Unit test sạch

❌ Test bẩn - nhiều khái niệm lộn xộn, fail = không biết vấn đề ở đâu

// test nay kiem tra nhieu thu cung luc - kho doc, kho debug
it('test order stuff', () => {
  const items = [{ price: 100000, qty: 2 }, { price: 50000, qty: 1 }];
  const total = calculateTotal(items);
  expect(total).toBe(250000);
  const discounted = applyDiscount(total, 10);
  expect(discounted).toBe(225000);
  expect(() => applyDiscount(total, -5)).toThrow();
  expect(() => applyDiscount(total, 110)).toThrow();
  const empty = calculateTotal([]);
  expect(empty).toBe(0);
  // Test fail: "Expected 0 to be 0" - o dong nao? Vi sao?
});

✅ Test sạch theo AAA - mỗi test một khái niệm, tên kể chuyện (Vitest)

import { describe, it, expect } from 'vitest';

function calculateTotal(items: { price: number; qty: number }[]): number {
  return items.reduce((sum, item) => sum + item.price * item.qty, 0);
}

function applyDiscount(total: number, discountPct: number): number {
  if (discountPct < 0 || discountPct > 100)
    throw new Error(`Phan tram giam gia khong hop le: ${discountPct}`);
  return total * (1 - discountPct / 100);
}

describe('calculateTotal', () => {
  it('tinh tong nhieu san pham co so luong khac nhau', () => {
    // Arrange
    const items = [{ price: 100000, qty: 2 }, { price: 50000, qty: 1 }];
    // Act
    const total = calculateTotal(items);
    // Assert
    expect(total).toBe(250000);
  });

  it('tra ve 0 khi danh sach rong', () => {
    // Arrange
    const items: { price: number; qty: number }[] = [];
    // Act
    const total = calculateTotal(items);
    // Assert
    expect(total).toBe(0);
  });
});

describe('applyDiscount', () => {
  it('ap dung dung phan tram giam gia vao tong tien', () => {
    // Arrange
    const total = 250000;
    const discountPct = 10;
    // Act
    const result = applyDiscount(total, discountPct);
    // Assert
    expect(result).toBe(225000);
  });

  it('bao loi khi phan tram giam gia am', () => {
    expect(() => applyDiscount(250000, -5))
      .toThrow('Phan tram giam gia khong hop le: -5');
  });

  it('bao loi khi phan tram giam gia vuot qua 100', () => {
    expect(() => applyDiscount(250000, 110))
      .toThrow('Phan tram giam gia khong hop le: 110');
  });
});

TDD mini: đỏ → xanh → refactor với Vitest

// Buoc 1 (Do): Viet test truoc - test nay fail vi ham chua ton tai
import { describe, it, expect } from 'vitest';
// import { formatVND } from './money'; // chua co - se fail compile

// Buoc 2 (Xanh): viet ham vua du de test xanh
function formatVND(amount: number): string {
  return amount.toLocaleString('vi-VN') + ' ₫';
}

describe('formatVND', () => {
  it('format so nguyen thanh tien Viet Nam', () => {
    expect(formatVND(1500000)).toBe('1.500.000 ₫');
  });

  it('format so 0', () => {
    expect(formatVND(0)).toBe('0 ₫');
  });

  it('format so lon', () => {
    expect(formatVND(10000000)).toBe('10.000.000 ₫');
  });
});

// Buoc 3 (Refactor): giu test xanh, lam code dep hon neu can
// (ham nay da ngan - khong can refactor; vong lap TDD ket thuc)