When working with file system in Node, you will need a fully qualified filename to do things like reading the content of a file:

require('fs').readFile(filename, function (err, data) {});

If you have the following directory structure:

.
├── app.js
├── data
│   └── file.txt
└── lib
    └── reader.js

There are two ways to read the content of data/file.txt from lib/reader.js:

Use the current directory of the script __dirname:

filename = __dirname + '/../data/file.txt';

or with path.join to normalize the resulting path:

filename = path.join(__dirname, '../data/file.txt');

The second method is to use the current working directory of the process process.cwd():

filename = process.cwd() + '/data/file.txt';

The second approach is more portable. If you move lib/reader.js to lib/utils/reader.js, no code change will be needed. But make sure the directory of the Node process is the application root directory where node app.js is being issued.