Introduce factory methods for franc and dollar

src/money/index.spec.ts

@@ -1,14 +1,51 @@
-import Dollar from '../dollar';
-import Franc from '../franc';
+import Money, { Dollar, Franc } from '../money';
 
 describe('Money', () => {
   describe('equality', () => {
     it('should consider an object representing $5 equals to another object representing $5', () => {
-      expect(new Franc(5).equals(new Franc(5))).toBe(true);
-      expect(new Franc(5).equals(new Franc(6))).toBe(false);
+      expect(Money.franc(5).equals(Money.franc(5))).toBe(true);
+      expect(Money.franc(5).equals(Money.franc(6))).toBe(false);
       expect(new Dollar(5).equals(new Dollar(5))).toBe(true);
       expect(new Dollar(5).equals(new Dollar(6))).toBe(false);
-      expect(new Franc(5).equals(new Dollar(5))).toBe(false);
+      expect(Money.franc(5).equals(new Dollar(5))).toBe(false);
+    });
+  });
+});
+
+describe('Dollar', () => {
+  it.todo('should sum $5 + 10CHF as being $10 if exchange rate is 2:1');
+
+  describe('multiplication', () => {
+    it('should multiply $5 by 2 and get $10', () => {
+      const five: Money = Money.dollar(5);
+      const product = five.times(2);
+      expect(product.equals(Money.dollar(10))).toBe(true);
+    });
+
+    it('should preserve the initial dollar amount when multiplying multiple times', () => {
+      const five = Money.dollar(5);
+      let product: Dollar = five.times(2);
+      expect(product.equals(Money.dollar(10))).toBe(true);
+      product = five.times(3);
+      expect(product.equals(Money.dollar(15))).toBe(true);
+    });
+  });
+});
+
+describe('Franc', () => {
+  describe('multiplication', () => {
+    it('should multiply $5 by 2 and get $10', () => {
+      const five = Money.franc(5);
+      const product = five.times(2);
+      expect(product.equals(Money.franc(10))).toBe(true);
+    });
+
+    it('should preserve the initial dollar amount when multiplying multiple times', () => {
+      const five = Money.franc(5);
+      let product: Franc = five.times(2);
+      expect(product.equals(Money.franc(10))).toBe(true);
+      product = five.times(3);
+      expect(product.equals(Money.franc(15))).toBe(true);
     });
   });
 });

src/money/index.ts

@@ -1,4 +1,4 @@
-export default class Money {
+abstract class Money {
   private _amount: number;
 
   constructor(amount: number) {
@@ -13,4 +13,36 @@ export default class Money {
     const money: Money = obj as Money;
     return this.amount == money.amount && obj.constructor.name == this.constructor.name;
   }
+
+  static dollar(amount: number): Money {
+    return new Dollar(amount);
+  }
+
+  static franc(amount: number): Money {
+    return new Franc(amount);
+  }
+
+  abstract times(multiplier: number): Money;
+}
+
+export class Dollar extends Money {
+  constructor(amount: number) {
+    super(amount);
+  }
+
+  times(multiplier: number): Dollar {
+    return new Dollar(this.amount * multiplier);
+  }
 }
+
+export class Franc extends Money {
+  constructor(amount: number) {
+    super(amount);
+  }
+
+  times(multiplier: number): Franc {
+    return new Franc(this.amount * multiplier);
+  }
+}
+
+export default Money;