Unit Testing

class BankAccount {
  constructor(initialBalance = 0) {
    if (initialBalance < 0) throw new Error("Negative balance not allowed");
    this.balance = initialBalance;
    this.transactions = [];
  }
  
  deposit(amount) {
    if (amount <= 0) throw new Error("Deposit amount must be positive");
    this.balance += amount;
    this.transactions.push({ type: "deposit", amount });
  }
  
  withdraw(amount) {
    if (amount > this.balance) throw new Error("Insufficient funds");
    this.balance -= amount;
    this.transactions.push({ type: "withdrawal", amount });
  }
}

// Test file
describe("BankAccount", () => {
  let account;
  
  beforeEach(() => {
    account = new BankAccount(1000); // fresh account before each test
  });
  
  test("creates account with initial balance", () => {
    expect(account.balance).toBe(1000);
  });
  
  test("deposit increases balance", () => {
    account.deposit(500);
    expect(account.balance).toBe(1500);
  });
  
  test("withdraw decreases balance", () => {
    account.withdraw(300);
    expect(account.balance).toBe(700);
  });
  
  test("withdraw more than balance throws", () => {
    expect(() => account.withdraw(2000)).toThrow("Insufficient funds");
  });
  
  test("records transaction history", () => {
    account.deposit(200);
    account.withdraw(100);
    expect(account.transactions).toHaveLength(2);
  });
});