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

Example:

const fs = require('fs')

const content = 'Some content!'

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

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

const fs = require('fs')

const content = 'Some content!'

try {
  const data = fs.writeFileSync('/Users/flavio/test.txt', content)
  //file written successfully
} catch (err) {
  console.error(err)
}

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/flavio/test.txt', content, { flag: 'a+' }, (err) => {})

The flags you’ll likely use are

• r+ open the file for reading and writing
• w+ open the file for reading and writing, positioning the stream at the beginning of the file. The file is created if not existing
• a open the file for writing, positioning the stream at the end of the file. The file is created if not existing
• a+ open the file for reading and writing, positioning the stream at the end of the file. The file is created if not existing

(you can find more flags at https://nodejs.org/api/fs.html#fs_file_system_flags)

Append to a file

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

const content = 'Some content!'

fs.appendFile('file.log', content, (err) => {
  if (err) {
    console.error(err)
    return
  }
  //done!
})

Using streams

All those methods write the full content to the file before returning the control back to your program (in the async version, this means executing the callback)

In this case, a better option is to write the file content using streams.