# How to append data to a file in Node.js

By Atta Ur Rehman Shah (https://attacomsian.com/about). Published Mar 30, 2020, updated Aug 14, 2022. Topics: Node.js.
Canonical URL: https://attacomsian.com/blog/nodejs-append-data-to-file

> A quick article to learn how to append data to a file in a Node.js application.

To append data to a file in Node.js:

1. Use the `fs.appendFile()` method to asynchronously append data to a file.
2. Pass the file name, the contents to append, and a callback function as first, second, and third parameters.
3. The `fs.appendFile()` automatically creates a new file if it doesn't exist. 
4. The callback method gets called with either an error object or `null`.

Here is an example:

```javascript
const fs = require('fs')

// append data to a file
fs.appendFile('file.txt', 'Hey there!', err => {
  if (err) {
    throw err
  }
  console.log('File is updated.')
})
```

If you are appending text to a file that has a different encoding scheme than the default operating system encoding, you should pass the encoding scheme as the 3rd argument:

```javascript
fs.appendFile('file.txt', 'Hey there!', 'utf8', (err) => { })
```

The `fs` module provides another method called `fs.appendFileSync()` to synchronously append text to a file. It [blocks the Node.js event loop](https://attacomsian.com/blog/java-pause-sleep-delay-wait) until the file append operation is finished. 

Here is an example:

```javascript
const fs = require('fs')

// append data to a file
try {
  fs.appendFileSync('file.txt', 'Hey there!')

  console.log('File is updated.')
} catch (error) {
  console.log(error)
}
```

Both `fs.appendFile()` and `fs.appendFileSync()` methods create a new file handle each time they are called. They are only good for a one-off append operation. 

If you want to append repeatedly to the same file, for example, writing in a log file, do not use these methods. Instead, you should use the `fs.createWriteStream()` method that creates a writable stream and reuses the file handle to append new data. 

Here is an example:

```javascript
const fs = require('fs')

// create a stream
const stream = fs.createWriteStream('file.txt', { flags: 'a' })

// append data to the file
;[...Array(100)].forEach((num, index) => {
  stream.write(`${index}\n`)
})

// end stream
stream.end()
```

Read [this article](https://attacomsian.com/blog/reading-writing-files-nodejs) to learn more about reading and writing files in a Node.js application.