Add jest, various tests

.gitignore

@@ -1,3 +1,4 @@
 node_modules
 yarn-error.log
-lib
+lib
+coverage

babel.config.js

@@ -0,0 +1,6 @@
+module.exports = {
+  presets: [
+    ["@babel/preset-env", { targets: { node: "current" } }],
+    "@babel/preset-typescript",
+  ],
+};

jest.config.js

@@ -0,0 +1,3 @@
+module.exports = {
+  collectCoverageFrom: ["src/**/*"],
+};

package.json

@@ -5,13 +5,22 @@
   "author": "Sam Brown (https://github.com/sgb-io)",
   "dependencies": {},
   "devDependencies": {
+    "@babel/core": "^7.18.2",
+    "@babel/preset-env": "^7.18.2",
+    "@babel/preset-typescript": "^7.17.12",
+    "@types/jest": "^27.5.1",
+    "babel-jest": "^28.1.0",
     "eslint": "^8.16.0",
+    "jest": "^28.1.0",
     "prettier": "^2.6.2",
     "typescript": "^4.7.2"
   },
   "scripts": {
     "build:watch": "tsc --watch",
-    "build": "tsc"
+    "build": "tsc",
+    "test": "jest",
+    "test:watch": "jest --watch",
+    "test:coverage": "jest --coverage"
   },
   "repository": {
     "type": "git",

src/incomeTax.test.ts

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+import { calculateIncomeTax } from "./incomeTax";
+import { calculatePersonalAllowance } from "./personalAllowance";
+
+const expectations = [
+  { taxableAnnualIncome: 15_000, basic: 486, higher: 0, additional: 0 },
+  { taxableAnnualIncome: 17_500, basic: 986, higher: 0, additional: 0 },
+  { taxableAnnualIncome: 20_000, basic: 1486, higher: 0, additional: 0 },
+  { taxableAnnualIncome: 22_500, basic: 1986, higher: 0, additional: 0 },
+  { taxableAnnualIncome: 25_000, basic: 2486, higher: 0, additional: 0 },
+  { taxableAnnualIncome: 50_000, basic: 7486, higher: 0, additional: 0 },
+  { taxableAnnualIncome: 55_000, basic: 7540, higher: 1892, additional: 0 },
+  { taxableAnnualIncome: 60_000, basic: 7540, higher: 3892, additional: 0 },
+  { taxableAnnualIncome: 75_000, basic: 7540, higher: 9892, additional: 0 },
+  { taxableAnnualIncome: 90_000, basic: 7540, higher: 15892, additional: 0 },
+  { taxableAnnualIncome: 110_000, basic: 7540, higher: 25892, additional: 0 },
+  { taxableAnnualIncome: 130_000, basic: 7540, higher: 36920, additional: 0 },
+  {
+    taxableAnnualIncome: 145_000,
+    basic: 7540,
+    higher: 39892,
+    additional: 3406.5,
+  },
+  {
+    taxableAnnualIncome: 160_000,
+    basic: 7540,
+    higher: 39892,
+    additional: 10156.5,
+  },
+  {
+    taxableAnnualIncome: 175_000,
+    basic: 7540,
+    higher: 39892,
+    additional: 16906.5,
+  },
+  {
+    taxableAnnualIncome: 200_000,
+    basic: 7540,
+    higher: 39892,
+    additional: 28156.5,
+  },
+  {
+    taxableAnnualIncome: 250_000,
+    basic: 7540,
+    higher: 39892,
+    additional: 50656.5,
+  },
+  {
+    taxableAnnualIncome: 500_000,
+    basic: 7540,
+    higher: 39892,
+    additional: 163156.5,
+  },
+  {
+    taxableAnnualIncome: 1_000_000,
+    basic: 7540,
+    higher: 39892,
+    additional: 388156.5,
+  },
+];
+
+describe("calculateIncomeTax", () => {
+  expectations.forEach((expectation) => {
+    const { taxableAnnualIncome, basic, higher, additional } = expectation;
+    test(taxableAnnualIncome.toString(), () => {
+      const personalAllowance = calculatePersonalAllowance({
+        taxableAnnualIncome,
+      });
+      expect(
+        calculateIncomeTax({ taxableAnnualIncome, personalAllowance })
+      ).toEqual({
+        basicRateTax: basic,
+        higherRateTax: higher,
+        additionalRateTax: additional,
+      });
+    });
+  });
+});

