(Medium) - How TDD Can Prevent Over-Engineering

MoveRangesIntoArray.diff

     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');
 
-    if (loanAmount.greaterThan(Money('$2000.00'))) {
-      interestAmount = interestAmount.plus(calculateInterest(loanAmount, greaterThan2000));
+    if (loanAmount.greaterThan(ranges[0].endOfRange)) {
+      interestAmount = interestAmount.plus(calculateInterest(loanAmount, ranges[0]));
     }
 
-    if (loanAmount.greaterThan(Money('$5000.00'))) {
-      interestAmount = interestAmount.plus(calculateInterest(loanAmount, greaterThan5000));
+    if (loanAmount.greaterThan(ranges[1].endOfRange)) {
+      interestAmount = interestAmount.plus(calculateInterest(loanAmount, ranges[1]));
     }
 
-    if (loanAmount.greaterThan(Money('$10000.00'))) {
-      interestAmount = interestAmount.plus(calculateInterest(loanAmount, greaterThan10000));
+    if (loanAmount.greaterThan(ranges[2].endOfRange)) {
+      interestAmount = interestAmount.plus(calculateInterest(loanAmount, ranges[2]));
     }
 
     return interestAmount;