「テスト駆動開発」JS（AVA）

main.js

class Bank {
  constructor() {
    this.rates = new Map();
  }

  reduce(source, to) {
    return source.reduce(this, to);
  }

  addRate(from, to, rate) {
    this.rates.set(`${from}_${to}`, rate);
  }

  rate(from, to) {
    if (from === to) return 1;
    return this.rates.get(`${from}_${to}`);
  }
}

class Sum {
  constructor(augend, addend) {
    this.augend = augend;
    this.addend = addend;
  }

  times(multiplier) {
    return new Sum(this.augend.times(multiplier), this.addend.times(multiplier));
  }

  plus(addend) {
    return new Sum(this, addend);
  }

  reduce(bank, to) {
    const amount = this.augend.reduce(bank, to).amount + this.addend.reduce(bank, to).amount;
    return new Money(amount, to);
  }
}

class Money {
  constructor(amount, currency) {
    this.amount = amount;
    this.currency = currency;
  }

  times(multiplier) {
    return new Money(this.amount * multiplier, this.currency);
  }

  plus(addend) {
    return new Sum(this, addend);
  }

  reduce(bank, to) {
    const rate = bank.rate(this.currency, to);
    return new Money(this.amount / rate, to);
  }

  getCurrency() {
    return this.currency;
  }

  equals(money) {
    return this.amount === money.amount && this.currency === money.getCurrency();
  }

  static dollar(amount) {
    return new Money(amount, 'USD');
  }

  static franc(amount) {
    return new Money(amount, 'CHF');
  }
}

export { Bank, Sum, Money }

test_main.js

import test from 'ava';
import 'babel-register';
import { Bank, Sum, Money } from '../src/js/main';

test('Test Multiplication.', t => {
  const five = Money.dollar(5);
  t.is(Money.dollar(10).amount, five.times(2).amount);
  t.is(Money.dollar(15).amount, five.times(3).amount);
});

test('Test Equality.', t => {
  t.true(Money.dollar(5).equals(Money.dollar(5)));
  t.false(Money.dollar(5).equals(Money.dollar(6)));
  // ドルとフランが一緒じゃないことをテストする
  t.false(Money.dollar(5).equals(Money.franc(5)));
});

test('Test Currency.', t => {
  t.is('USD', Money.dollar(1).getCurrency());
  t.is('CHF', Money.franc(1).getCurrency());
});

test('Test Simple Addition.', t => {
  const five = Money.dollar(5);
  const sum = five.plus(five);
  const bank = new Bank();
  const reduced = bank.reduce(sum, 'USD');
  t.true(Money.dollar(10).equals(reduced));
});

test('Test Plus Returns Sum.', t => {
  const five = Money.dollar(5);
  const sum = five.plus(five);
  t.true(five.equals(sum.augend));
  t.true(five.equals(sum.addend));
});

test('Test Reduce Sum.', t => {
  const sum = new Sum(Money.dollar(3), Money.dollar(4));
  const bank = new Bank();
  const result = bank.reduce(sum, 'USD');
  t.true(Money.dollar(7).equals(result));
});

test('Test Reduce Money.', t => {
  const bank = new Bank();
  const result = bank.reduce(Money.dollar(1), 'USD');
  t.true(Money.dollar(1).equals(result));
});

test('Test Reduce Money Different Currency.', t => {
  const bank = new Bank();
  bank.addRate('CHF', 'USD', 2);
  const result = bank.reduce(Money.franc(2), 'USD');
  t.true(Money.dollar(1).equals(result));
});

test('Test Identity Rate.', t => {
  t.is(1, new Bank().rate('USD', 'USD'));
});

test('Test Identity Rate.', t => {
  const bank = new Bank();
  bank.addRate('CHF', 'USD', 2);
  t.is(2, bank.rate('CHF', 'USD'));
});

test('Test Mixed Addision.', t => {
  const fiveBucks = Money.dollar(5);
  const tenFrancs = Money.franc(10);
  const bank = new Bank();
  bank.addRate('CHF', 'USD', 2);
  const result = bank.reduce(fiveBucks.plus(tenFrancs), 'USD');
  t.true(Money.dollar(10).equals(result));
});

test('Test Sum Plus Money.', t => {
  const fiveBucks = Money.dollar(5);
  const tenFrancs = Money.franc(10);
  const bank = new Bank();
  bank.addRate('CHF', 'USD', 2);
  const sum = new Sum(fiveBucks, tenFrancs).plus(fiveBucks);
  const result = bank.reduce(sum, 'USD');
  t.true(Money.dollar(15).equals(result));
});

test('Test Sum Times.', t => {
  const fiveBucks = Money.dollar(5);
  const tenFrancs = Money.franc(10);
  const bank = new Bank();
  bank.addRate('CHF', 'USD', 2);
  const sum = new Sum(fiveBucks, tenFrancs).times(2);
  const result = bank.reduce(sum, 'USD');
  t.true(Money.dollar(20).equals(result));
});