src/nationalInsurance.test.ts

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+import { calculateEmployeeNationalInsurance } from "./nationalInsurance";
+
+const expectations = [
+  { taxableAnnualIncome: 15_000, nics: 678.4 },
+  { taxableAnnualIncome: 17_500, nics: 1009.6500000000001 },
+  { taxableAnnualIncome: 20_000, nics: 1340.9000000000003 },
+  { taxableAnnualIncome: 22_500, nics: 1672.15 },
+  { taxableAnnualIncome: 25_000, nics: 2003.3999999999999 },
+  { taxableAnnualIncome: 50_000, nics: 5315.900000000001 },
+  { taxableAnnualIncome: 55_000, nics: 5506.8 },
+  { taxableAnnualIncome: 60_000, nics: 5669.3 },
+  { taxableAnnualIncome: 75_000, nics: 6156.8 },
+  { taxableAnnualIncome: 90_000, nics: 6644.3 },
+  { taxableAnnualIncome: 110_000, nics: 7294.3 },
+  { taxableAnnualIncome: 130_000, nics: 7944.3 },
+  { taxableAnnualIncome: 150_000, nics: 8594.300000000001 },
+  { taxableAnnualIncome: 175_000, nics: 9406.800000000001 },
+  { taxableAnnualIncome: 200_000, nics: 10219.300000000001 },
+  { taxableAnnualIncome: 250_000, nics: 11844.300000000001 },
+  { taxableAnnualIncome: 500_000, nics: 19969.3 },
+  { taxableAnnualIncome: 1_000_000, nics: 36219.299999999996 },
+];
+
+describe("calculateEmployeeNationalInsurance", () => {
+  expectations.forEach((expectation) => {
+    const { taxableAnnualIncome, nics } = expectation;
+    test(taxableAnnualIncome.toString(), () => {
+      expect(
+        calculateEmployeeNationalInsurance({ taxableAnnualIncome })
+      ).toEqual(nics);
+    });
+  });
+});

src/personalAllowance.test.ts

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+import { calculatePersonalAllowance } from "./personalAllowance";
+
+const expectations = [
+  { taxableAnnualIncome: 15_000, allowance: 12570 },
+  { taxableAnnualIncome: 17_500, allowance: 12570 },
+  { taxableAnnualIncome: 20_000, allowance: 12570 },
+  { taxableAnnualIncome: 22_500, allowance: 12570 },
+  { taxableAnnualIncome: 25_000, allowance: 12570 },
+  { taxableAnnualIncome: 50_000, allowance: 12570 },
+  { taxableAnnualIncome: 55_000, allowance: 12570 },
+  { taxableAnnualIncome: 60_000, allowance: 12570 },
+  { taxableAnnualIncome: 75_000, allowance: 12570 },
+  { taxableAnnualIncome: 90_000, allowance: 12570 },
+  { taxableAnnualIncome: 110_000, allowance: 7570 },
+  { taxableAnnualIncome: 120_000, allowance: 2570 },
+  { taxableAnnualIncome: 124_500, allowance: 320 },
+  { taxableAnnualIncome: 125_000, allowance: 70 },
+  { taxableAnnualIncome: 130_000, allowance: 0 },
+  { taxableAnnualIncome: 145_000, allowance: 0 },
+  { taxableAnnualIncome: 160_000, allowance: 0 },
+  { taxableAnnualIncome: 175_000, allowance: 0 },
+  { taxableAnnualIncome: 200_000, allowance: 0 },
+  { taxableAnnualIncome: 250_000, allowance: 0 },
+  { taxableAnnualIncome: 500_000, allowance: 0 },
+  { taxableAnnualIncome: 1_000_000, allowance: 0 },
+];
+
+describe("calculatePersonalAllowance", () => {
+  expectations.forEach((expectation) => {
+    const { taxableAnnualIncome, allowance } = expectation;
+    test(taxableAnnualIncome.toString(), () => {
+      expect(calculatePersonalAllowance({ taxableAnnualIncome })).toEqual(
+        allowance
+      );
+    });
+  });
+});