words.js

const Fs = require('fs');
const Os = require('os');
const Path = require('path');

const FILTERED_WORDS_FILE = 'filtered.txt';
const STARTS_WITH = 'h';
const TOTAL_LENGTH = 12;
const WORDS_FILE = '20k.txt'; // https://github.com/first20hours/google-10000-english/blob/master/20k.txt

const addendPairs = [];
const addends = {};

for (let i = 1; i <= Math.floor(TOTAL_LENGTH / 2); ++i) {
  const pair = [i, TOTAL_LENGTH - i];
  addendPairs.push(pair);
  addends[pair[0]] = [];
  addends[pair[1]] = [];
}

const fileContents = Fs.readFileSync(Path.resolve(process.cwd(), WORDS_FILE), 'utf8');
const words = fileContents.split('\n');

const filteredByStartsWith = words.filter(word => word.startsWith(STARTS_WITH));

filteredByStartsWith.forEach(word => {
  const length = word.length;
  if (addends.hasOwnProperty(length)) {
    addends[length.toString()].push(word);
  }
});

const wordPairs = [];

addendPairs.forEach(pair => {
  const left = pair[0];
  const right = pair[1];

  addends[left.toString()].forEach(leftWord => {
    addends[right.toString()].forEach(rightWord => {
      wordPairs.push([leftWord, rightWord]);
    });
  });
});

Fs.writeFileSync(Path.resolve(process.cwd(), FILTERED_WORDS_FILE), wordPairs.join('\n'), 'utf8');