https://www.codewars.com/kata/recover-a-secret-string-from-random-triplets/javascript solution

recoverSecret.js

class Letter {
  constructor(letter) {
    this._letter = letter;
    this._before = [];
    this._after = [];
  }

  get letter() {
    return this._letter;
  }

  get before() {
    return this._before;
  }

  get after() {
    return this._after;
  }

  isBefore(other) {
    if (this.before.indexOf(other) === -1) {
      this.before.push(other);
    }
  }

  isAfter(other) {
    if (this.after.indexOf(other) === -1) {
      this.after.push(other);
    }
  }

  toString() {
    return this.letter;
  }
}

class Triplets {
  constructor() {
    this._letters = [];
  }

  get letters() {
    return this._letters;
  }

  add(first, sec, last) {
    this.addOne(first, sec);
    this.addOne(sec, last, first);
    this.addOne(last, null, sec);
    return this;
  }

  addLetter(letter) {
    const l = new Letter(letter);
    this.letters.push(l);
    return l;
  }

  findLetter(letter) {
    if (!letter) {
      return
    }

    return this.letters.find(l => l.letter === letter) || this.addLetter(letter);
  }

  addOne(letter, before, after) {
    letter = this.findLetter(letter);
    before = this.findLetter(before);
    after = this.findLetter(after);
    if (before) {
      letter.isBefore(before)
    }
    if (after) {
      letter.isAfter(after)
    }
  }

  deleteFromArray(arr, target) {
    const item = arr.find(l => l.letter === target.letter);
    const index = arr.indexOf(item);
    arr.splice(index, 1);
  }

  findNextLetter(from) {
    const res = from.before.find(l => l.after.length === 1 && l.after[0].letter == from.letter);
    from.before.forEach(bef => this.deleteFromArray(bef.after, from));
    this.deleteFromArray(this.letters, from);
    return res;
  }

  toString() {
    const res = this.letters.filter(l => l.after.length === 0);
    while(this.letters.length > 1) {
      res.push(this.findNextLetter(res[res.length-1]));
    }
    return res.map(l => l.toString()).join('');
  }
}

var recoverSecret = function(triplets) {
  return triplets.reduce((t, l) => t.add(...l), new Triplets()).toString()
}