(Medium) - How TDD Can Prevent Over-Engineering

FinalCode.js

const interestToPayFor = (loanAmount) => {
  const greaterThan2000 = { endOfRange: Money('$2000.00'), interestPerDollar: Money('$0.09'), previousInterestPerDollar: Money('$0.00') };
  const greaterThan5000 = { endOfRange: Money('$5000.00'), interestPerDollar: Money('$0.14'), previousInterestPerDollar: Money('$0.09') };
  const greaterThan10000 = { endOfRange: Money('$10000.00'), interestPerDollar: Money('$0.21'), previousInterestPerDollar: Money('$0.14') };
  // You can move this into a configuration file so that changes
  // in the behavior doesn't require code changes
  const ranges = [greaterThan2000, greaterThan5000, greaterThan10000];
  let interestAmount = Money('$0.00');
  for (range of ranges) {
    if (loanAmount.greaterThan(range.endOfRange)) {
      interestAmount = interestAmount.plus(calculateInterest(loanAmount, range));
    }
  }
  return interestAmount;
};