How to create a directory if it doesn't exist using Node.js?

var fs = require('fs');
var dir = './tmp';

if (!fs.existsSync(dir)){
    fs.mkdirSync(dir);
}

No, for multiple reasons.

1. The path module does not have an exists/existsSync method. It is in the fs module. (Perhaps you just made a typo in your question?)

2. The docs explicitly discourage you from using exists.

   fs.exists() is an anachronism and exists only for historical reasons. There should almost never be a reason to use it in your own code.

   In particular, checking if a file exists before opening it is an anti-pattern that leaves you vulnerable to race conditions: another process may remove the file between the calls to fs.exists() and fs.open(). Just open the file and handle the error when it's not there.

   Since we're talking about a directory rather than a file, this advice implies you should just unconditionally call mkdir and ignore EEXIST.

3. In general, You should avoid the *Sync methods. They're blocking, which means absolutely nothing else in your program can happen while you go to the disk. This is a very expensive operation, and the time it takes breaks the core assumption of node's event loop.

   The *Sync methods are usually fine in single-purpose quick scripts (those that do one thing and then exit), but should almost never be used when you're writing a server: your server will be unable to respond to anyone for the entire duration of the I/O requests. If multiple client requests require I/O operations, your server will very quickly grind to a halt.

   ---

   The only time I'd consider using *Sync methods in a server application is in an operation that happens once (and only once), at startup. For example, require actually uses readFileSync to load modules.

   Even then, you still have to be careful because lots of synchronous I/O can unnecessarily slow down your server's startup time.

   ---

   Instead, you should use the asynchronous I/O methods.

So if we put together those pieces of advice, we get something like this:

function ensureExists(path, mask, cb) {
    if (typeof mask == 'function') { // allow the `mask` parameter to be optional
        cb = mask;
        mask = 0777;
    }
    fs.mkdir(path, mask, function(err) {
        if (err) {
            if (err.code == 'EEXIST') cb(null); // ignore the error if the folder already exists
            else cb(err); // something else went wrong
        } else cb(null); // successfully created folder
    });
}

And we can use it like this:

ensureExists(__dirname + '/upload', 0744, function(err) {
    if (err) // handle folder creation error
    else // we're all good
});

---

Of course, this doesn't account for edge cases like

• What happens if the folder gets deleted while your program is running? (assuming you only check that it exists once during startup)
• What happens if the folder already exists but with the wrong permissions?

The mkdir method has the ability to recursively create any directories in a path that don't exist, and ignore the ones that do.

From the Node v10/11 docs:

// Creates /tmp/a/apple, regardless of whether `/tmp` and /tmp/a exist.
fs.mkdir('/tmp/a/apple', { recursive: true }, (err) => {
    if (err) throw err;
});

NOTE: You'll need to import the built-in fs module first.

Now here's a little more robust example that leverages native ES Modules (with flag enabled and .mjs extension), handles non-root paths, and accounts for full pathnames:

import fs from 'fs';
import path from 'path';

createDirectories(pathname) {
   const __dirname = path.resolve();
   pathname = pathname.replace(/^\.*\/|\/?[^\/]+\.[a-z]+|\/$/g, ''); // Remove leading directory markers, and remove ending /file-name.extension
   fs.mkdir(path.resolve(__dirname, pathname), { recursive: true }, e => {
       if (e) {
           console.error(e);
       } else {
           console.log('Success');
       }
    });
}

You can use it like createDirectories('/components/widget/widget.js');.

And of course, you'd probably want to get more fancy by using promises with async/await to leverage file creation in a more readable synchronous-looking way when the directories are created; but, that's beyond the question's scope.