(Medium) - How TDD Can Prevent Over-Engineering
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Summary (TL;DR)
The function calculates interest on a loan amount using tiered rates. Up to $2000, interest is $0. For $2001-$5000, interest is $0.09 per dollar above $2000. For $5001-$10000, interest includes the previous tier plus $0.05 per dollar above $5000 ($0.14-$0.09). Above $10000, interest includes previous tiers plus $0.07 per dollar above $10000 ($0.21-$0.14).
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| const interestToPayFor = (loanAmount) => { | |
| const END_OF_FIRST_RANGE = Money('$2000.00'); | |
| const END_OF_SECOND_RANGE = Money('$5000.00'); | |
| const END_OF_THIRD_RANGE = Money('$10000.00'); | |
| const CENTS_FOR_FIRST_RANGE = Money('$0.00'); | |
| const CENTS_FOR_SECOND_RANGE = Money('$0.09'); | |
| const CENTS_FOR_THIRD_RANGE = Money('$0.14'); | |
| const CENTS_FOR_FOURTH_RANGE = Money('$0.21'); | |
| if (loanAmount.greaterThan(END_OF_FIRST_RANGE) && loanAmount.lessThan(Money('$5001.00'))) { | |
| const dollarsAboveThreshold = loanAmount.minus(END_OF_FIRST_RANGE); | |
| const interestAmount = CENTS_FOR_SECOND_RANGE.multipliedBy(dollarsAboveThreshold); | |
| return interestAmount; | |
| } | |
| if (loanAmount.greaterThan(END_OF_SECOND_RANGE) && loanAmount.lessThan(Money('$10001.00'))) { | |
| let interestAmount = Money('$0.00'); | |
| const dollarsAboveThresholdForSecondRange = loanAmount.minus(END_OF_FIRST_RANGE); | |
| const interestAmountForSecondRange = CENTS_FOR_SECOND_RANGE.multipliedBy(dollarsAboveThresholdForSecondRange); | |
| interestAmount = interestAmount.plus(interestAmountForSecondRange); | |
| const dollarsAboveThresholdForThirdRange = loanAmount.minus(END_OF_SECOND_RANGE); | |
| const interestAmountForThirdRange = CENTS_FOR_THIRD_RANGE.minus(CENTS_FOR_SECOND_RANGE).multipliedBy(dollarsAboveThresholdForThirdRange); | |
| interestAmount = interestAmount.plus(interestAmountForThirdRange); | |
| return interestAmount; | |
| } | |
| if (loanAmount.greaterThan(END_OF_THIRD_RANGE)) { | |
| let interestAmount = Money('$0.00'); | |
| const dollarsAboveThresholdForSecondRange = loanAmount.minus(END_OF_FIRST_RANGE); | |
| const interestAmountForSecondRange = CENTS_FOR_SECOND_RANGE.multipliedBy(dollarsAboveThresholdForSecondRange); | |
| interestAmount = interestAmount.plus(interestAmountForSecondRange); | |
| const dollarsAboveThresholdForThirdRange = loanAmount.minus(END_OF_SECOND_RANGE); | |
| const interestAmountForThirdRange = CENTS_FOR_THIRD_RANGE.minus(CENTS_FOR_SECOND_RANGE).multipliedBy(dollarsAboveThresholdForThirdRange); | |
| interestAmount = interestAmount.plus(interestAmountForThirdRange); | |
| const dollarsAboveThresholdForFourthRange = loanAmount.minus(END_OF_THIRD_RANGE); | |
| const interestAmountForFourthRange = CENTS_FOR_FOURTH_RANGE.minus(CENTS_FOR_THIRD_RANGE).multipliedBy(dollarsAboveThresholdForFourthRange); | |
| interestAmount = interestAmount.plus(interestAmountForFourthRange); | |
| return interestAmount; | |
| } | |
| return CENTS_FOR_FIRST_RANGE; | |
| }; |
