs");  

console.log("Going to open file!");  
fs.open('input.txt', 'r+', function(err, fd) {  
   if (err) {  
       return console.error(err);  
   }  
   console.log("File opened successfully!");       
});

Run the program using:

node main.js

Node.js – File Information

You can get detailed information about a file using the fs.stat() method.

Syntax:

fs.stat(path, callback)

Parameters:

path: The name or path of the file.
callback: Function with two parameters — err and stats. The stats object is an instance of fs.Stats and provides various file properties and methods.

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Common `fs.Stats` Methods

Method	Description
`stats.isFile()`	Returns `true` if the file is a regular file.
`stats.isDirectory()`	Returns `true` if it’s a directory.
`stats.isBlockDevice()`	Returns `true` if it’s a block device.
`stats.isCharacterDevice()`	Returns `true` if it’s a character device.
`stats.isSymbolicLink()`	Returns `true` if it’s a symbolic link.
`stats.isFIFO()`	Returns `true` if it’s a FIFO.
`stats.isSocket()`	Returns `true` if it’s a socket.

Example:

File: main.js

var fs = require("fs");  

console.log("Going to get file info!");  
fs.stat('input.txt', function (err, stats) {  
   if (err) {  
       return console.error(err);  
   }  
   console.log(stats);  
   console.log("Got file info successfully!");  
   console.log("isFile ? " + stats.isFile());  
   console.log("isDirectory ? " + stats.isDirectory());      
});

Run the script using:

node main.js

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Node.js File System (FS)