Read and parse files using streams in node.js to build a dictionary object mapping word=>frequency.

dictionary.js

const fs = require('fs');
const Parse = require('./parser');

//constructor
function CreateDictionary(){
  this.dictionary = {};
}
// reads files from directory, wait for files to get parsed and then return dictionary
CreateDictionary.prototype.parseDirectory = function(dirpath){
  return new Promise((resolve,reject) => {
    return fs.readdir(dirpath,(err,files) => {
      if(err) reject(err);
      else{
        const fns = files
        .filter((file)=> /.*\.txt$/g.test(file))
        .map((file) => this.parseFile(dirpath+'/'+file)); 

        return Promise.all([...fns]).then((values,err) => {
          if(err) reject(err);
          console.log(`parsing complete.`);
          resolve(this.dictionary);
        });
      }
    });
  });
}

//read a single file, wait for it to get parsed and return dictionary
CreateDictionary.prototype.parseFile = function(filepath){
  return new Promise((resolve,reject) => {
    const read = fs.createReadStream(filepath, {encoding: 'utf-8'});
    const parser = new Parse(this.dictionary);
    console.log(`reading file ${filepath}`);  
    read.pipe(parser)
    .on('error',(err) => reject(err))
    .on('finish',() => {
      console.log(`finished parsing file ${filepath}`);
      resolve(this.dictionary);
    });  
  });
}

module.exports =  CreateDictionary;

parser.js

const Transform = require('stream').Transform;

/** returns a transform stream object that expects chunk of data from a read stream, 
* converts the chunk into string, 
* cleans the string of any unwanted characters,
* splits the string into an array of words
* if word is present in the dictionary increase frequency of the word, else initialize frequency to 1
**/
function Parse (dictionary) {
  const parser = new Transform();
  parser._transform = (data, encoding, done) => {
    cleanString(data.toString())
    .split(/[\n]/g)
    .filter((line) => line!=='')
    .map((elem) => {
      elem = elem.toLowerCase();
      dictionary[elem] = (!dictionary[elem]) ? 1 : dictionary[elem]+1 ;
    });        
    done();
  }
  return parser;
}

function cleanString(str){
  str = str.replace(/[^a-zA-Z]/g,'\n');
  return str;
}

module.exports =  Parse;

script.js

const CreateDictionary = require('./dictionary.js');
const dict =  new CreateDictionary();

//test parseDirectory
dict.parseDirectory('some-folder-path')
.then((dictionary) => 
  console.log(`
    number of dictionary elements : ${Object.keys(dictionary).length}
    dictionary: ${JSON.stringify(dictionary,null,2)}
  `)
).catch((err) => console.log(err.message));

//test parseFile
dict.parseFile('some-file-path')
.then((dictionary) => 
  console.log(`
    number of dictionary elements : ${Object.keys(dictionary).length}
    dictionary: ${JSON.stringify(dictionary,null,2)}
  `)
).catch((err) => console.log(err.message));