JS Bin// source https://jsbin.com/liboref

Interest Gained Calculator

/*
* A calculator for determining how much interest will be gained on an investment in a year
* Inputs: Principle, interest rate of the loan, additional amount invested every month, and the number of months to calculate interest for
*/

const PRINCIPAL = 30000
const INTEREST_RATE = .0139
const ADDITIONS_PER_MONTH = 0
const NUM_MONTHS = 12

const calculateInterest = (principal, interestRate, additionsPerMonth, numMonths) => {
  let accountSum = principal
  let totalInterest = 0
  
  for(let i=0; i<numMonths; i++) {
    const monthInterest = (accountSum * interestRate) * (1/12)
    totalInterest += monthInterest
    accountSum += monthInterest + additionsPerMonth
  }
  console.log('Account Sum: ' + accountSum)
  console.log('Interest Gained: ' + totalInterest)
}

console.log('PRINCIPAL ' + PRINCIPAL + ' Interest rate ' + INTEREST_RATE + ' Additions per month ' + ADDITIONS_PER_MONTH)
calculateInterest(PRINCIPAL, INTEREST_RATE, ADDITIONS_PER_MONTH, NUM_MONTHS)