✨ 【第14章 学習用テストと回帰テスト】 文字列をユニオン型で持たせるようにし取り回しやすいようにする

src/bank.ts

@@ -1,15 +1,15 @@
 import { Expression } from './expression';
-import { Money } from './money';
+import { Currency, Money } from './money';
 
 export class Bank {
-  reduce(source: Expression, to: string):Money {
+  reduce(source: Expression, to: Currency):Money {
     return source.reduce(this, to);
   }
 
   addRate(from: string, to: string, rate: number) {
   }
 
-  rate(from: string, to: string): number {
+  rate(from: Currency, to: Currency): number {
     return from === 'CHF' && to === 'USD' ? 2 : 1
   }
 }

src/expression.ts

@@ -1,6 +1,6 @@
 import { Bank } from "./bank";
-import { Money } from "./money";
+import { Currency, Money } from "./money";
 
 export interface Expression{
-  reduce(bank: Bank, to: string): Money;
+  reduce(bank: Bank, to: Currency): Money;
 }

src/money.ts

@@ -2,8 +2,10 @@ import { Bank } from "./bank";
 import { Expression } from "./expression";
 import { Sum } from "./sum";
 
+export type Currency = 'CHF' | 'USD';
+
 export class Money implements Expression {
-  constructor(public readonly amount: number, public readonly currency: string) {
+  constructor(public readonly amount: number, public readonly currency: Currency) {
   }
 
   static dollar(amount: number):Money {
@@ -34,7 +36,7 @@ export class Money implements Expression {
     return new Sum(this, addend);
   }
 
-  reduce(bank: Bank,to: string): Money {
+  reduce(bank: Bank,to: Currency): Money {
     const rate = bank.rate(this.currency, to);
     return new Money(this.amount / rate, to);
   }

src/sum.ts

@@ -1,12 +1,12 @@
 import { Bank } from "./bank";
 import { Expression } from "./expression";
-import { Money } from "./money";
+import { Currency, Money } from "./money";
 
 export class Sum implements Expression {
   constructor(public augend: Money, public addend: Money) {
   }
 
-  reduce(bank: Bank, to: string):Money {
+  reduce(bank: Bank, to: Currency):Money {
     const amount :number = this.augend.amount + this.addend.amount;
     return new Money(amount, to);
   }