How to writing to files in Node.js ?

To write to files in Node.js, use the fs (File System) module. Here are the key methods with examples:

1. fs.writeFile() (Asynchronous – Non-Blocking)

Writes data to a file, replacing it if it exists.

const fs = require('fs');

// Basic write (UTF-8 encoding)
fs.writeFile('file.txt', 'Hello World', (err) => {
  if (err) throw err;
  console.log('File saved!');
});

// With options (append instead of overwrite)
fs.writeFile('log.txt', 'New log entry\n', { flag: 'a' }, (err) => {
  if (err) throw err;
});

2. fs.writeFileSync() (Synchronous – Blocking)

Synchronous version of writeFile:

const fs = require('fs');

try {
  fs.writeFileSync('sync-file.txt', 'Blocking write');
  console.log('File saved synchronously');
} catch (err) {
  console.error(err);
}

3. fs.appendFile() (Append Content Asynchronously)

Adds content to the end of a file:

const fs = require('fs');

fs.appendFile('notes.txt', 'New note\n', (err) => {
  if (err) throw err;
  console.log('Note appended');
});

4. fs.createWriteStream() (Stream for Large Files)

Efficient for writing large datasets incrementally:

const fs = require('fs');
const stream = fs.createWriteStream('bigfile.txt', { flags: 'a' });

stream.write('Chunk 1\n');
stream.write('Chunk 2\n');
stream.end(() => console.log('Stream finished'));

// Handle errors
stream.on('error', (err) => console.error('Stream error:', err));

5. fs.promises.writeFile() (Promise-Based API)

Modern async/await syntax:

const fs = require('fs/promises');

async function saveData() {
  try {
    await fs.writeFile('data.json', JSON.stringify({ key: 'value' }));
    console.log('Data saved');
  } catch (err) {
    console.error('Error:', err);
  }
}

saveData();

Flags for File Operations

Flag	Description
w	Overwrite (default for writeFile)
a	Append to file
wx	Write only if file doesn’t exist
r+	Read and write

Key Notes

1. Encoding
   Specify encoding in options (e.g., { encoding: 'utf8' }). Default is UTF-8.
2. Error Handling
   Always handle errors in callbacks or use try/catch for synchronous code.
3. Performance

• Use streams for large files to avoid memory overload.
• Prefer asynchronous methods for non-blocking operations.

Full Example: Write & Append

const fs = require('fs');

// Write initial file
fs.writeFile('example.txt', 'Start of log\n', (err) => {
  if (err) throw err;

  // Append after initial write
  fs.appendFile('example.txt', 'Log entry 1\n', (err) => {
    if (err) throw err;
    console.log('Log updated');
  });
});

Comparison Table

Method	Use Case	Async/Sync
writeFile()	Simple writes, small files	Async
writeFileSync()	Scripts needing blocking I/O	Sync
appendFile()	Adding to logs or existing files	Async
createWriteStream()	Large files (e.g., videos, CSVs)	Async