The double underscores in __dirname looks intimidating but it’s not! It’s a useful variable that’s been around since the beginnging of the NodeJS project. Why’s it such a core feature of Node.js? __dirname tells you the absolute path of the directory containing the currently executing file.

Here’s a simple example:

gator-app
  ├──index.js
  ├──public
  ├──src
  │  ├──helpers.js
  │  └──api
  │      └──crocodile.js
  ├──cronjobs
  │  ├──swamp-pix
  │  └──get-latest-gators.js
  └──package.json

console.log(__dirname)      // "/Users/Sam/gator-app/src/api"
console.log(process.cwd())  // "/Users/Sam/gator-app"

console.log(__dirname)     // "/Users/Sam/gator-app/cronjobs"
console.log(process.cwd()) // "/Users/Sam/gator-app"

If you noticed, __dirname has a different value depending on which file you invoked it in, whereas process.cwd() (another popular Node.js utility) is different. It always returns the same value: the absolute path of where you started the Node.js process (eg., $ node index.js).

When Should I Use It?

__dirname is useful when you want to know the immediate containing folder. You may want to get this path for several reasons:

Making new directories

const fs = require('fs');
const path = require('path');
const dirPath = path.join(__dirname, '/swamp-pix');

fs.mkdirSync(dirPath); // sibling directory was created named "swamp-pix"

Pointing to directories

express.static(path.join(__dirname, '/public'));

Adding files to a directory

const fs = require('fs');
const path = require('path');
const filePath = path.join(__dirname, '/swamp-pix/bayou.jpeg');

fs.openSync(filePath, 'a'); // creates file if it doesn't exist

Once you get familiar with Node.js, interacting with your filesystem is easy when you use __dirname.

Plus, __dirname is a global object that’s available to use in all your Node.js modules, without having to import anything. That’s because Node modules get wrapped when executed and given access to a series of global objects.