Qs: Testing

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Why Testing?

Reduction in bugs reported after deployments, which led to faster release cycles and improved customer satisfaction.

Instill a culture of quality within the team.

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How would you implement a more robust testing strategy?

Included both unit tests and end-to-end tests.

Assess current testing practices and identify key areas where additional testing could be most beneficial.

Used mental models like the Testing Pyramid to design our testing efforts and agreed on a desired code coverage.

In the initial step, we would be happy with anything above 60%.

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What's A Testing Pyramid?

Metaphor for structuring automated tests.

Shape: pyramid — wide at the bottom, narrow at the top.

Core Idea:

1. Lots of unit tests (fast, isolated).

2. Fewer integration tests.

3. Even fewer E2E/UI tests (slow, brittle).

From top to bottom:

• Manual and Exploratory Tests

• UI and API Testing

• Integration Testing

• Component Testing

• Unit Testing

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What's Unit Testing?

Individual pieces of code — the smaller the better.

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What's Component Testing?

Tests objects independently, such as modules, classes, and programs. More thorough than unit testing. Often need access to a database or other components. Slower than unit tests.

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What's Integration Testing?

Combines the individual software modules and tests them together as a group.

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What's Manual and Exploratory Tests?

TODO

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Code Coverage

Software testing metric that measures the percentage of a program's source code that is executed when a test suite is run.

Measure % of functions that are tested.

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AAA Pattern

Technique to write adequate test cases.

A = Arrange A = Act A = Assert

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"Given > When > Then" Pattern

technique to write adequate test cases.

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What is a Test Double?

A generic term (Umbrella Term) for any kind of object that replaces a real component in testing.

Includes dummy, stub, fake, mock, and spy.

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Why Test Doubles?

To isolate the system under test (SUT) from external complexity.

Typically: test a function in isolation.

To do so, we need to provide a placeholder or replacement.

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Types of Test Doubles?

Type	Purpose	Behavior	Example Use Case
Dummy	Placeholder	Does nothing	Fill unused params
Stub	Provide test data	Returns predefined values	Feed input to SUT
Fake	Lightweight version	Works but not production-ready	In-memory DB
Mock	Verify interactions	Checks if calls were made	Ensure method was called

Summary:

1. Dummy = exists but unused

2. Stub = provides answers

3. Fake = simplified real thing

4. Mock = checks expectations

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What's a Dummy?

Passed around only because the method signature requires it.

Never actually used in the test.

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What's a Stub?

Provides predefined responses.

Used when the SUT needs some data.

No verification of interactions.

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What's a Fake?

Has a simplified but working implementation.

Not suitable for production.

Example: in-memory DB.

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Why use Mocks in Unit Tests?

If we do not provide a mock, the test will become an INTEGRATION TEST - not a unit test - as it will evaluate the function and its dependencies together.

Used to verify interactions with dependencies.

Typically created with a mocking framework (like Jest).

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Are test doubles only used in Unit Testing?

No — test doubles are not only used in unit testing.

But unit testing is where they are most common and essential.

Test doubles can also be useful outside of unit tests:

• Integration Testing: Sometimes you still want to isolate part of the system, e.g., replace a payment provider with a fake service to avoid hitting real APIs.

• System / End-to-End (E2E) Testing: Often discouraged, but still possible when external services are too costly or unstable. Example: Using a mocked email server to capture emails instead of sending real ones.

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To write a unit test for the following function we need to also ...

import { User } from "./types";
import validateUser from "./validateUser";
async function getUser(url: string): Promise<User | false> {
  const response = await fetch(url);
  const user = (await response.json()) as User;

  if (validateUser(user)) return user;

  return false;
}
export default getUser;

1. mock the validateUser function so the test is constrained to the current function

2. stub the fetch call, and return mock data to make the test independent of the availability/response of the backend

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What are Spies?

Spy function records the calls to the external function

For instance, to spy on a specific method, and make sure it was called with the right inputs.

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