Get input from stdin or filename

do-stuff.js

#!/usr/bin/env node

const fs = require('fs');

function getInput() {
    if (process.stdin.isTTY) {
        const inputFileName = process.argv[2];
        return fs.readFileSync(inputFileName, {encoding: 'utf8'});
    } else {
        return fs.readFileSync(0, {encoding: 'utf8'});
    }
}