The Node fs module

The fs module gives you access to the file system. You do not install it. It is part of Node core:

const fs = require('fs')

Once you require it, you get dozens of methods, including:

• fs.access(): check if the file exists and Node can access it with its permissions
• fs.appendFile(): append data to a file. If the file does not exist, it’s created
• fs.chmod(): change the permissions of a file specified by the filename passed. Related: fs.lchmod(), fs.fchmod()
• fs.chown(): change the owner and group of a file specified by the filename passed. Related: fs.fchown(), fs.lchown()
• fs.close(): close a file descriptor
• fs.copyFile(): copies a file
• fs.createReadStream(): create a readable file stream
• fs.createWriteStream(): create a writable file stream
• fs.link(): create a new hard link to a file
• fs.mkdir(): create a new folder
• fs.mkdtemp(): create a temporary directory
• fs.open(): set the file mode
• fs.readdir(): read the contents of a directory
• fs.readFile(): read the content of a file. Related: fs.read()
• fs.readlink(): read the value of a symbolic link
• fs.realpath(): resolve relative file path pointers (., ..) to the full path
• fs.rename(): rename a file or folder
• fs.rmdir(): remove a folder
• fs.stat(): returns the status of the file identified by the filename passed. Related: fs.fstat(), fs.lstat()
• fs.symlink(): create a new symbolic link to a file
• fs.truncate(): truncate to the specified length the file identified by the filename passed. Related: fs.ftruncate()
• fs.unlink(): remove a file or a symbolic link
• fs.unwatchFile(): stop watching for changes on a file
• fs.utimes(): change the timestamp of the file identified by the filename passed. Related: fs.futimes()
• fs.watchFile(): start watching for changes on a file. Related: fs.watch()
• fs.writeFile(): write data to a file. Related: fs.write()

One peculiar thing about fs: every method is asynchronous by default, but you can also call a synchronous version by appending Sync:

• fs.rename()
• fs.renameSync()
• fs.write()
• fs.writeSync()

That choice has a huge impact on your application flow.

Node 10 includes experimental support for a promise based API

Let’s look at fs.rename(). The asynchronous API takes a callback:

const fs = require('fs')

fs.rename('before.json', 'after.json', (err) => {
  if (err) {
    return console.error(err)
  }

  //done
})

The synchronous version uses try/catch:

const fs = require('fs')

try {
  fs.renameSync('before.json', 'after.json')
  //done
} catch (err) {
  console.error(err)
}

The key difference: in the second example, your script blocks until the file operation finishes.