introduce taxableChargeFn function

Author: kaiosilveira

src/tax-utils/index.js

@@ -25,4 +25,14 @@ function taxThreshold(year) {
   return THRESHOLDS_BY_YEAR[year];
 }
 
-module.exports = { baseRate, taxThreshold, RATES_BY_MONTH_AND_YEAR, THRESHOLDS_BY_YEAR };
+function taxableChargeFn(aReading) {
+  return Math.max(0, aReading.baseCharge - taxThreshold(aReading.year));
+}
+
+module.exports = {
+  baseRate,
+  taxThreshold,
+  taxableChargeFn,
+  RATES_BY_MONTH_AND_YEAR,
+  THRESHOLDS_BY_YEAR,
+};

src/tax-utils/index.test.js

@@ -1,4 +1,10 @@
-const { baseRate, RATES_BY_MONTH_AND_YEAR, taxThreshold, THRESHOLDS_BY_YEAR } = require('.');
+const {
+  baseRate,
+  RATES_BY_MONTH_AND_YEAR,
+  taxThreshold,
+  THRESHOLDS_BY_YEAR,
+  taxableChargeFn,
+} = require('.');
 
 describe('tax-utils', () => {
   describe('baseRate', () => {
@@ -12,4 +18,14 @@ describe('tax-utils', () => {
       expect(taxThreshold(2017)).toEqual(THRESHOLDS_BY_YEAR[2017]);
     });
   });
+
+  describe('taxableChargeFn', () => {
+    it('should return zero if result is negative', () => {
+      expect(taxableChargeFn({ baseCharge: 0, year: 2017 })).toEqual(0);
+    });
+
+    it('should calculate the correct taxable charge', () => {
+      expect(taxableChargeFn({ baseCharge: 1, year: 2017 })).toEqual(0.5);
+    });
+  });
 });