Writing files with Node.js

Writing a file

The easiest way to write to files in Node.js is to use the fs.writeFile() API.

const fs = require('node:fs')
const content = 'Some content!'

fs.writeFile('/Users/joe/test.txt', content, err => {
  if (err) {
    console.error(err)
  } else {
    // file written successfully
  }
})

Writing a file synchronously

Alternatively, you can use the synchronous version fs.writeFileSync:

const fs = require('node:fs')
const content = 'Some content!'

try {
  fs.writeFileSync('/Users/joe/test.txt', content)
} catch (err) {
  console.error(err)
}

You can also use the promise-based fsPromises.writeFile() method offered by the fs/promises module:

const fs = require('node:fs/promises')
async function example() {
  try {
    const content = 'Some content!'
    await fs.writeFile('/Users/joe/test.txt', content)
  } catch (err) {
    console.log(err)
  }
}

example()

By default, this API will replace the contents of the file if it does already exist.

You can modify the default by specifying a flag:

fs.writeFile('/Users/joe/test.txt', content, { flag: 'a+' }, err => [])

The flags you'll likely use are:

Flag	Description	File gets created if it doesn't exist
r+	This flag opens the file for reading and writing	❌
w+	This flag opens the file for reading and writing and it also positions the stream at the beginning of the file	✅
a	This flag opens the file for writing and it also positions the stream at the end of the file	✅
a+	This stream opens the file for reading and writing and it also positions the stream at the end of the file	✅

You can find more information about the flags in the fs documentation.

Appending content to a file

Appending to files is handy when you don't want to overwrite a file with new content, but rather add to it.

Examples

A handy method to append content to the end of a file is fs.appendFile() (and its fs.appendFileSync() counterpart):

const fs = require('node:fs')
const content = 'Some content!'

fs.appendFile('file_log', content, err => {
  if (err) {
    console.error(err)
  } else {
    // done!
  }
})

Example with Promises

Here's a fsPromises.appendFile() example:

const fs = require('node:fs/promises')
async function example() {
  try {
    const content = 'Some content!'
    await fs.appendFile('/Users/joe/test.txt', content)
  } catch (err) {
    console.log(err)
  }
}

example()