You may want to create a Node module that is also an executable. The convention is to create two files, one for lib and the other for bin, but I think it's OK for simple modules.

To achieve it, you need to detect whether it's executed as an entry point. According to the official documentation of Node:

When a file is run directly from Node.js, require.main is set to its module. That means that you can determine whether a file has been run directly by testing

require.main === module

For example:

#!/usr/bin/env node

function sum(a, b) {
  return a + b;
}

module.exports = sum;

if (require.main === module) {
  var a = parseInt(process.argv[2], 10);
  var b = parseInt(process.argv[3], 10);
  console.log(sum(a, b));
}

Then you can use it as an executable and a module.

$ node sum.js 3 4
7
$ node
> require('./sum')(3, 4)
7