✨ 【第13章 実装を導くテスト】 reduce メソッドで為替の変更ができるようにする

・plus の結果を確かめるために Sum クラスの被加算数と加数が一致することのテストを追加している

src/__tests__/money.test.ts

@@ -35,14 +35,14 @@ test('times', () => {
   expect(five.times(3)).toEqual(Money.dollar(15));
 });
 
-test('simple addition', () => {
-  const sum  = Money.dollar(5).plus(Money.dollar(5));
+test('test reduce sum', () => {
+  const sum  = Money.dollar(3).plus(Money.dollar(4));
   const bank = new Bank();
   const reduced = bank.reduce(sum, 'USD');
-  expect(Money.dollar(10)).toEqual(reduced);
+  expect(Money.dollar(7)).toEqual(reduced);
 });
 
-test('testPlusReturnsSum', () => {
+test('test plus returns sum', () => {
   const five = Money.dollar(5);
   const result = five.plus(five)
   const sum = result as Sum

src/bank.ts

@@ -1,8 +1,11 @@
 import { Expression } from './expression';
 import { Money } from './money';
+import { Sum } from './sum';
 
 export class Bank {
-  reduce(sum: Expression, currency: string):Money {
-    return Money.dollar(10);
+  reduce(source: Expression, to: string):Money {
+    const sum = source as Sum
+    const amount = sum.augend.amount + sum.addend.amount;
+    return new Money(amount, to);
   }
 }

src/money.ts

@@ -2,7 +2,7 @@ import { Expression } from "./expression";
 import { Sum } from "./sum";
 
 export class Money implements Expression {
-  constructor(protected readonly amount: number, public readonly currency: string) {
+  constructor(public readonly amount: number, public readonly currency: string) {
   }
 
   static dollar(amount: number):Money {