readWords.js

import fs from 'fs';
import readline from 'readline';

const words = {};
const wordReader = readline.createInterface({
  input: fs.createReadStream('words.italian.txt','utf8')
});

wordReader.on('line',( word ) => {
  if ( word.length ) {
    if ( !words[word.length] ) {
      words[word.length] = [];
    }
    words[word.length].push(word);
  }
});

wordReader.on('close', () => {
  console.log('Readed all!');
});