Interest Payment Calculator

/*
* A calculator for determining how much interest will be paid on a loan and an estimate on how long the loan will be for.
* Inputs: Principle, interest rate of the loan, and monthly payment
*/

const PRINCIPAL = 4000
const INTEREST_RATE = .0525
const MONTHLY_PAYMENT = 300

const calculateInterest = (principal, interestRate, monthlyPayment) => {
  let principalLeft = principal
  let totalInterest = 0
  let totalPayment = 0
  let numMonths = 0
  
  while (principalLeft >= 0) {
    const monthInterest = (principalLeft * interestRate) * (1/12)
    const appliedToPrincipal = monthlyPayment - monthInterest
    const nextPrincipal = principalLeft - appliedToPrincipal
    
    
    totalPayment += nextPrincipal < 0 ? principalLeft + monthInterest : monthlyPayment
    totalInterest += monthInterest
    principalLeft = nextPrincipal
    numMonths++
    /*
    console.log(principalLeft+': month '+ numMonths+ ' interest ' + monthInterest + ' totalInterest ' + totalInterest + ' applied principal '+ appliedToPrincipal +' principal left ' + principalLeft + ' ')
    */
  }
  console.log('Total Paid: ' + totalPayment)
  console.log('Interest: ' + totalInterest)
  console.log('Months: ' + numMonths)
}

console.log('PRINCIPAL ' + PRINCIPAL + ' Interest rate ' + INTEREST_RATE + ' Total Payment ' + MONTHLY_PAYMENT)
calculateInterest(PRINCIPAL, INTEREST_RATE, MONTHLY_PAYMENT)