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Node JS fs.readFileSync() Method - GeeksforGeeks

Last Updated : 12 Jul, 2025

In NodeJS, the fs.readFileSync() method is used to read files from the filesystem in a synchronous manner. This means that when you use this method, your program will pause and wait until the file has been completely read before moving on to the next task.

This behavior can be useful when you need to ensure that the file's content is available immediately, such as during the initial setup of your application.​

Syntax:

fs.readFileSync(path, options);

In this syntax:

• path: A string representing the path to the file you want to read. This can be a relative or absolute path.​
• options (optional): An object or string that specifies the encoding and flag. If not provided, it defaults to null for encoding (returning a buffer) and 'r' for the flag (open for reading).

Return Value:

• Returns the content of the file. If no encoding is specified, it returns a Buffer object; otherwise, it returns a string.

When you call fs.readFileSync(), NodeJS reads the file from the disk and blocks the rest of the program from executing until the file has been completely read. This ensures that the file's data is available immediately after the method call, which can be beneficial in certain situations but may affect performance if used improperly.​

Now, let us understand with the help of the example:

Suppose you have a file named input.txt with the following content:

This is some text data stored in input.txt.

You can read this file synchronously using fs.readFileSync() as follows:

// Include the fs module
const fs = require('fs');

// Read the file synchronously
const data = fs.readFileSync('./input.txt', { encoding: 'utf8', flag: 'r' });

// Display the file content
console.log(data);

Output:

This is some text data stored in input.txt.

Understanding the difference between fs.readFileSync() and fs.readFile() is important for effective file handling in NodeJS:​

• fs.readFileSync(): Reads the file synchronously, blocking the event loop until the operation is complete. This is suitable when you need the file's content immediately and can afford to pause the program's execution.​
• fs.readFile(): Reads the file asynchronously, allowing other operations to continue while the file is being read. This is preferable in performance-critical applications where non-blocking behavior is desired.

Now, let us understand with the help of the example:

// Include the fs module
const fs = require('fs');

// Asynchronously read 'input1.txt'
fs.readFile('./input1.txt', { encoding: 'utf8', flag: 'r' }, (err, data1) => {
  if (err) {
    console.error('Error reading input1.txt:', err);
  } else {
    console.log('input1.txt content:', data1);
  }
});

// Synchronously read 'input2.txt'
try {
  const data2 = fs.readFileSync('./input2.txt', { encoding: 'utf8', flag: 'r' });
  console.log('input2.txt content:', data2);
} catch (err) {
  console.error('Error reading input2.txt:', err);
}

Output:

input1.txt content: (Content of input1.txt)
input2.txt content: (Content of input2.txt)

• fs.readFile() initiates an asynchronous read and uses a callback to handle the data once it's available. The program continues executing other code without waiting for the file read to complete.​
• fs.readFileSync() blocks the execution until the file is fully read, ensuring that the data is available immediately after the call.

• Working with small files, such as configuration or initialization files, where blocking the execution has minimal impact.​
• Ensuring that the file's content is available immediately before proceeding with other operations.

• Handling large files or performing operations where non-blocking I/O is essential to maintain application responsiveness.​
• Developing applications that require high performance and cannot afford to block the event loop, such as web servers handling multiple requests.

• fs.readFileSync() blocks the program until it finishes reading a file, it can slow things down when reading large files or when multiple tasks need to be handled at the same time. For better performance, use fs.readFile() (asynchronous) or promises.

• If you're building a web server with NodeJS (like using Express), fs.readFileSync() can make your server slower. Each request might get blocked while waiting for the file to be read, slowing down the server's response time.

The fs.readFileSync() method in NodeJS provides a straightforward way to read files synchronously, ensuring that the file's content is available immediately after the method call. However, it's important to use this method judiciously, as it blocks the event loop and can impact performance, especially in applications that require high concurrency. For non-blocking file reading, consider using the asynchronous fs.readFile() method.

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