add plus operation to Expression interface

Task list:
- $5 + 10CHF = $10 if rate is 2:1 🎯 🎉 ✨
- $5 + $5 = $10 ✅
- Return Money from $5 + $5
- Bank.reduce(Money) ✅
- Reduce Money with conversion ✅
- Reduce(Bank, String) ✅
- Sum.plus
- Expression.times

src/expression/index.ts

@@ -3,6 +3,7 @@ import Money from '../money';
 
 export default interface Expression {
   reduce(bank: Bank, to: string): Money;
+  plus(addend: Expression): Expression;
 }
 
 export class Sum implements Expression {
@@ -27,4 +28,8 @@ export class Sum implements Expression {
       this.augend.reduce(bank, to).amount + this.addend.reduce(bank, to).amount;
     return new Money(amount, to);
   }
+
+  plus(addend: Expression): Expression {
+    throw new Error('Method not implemented.');
+  }
 }

src/money/index.spec.ts

@@ -4,8 +4,8 @@ import Bank from '../bank';
 
 describe('Money', () => {
   it('should sum $5 + 10CHF as being $10 if exchange rate is 2:1', () => {
-    const fiveBucks: Money = Money.dollar(5);
-    const tenFrancs: Money = Money.franc(10);
+    const fiveBucks: Expression = Money.dollar(5);
+    const tenFrancs: Expression = Money.franc(10);
     const bank: Bank = new Bank();
     bank.addRate('CHF', 'USD', 2);