Examples of using Node's fs to read a file from disk.

Standard fs.readfile & fs.createReadStream

//NB: assumes encoding is 'utf8'; adjust as necessary

const readFromDiskAsync = (file) => {
  fs.readFile(file, { encoding:'utf8'}, function read(err, data) {
    if (err) {throw err;}
    console.log('The data is --> ', data)
  })
}

const readFromDiskStream = (file) => {
  let newReadStream = fs.createReadStream(file, { encoding: 'utf8' })
  let results = '';
  newReadStream
    .on('ready', console.log('ready to stream'))
    .on('data', function processChunk(chunk) {
      console.log('processing a chunk')
      results += chunk;
    })
    .on('end', (results) => { console.log('The results are --> ', results)} )
}

Wrapping a ReadStream in a Promise

// Instead of putting everything into the .on('end') event, use a Promise Chain
// NB: Only works if the logString is not *too* big to hold in memory.
// If that's *not* the case, make sure to be modifying the stream as needed during the .on('data') events. 

const fs = require ('fs');
const path = require ('path');
const sampleInput = path.resolve(__dirname, './sample-input')

const readPuzzleInput = (file) => {
  // I: A file with sample data
  // O: An array of numbers
  return new Promise ((resolve, reject) => {

    let readStream = fs.createReadStream(file, {encoding:'utf8'});
    
    readStream
    .on('data', function processChunk(chunk) {
      logString += chunk;
    })
    .on('end', () => { 
      logEntries = logString.split('\n')
      resolve(logEntries)
    })
  })
};

Promise.resolve(readPuzzleInput(sampleInput))
.then(()=> console.log(logEntries))