Before you’re able to interact with a file that sits in your filesystem, you must get a file descriptor.

A file descriptor is what’s returned by opening the file using the open() method offered by the fs module:

const fs = require('fs')

fs.open('/Users/flavio/test.txt', 'r', (err, fd) => {
  //fd is our file descriptor
})

Notice the r we used as the second parameter to the fs.open() call.

That flag means we open the file for reading.

Other flags you’ll commonly 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 also open the file by using the fs.openSync method, which instead of providing the file descriptor object in a callback, it returns it:

const fs = require('fs')

try {
  const fd = fs.openSync('/Users/flavio/test.txt', 'r')
} catch (err) {
  console.error(err)
}

Once you get the file descriptor, in whatever way you choose, you can perform all the operations that require it, like calling fs.open() and many other operations that interact with the filesystem.