A cash class which uses integer math to maintain precision in javascript.

cashIntMath.js

class CashAmount{
  constructor(cashDecimalValue) {
    this.cashDecimalValue = cashDecimalValue;

    this.totalInPennies = this.totalInPennies.bind(this);
    this.addDoubleAmount = this.addDoubleAmount.bind(this);
    this.toDouble = this.toDouble.bind(this);
    this.toDoubleString = this.toDoubleString.bind(this);
    this.quantityOfEachDenomination = this.quantityOfEachDenomination.bind(this);

    this.denominations = {
      'hundreds': 100,
      'fifties': 50,
      'twenties': 20,
      'tens': 10,
      'fives': 5,
      'ones': 1,
      'quarters': .25,
      'dimes': .10,
      'nickels': .05,
      'pennies': .01
    }
  }

  totalInPennies(cashAmount) {
    if (cashAmount) {
      return cashAmount * 100
    }
    return this.cashDecimalValue * 100;
  }

  addDoubleAmount(cashValue) {
    this.cashDecimalValue = (this.totalInPennies(this.cashDecimalValue) + this.totalInPennies(cashValue)) / 100;
  }

  quantityOfEachDenomination() {
    const denominationsResults = {}
    let cashAmountInPenniesLeft = this.totalInPennies(this.cashDecimalValue);
    for (const key in this.denominations) {
      if (this.denominations.hasOwnProperty(key)) {
        const currentDenominationInPennies = this.totalInPennies(this.denominations[key]);
        const amountOfCurrentDenomination = Math.floor(cashAmountInPenniesLeft/currentDenominationInPennies)
        cashAmountInPenniesLeft = cashAmountInPenniesLeft - (amountOfCurrentDenomination * currentDenominationInPennies);
        denominationsResults[key] = amountOfCurrentDenomination;
      }
    }
    return denominationsResults;
  }

  toDouble() {
    return this.cashDecimalValue;
  }

  toDoubleString() {
    const totalInString = `${this.cashDecimalValue}`;
    return totalInString;
  }
}


// Tests

const totalInPenniesTest = () => {
  const cash = new CashAmount(10.50);
  console.log('cash.totalInPennies() should equal 1050. Result:', cash.totalInPennies())
}

const addDoubleAmountTest = () => {
  const cash = new CashAmount(10.50);
  cash.addDoubleAmount(29.33);
  console.log('should equal 3983. Result:', cash.totalInPennies())
}

const checkDenominationTest = () => {
  const cash = new CashAmount(967.93);
  console.log(`should equal 
  {
    'hundreds': 9,
    'fifties': 1,
    'twenties': 0,
    'tens': 1,
    'fives': 1,
    'ones': 2,
    'quarters': 3,
    'dimes': 1,
    'nickels': 1,
    'pennies': 3
  }. 
  Result:
  `, cash.quantityOfEachDenomination())
}

const toDoubleTest = () => {
  const cash = new CashAmount(10.50);
  cash.addDoubleAmount(29.33);
  console.log('should equal 39.83. Result:', cash.toDouble());
}

const toDoubleStringTest = () => {
  const cash = new CashAmount(10.50);
  cash.addDoubleAmount(29.33);
  console.log('should equal "39.83". Result:', cash.toDoubleString(), 'Type of Response:', typeof cash.toDoubleString());
}


const tests = [
  totalInPenniesTest,
  addDoubleAmountTest,
  checkDenominationTest,
  toDoubleTest,
  toDoubleStringTest,
]

for (let i = 0; i < tests.length; i++) {
  tests[i](); 
}