Checking file existence

A fairly common task in systems and server side programming, is checking whether a file exists or not.

There is a method, fs.exists (and its sync cousin, fs.existsSync), which allows us to perform this very action. However, it has been deprecated since Node version 4, and therefore isn't future-safe.

A better practice is to use fs.access (added when fs.exists was deprecated).

By default fs.access checks purely for file visibility, which is essentially the equivalent of checking for existence.

Let's write a file called check.js; we can pass it a file and it will tell us whether the file exists or not:

const fs = require('fs') 
 
const exists = (file) => new Promise((resolve, reject) => { 
  fs.access(file, (err) => { 
    if (err) { 
      if (err.code !== 'ENOENT') { return reject(err) } 
      return resolve({file, exists: false}) 
    } 
    resolve({file, exists: true}) 
  })   
}) 
 
exists(process.argv[2]) 
  .then(({file, exists}) => console.log(`"${file}" does${exists ? '' : ' not'} exist`)) 
  .catch(console.error) 

Now if we run the following:

$ node check.js non-existent-file 
"non-existent-file" does not exist 

But if we run:

$ node check.js check.js 
"check.js" does exist 

The fs.access method is more versatile than fs.exists. It can be passed different modes (fs.F_OK (default), fs.R_OK, fs.W_OK, and fs.X_OK) to alter access checks being made. The mode is a number, and the constants of fs (ending in _OK) are numbers, allowing for a bitmask approach.

For instance, here's how we can check if we have permissions to read, write, and execute a file (this time with fs.accessSync):

fs.access('/usr/local/bin/node', fs.R_OK | fs.W_OK | fs.X_OK, console.log) 

If there's a problem accessing, an error will be logged, if not, null will be logged.