charactersCounter.js

class charactersCounter {
  constructor(input) {
    this.inputArr = input.split('');
    this.availableLetters = [...new Set(this.inputArr)];
    this.mostPopular = {
      letter: '',
      amount: 0
    }
  }
  
  getOccurrence(arr, val) {
    return arr.filter(v => (v === val)).length;
  }
  
  addLetterAmount(targetArray) {
    for(let i = 0, l = targetArray.length; i < l; i++) {
      targetArray[i] = {
        letter: targetArray[i],
        amount: this.getOccurrence(this.inputArr, targetArray[i])
      }
    }
  }
  
  getMostPopular(fromArray) {
    for(let i = 0, l = fromArray.length; i < l; i++) {
      if(fromArray[i].amount > this.mostPopular.amount) {
        console.log(fromArray[i].amount, this.mostPopular)  
        this.mostPopular.letter = fromArray[i].letter;
        this.mostPopular.amount = fromArray[i].amount;
      }
    }
  }
  
  init() {
    this.addLetterAmount(this.availableLetters);
    this.getMostPopular(this.availableLetters);
    return this.mostPopular.letter;
  }
}

const charCounter = new charactersCounter('hello worlddd');
console.log(charCounter.init());