Jest AAA 測試原則入門教學筆記 | 學習筆記

範例測試：saveMoney 方法

atm.js：

class ATM {
  constructor(balance) {
    this.balance = balance;
  }
  saveMoney(amount) {
    if (amount <= 0) {
      throw new Error('Amount must be positive');
    }
    this.balance += amount;
    return this.balance;
  }
  withdrawMoney(amount) {
    if (amount <= 0) {
      throw new Error('Amount must be positive');
    }
    if (amount > this.balance) {
      throw new Error('Insufficient balance');
    }
    this.balance -= amount;
    return this.balance;
  }
  getBalance() {
    return this.balance;
  }
  reset() {
    this.balance = 0;
  }
  setBalance(balance) {
    if (balance < 0) {
      throw new Error('Balance cannot be negative');
    }
    this.balance = balance;
  }
}

export default ATM;

atm.test.js：

import ATM from '../atm.js';

test('saveMoney adds money to balance', () => {
  // Arrange: 建立一個 ATM 實例，初始餘額 0
  const atm = new ATM(0);

  // Act: 存入 1000
  atm.saveMoney(1000);

  // Assert: 餘額應該變成 1000
  expect(atm.balance).toBe(1000);
});

逐步對應：

步驟	內容
Arrange	建立 ATM 物件並給初始值
Act	呼叫 saveMoney(1000)
Assert	驗證 atm.balance 是否等於 1000

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另一個範例：檢查錯誤拋出

如果要測試當金額是負數時會丟錯誤：

test('saveMoney throws error when amount <= 0', () => {
  // Arrange
  const atm = new ATM(0);

  // Act & Assert
  expect(() => atm.saveMoney(0)).toThrow('Amount must be positive');
});

這裡因為 act 跟 assert 綁在一起，所以在 expect 裡包了一個 function，來驗證是否拋出錯誤。

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完整測試檔（用 AAA 標註）

import ATM from '../atm.js';

test('saveMoney adds money to balance', () => {
  // Arrange
  const atm = new ATM(0);

  // Act
  atm.saveMoney(1000);

  // Assert
  expect(atm.balance).toBe(1000);
});

test('saveMoney throws error when amount <= 0', () => {
  // Arrange
  const atm = new ATM(0);

  // Act & Assert
  expect(() => atm.saveMoney(0)).toThrow('Amount must be positive');
});