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✅ Testing

Testing is the process of checking software to ensure it works correctly, meets requirements, and is free of bugs.

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📌 Why Testing is Important (Key Points):

1. ✅ Finds bugs early
2. ✅ Ensures software quality
3. ✅ Prevents crashes and failures
4. ✅ Saves time and cost
5. ✅ Improves security
6. ✅ Builds confidence
7. ✅ Enhances user experience
8. ✅ Validates requirements

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Some types:

✅ 1. Unit Testing

• What it is: Testing individual components (like functions, methods, or classes) in isolation.
• Purpose: To ensure that each unit of code works correctly on its own.
• Example: Testing a calculateTotal(price, tax) function to see if it returns the correct result.
• Tools: Jest, Mocha, JUnit, NUnit, PyTest, etc.
• Done by: Developers.

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🔗 2. Integration Testing

• What it is: Testing the interaction between multiple components or modules.
• Purpose: To verify that integrated units work together correctly.
• Example: Testing a user login flow where the frontend form, backend API, and database are all involved.
• Tools: Postman, JUnit, Supertest, TestNG, etc.
• Done by: Developers or QA engineers.

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🌐 3. End-to-End (E2E) Testing

• What it is: Testing the entire application flow from the user's perspective.
• Purpose: To ensure the whole system works as expected across all layers (frontend, backend, database).
• Example: Simulating a real user who signs up, logs in, and completes a purchase.
• Tools: Cypress, Playwright, Selenium, Puppeteer.
• Done by: QA engineers or automation testers.

some Nodejs testing library

• jest
• moka
• Nodejs build-in

Mocking

✅ What is Mocking?

Mocking is a technique used in testing where you create fake versions of functions, modules, or services so you can:

• Isolate the unit you're testing
• Avoid calling real dependencies (e.g., databases, APIs)
• Simulate different scenarios and behaviors

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📌 Why Use Mocking?

1. ✅ Test in isolation – Focus only on the component being tested
2. 🚫 Avoid side effects – No real API calls, DB writes, or external service usage
3. ⏱️ Improve test speed – Mocks are faster than real operations
4. ⚙️ Control behavior – You decide what the mock returns (success, error, etc.)

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🧪 Example in JavaScript (Jest):

// real function
function fetchUser(id) {
  return axios.get(`/user/${id}`);
}

// test with mock
jest.mock('axios'); // mock axios

test('fetchUser returns data', async () => {
  axios.get.mockResolvedValue({ data: { name: 'Bikram' } });
  
  const response = await fetchUser(1);
  expect(response.data.name).toBe('Bikram');
});

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🔁 Types of Test Doubles:

Type	Description
Mock	Fake with behavior you define/test
Stub	Predefined responses (no logic)
Spy	Tracks function calls and arguments
Fake	Simpler implementation of a real one
Dummy	Passed but not used (e.g., null obj)

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