Node.js fs.writeFileSync() Method

fs.writeFileSync() is a method in Node.js used to write data to a file synchronously. It blocks the execution of further code until the file writing operation is completed.

• Writes data to a file and creates the file if it does not exist.
• Overwrites existing content by default (unless flags are specified).
• Being synchronous, it can block the event loop during execution.

Node.js File System (fs) Module

Node.js File System (fs) Module is a built-in module that allows developers to work with the file system on their computer. It provides methods to create, read, update, delete, and manage files and directories.

• Supports both synchronous and asynchronous file operations.
• Used for tasks like reading files, writing files, renaming, deleting, and managing folders.
• Commonly used in server-side applications to handle file storage and data management.

Node.js fs.writeFileSync()

fs.writeFileSync() is a method in the Node.js File System module used to write data to a file synchronously. It ensures that the file writing process is completed before moving to the next line of code.

• Creates a new file if it does not exist.
• Overwrites existing file content by default.
• Blocks the execution (event loop) until the write operation finishes.

Syntax:

fs.writeFileSync( file, data, options )

Parameters: This method accepts three parameters as mentioned above and described below:

1. file: It is a string, Buffer, URL, or file description integer that denotes the path of the file where it has to be written. Using a file descriptor will make it behave similarly to the fs.write() method.

2. data: It is a string, Buffer, TypedArray, or DataView that will be written to the file.

3. options: It is a string or object that can be used to specify optional parameters that will affect the output. It has three optional parameters:

• encoding: It is a string that specifies the encoding of the file. The default value is 'utf8'.
• mode: It is an integer that specifies the file mode. The default value is 0o666.
• flag: It is a string that specifies the flag used while writing to the file. The default value is 'w'.

Features

• Synchronous Operation: Ensures that file writing is completed before moving on to the next operation.
• File Creation and Overwriting: Creates a file if it doesn't exist, or overwrites the file if it does.
• Flexible Data Input: Can handle strings, buffers, and other data types.
• Error Handling: Throws errors for invalid file paths or permission issues.
• Custom Encoding: Supports custom encoding (default is UTF-8).
• File Permissions: Allows you to set file permissions.

Here, we implement the fs.writeFileSync() method in Node.js.

// Filename - index.js

// Node.js program to demonstrate the 
// fs.writeFileSync() method 

// Import the filesystem module 
const fs = require('fs'); 

let data = "This is a file containing a collection"
		+ " of programming languages.\n"
+ "1. C\n2. C++\n3. Python"; 

fs.writeFileSync("programming.txt", data); 
console.log("File written successfully\n"); 
console.log("The written has the following contents:"); 
console.log(fs.readFileSync("programming.txt", "utf8")); 

Output:

File written successfully

The written has the following contents:
This is a file containing a collection of programming languages.
1. C
2. C++
3. Python

Example 2: This appends the names of five movies to a file named "movies.txt" five times and then reads and displays the file's contents.

// Filename - index.js

// Node.js program to demonstrate the 
// fs.writeFileSync() method 

// Import the filesystem module 
const fs = require('fs'); 

// Writing to the file 5 times 
// with the append file mode 
for (let i = 0; i < 5; i++) { 
fs.writeFileSync("movies.txt", 
	"Movie " + i + "\n", 
	{ 
	encoding: "utf8", 
	flag: "a+", 
	mode: 0o666 
	}); 
} 

console.log("File written successfully 5 times\n"); 
console.log("The written file has the following contents:"); 
console.log(fs.readFileSync("movies.txt", "utf8"));

Output:

File written successfully 5 times

The written file has the following contents:
Movie 0
Movie 1
Movie 2
Movie 3
Movie 4

Example 3: Taking runtime input from users for file name and file data using readline module 

// Filename - index.js

let readline = require('readline-sync');
let fs = require("fs");

let path = readline.question("Enter file name/path: ");

console.log("Entered path : " + path);

let data = readline.question("Enter file data: ");

//synchronous functions may throw errors 
//which can be handled using try-catch block
try {
    fs.writeFileSync(path, data, { flag: 'a+' }); //'a+' is append mode
    console.log("File written successfully");
} catch (err) {
    console.error(err);
}
console.log("-----------------------------------------------");
try {
    const data = fs.readFileSync(path, { encoding: "utf8" });
    console.log("File content is as follows:");
    // Display the file data 
    console.log(data);
} catch (err) {
    console.log(err);
}

Output 

Example 4: Taking runtime input from users for file data using readline module using buffer.

// Filename - index.js

let fs = require("fs");
let readline = require('readline-sync');
let path = readline.question("Enter file name/path: ");

console.log("Entered path : " + path);

// 1024 specifies the buffer size. We can limit 
// the data size by this approach
let buf = new Buffer.alloc(1024);
buf = readline.question("Enter data:");


try {
    fs.writeFileSync(path, buf, { flag: 'a+' });
    console.log("File written successfully");
} catch (err) {
    console.error(err);
}
console.log("-----------------------------------------------");
try {
    const data = fs.readFileSync(path, { encoding: "utf8" });
    console.log("File content is as follows:");
    // Display the file data 
    console.log(data);
} catch (err) {
    console.log(err);
}

Output