Mastering Unit Testing: 9 Simplified Principles Every Aspiring Full-Stack Developer Should Know

Unit testing, simply put, is the process of testing small, isolated parts (or units) of your code to confirm they function correctly.

It ensures that each code component works properly before integrating them into the system.

This article serves as a comprehensive guide for aspiring full-stack developers, providing nine essential principles to enhance your coding skills.

Principle 1: Understanding the Basics of Unit Testing

Unit testing verifies whether individual parts of your code—called units—function as intended.

Why is this important?

Well, consider building a car. Before putting all the pieces together, you’d want to make sure each part works flawlessly on its own—the engine starts, the brakes function, and so on.

What is Unit Testing, Why We Use It, and Sample Test Cases by Lets Build That App

Unit testing does the same for your code, checking that each component performs its job correctly.

// Let's say we have a simple function to add two numbers.
function add(a, b) {
  return a + b;

// A corresponding unit test might look like this.
test('Addition function should return the correct sum', () => {
  const result = add(2, 3);
  assert.equal(result, 5);

In this example, we’re testing a basic addition function. The test checks if the function returns the correct sum for given inputs.

This simple test is the building block of unit testing.

The value of unit testing. And how to use it wisely.
Everyone unit tests, they don’t necessarily realize it.

Principle 2: Writing Testable Code

Testable code is like a well-organized toolbox—it makes your job easier.

Characteristics of Testable Code:

  1. Modularity: Break down your code into small, manageable modules. Each module should have a specific responsibility, making it easier to test in isolation.
// Instead of a monolithic function, break it down.
function calculateTotal(price, quantity) {
  return price * quantity;

// Testable version with modular functions.
function calculatePrice(price, quantity) {
  return price * quantity;

function calculateTax(price, taxRate) {
  return price * taxRate;

  1. Minimized Dependencies: Reduce dependencies between different parts of your code. This way, you can test one unit without worrying about the intricate details of others.
// Avoid unnecessary dependencies.
function calculateTotal(price, quantity, taxRate) {
  const priceWithTax = calculatePrice(price, quantity) + calculateTax(price, taxRate);
  return priceWithTax;

// Testable version without unnecessary dependencies.
function calculateTotal(price, quantity, taxRate) {
  const priceWithTax = calculatePrice(price, quantity) + (price * taxRate);
  return priceWithTax;

By adopting these qualities, you create the path for better unit testing.

Principle 3: Choosing the Right Testing Frameworks

These frameworks provide a structured environment for creating and executing tests, ensuring a smooth testing experience.

Popular Testing Frameworks for Full-Stack Development:

  1. Jest: a framework that is well-known for its simplicity and speed. It is particularly popular in the JavaScript ecosystem.
Jest Testing Framework - Popular Unit Testing Tools Used by DevOps Engineer
// A simple Jest test.
test('Adding 2 + 3 equals 5', () => {
  expect(add(2, 3)).toBe(5);

  1. JUnit: Widely used in the Java world, JUnit is a powerful framework for testing Java applications.
// A basic JUnit test.
public void additionTest() {
  assertEquals(5, Calculator.add(2, 3));

  1. PyTest: If you’re working with Python, PyTest is a user-friendly framework that simplifies the testing process.
# A straightforward PyTest example.
def test_addition():
  result = add(2, 3)
  assert result == 5

Choosing the Most Suitable Framework:

When selecting a framework, consider factors like community support, ease of use, and compatibility with your tech stack.

Jest, for instance, is great for JavaScript projects, while JUnit is a natural fit for Java applications.

Principle 4: Creating Isolated Test Cases

This principle ensures that your tests remain reliable and provide accurate feedback on specific units of code.

Why Is Isolation Important?

  1. Independence: Each test should operate independently of others, preventing a ripple effect if one test fails. This independence makes it easier to pinpoint and fix issues.
// An isolated test case using Jest.
test('Multiplying 4 by 2 should equal 8', () => {
  expect(multiply(4, 2)).toBe(8);

test('Multiplying 3 by 5 should equal 15', () => {
  expect(multiply(3, 5)).toBe(15);

  1. Consistency: By isolating tests, you ensure that they produce consistent results regardless of the order in which they’re executed.
// Independent JUnit test cases.
public void multiplicationTest1() {
  assertEquals(8, Calculator.multiply(4, 2));

public void multiplicationTest2() {
  assertEquals(15, Calculator.multiply(3, 5));

Creating separate test cases may demand additional effort, but the advantages in terms of dependability and manageability are definitely worthwhile.

Principle 5: Test Automation

Automating your unit tests means you can run them with minimal manual intervention, saving time and ensuring consistency.

Automation Testing vs Manual Testing | Manual vs Automation Testing | Intellipaat

Benefits of Test Automation:

  1. Time Savings: Automated tests can be executed rapidly, allowing you to run them frequently without a significant time investment.
// Automating a test suite in Jest.
npm test

  1. Consistency: Automated tests provide consistent and reproducible results, reducing the likelihood of human error.
// Running JUnit tests from the command line.
java -cp junit-4.13.2.jar:. org.junit.runner.JUnitCore TestSuite

  1. Early Detection of Issues: Automated tests catch issues early in the development process, preventing bugs from reaching production.
# Running PyTest from the terminal.

By adding test automation to your workflow, you can guarantee the reliability of your code and maintain an agile development process.

Principle 6: Continuous Integration and Unit Testing

Continuous Integration (CI) is like having a diligent assistant checking your work every time you make a change.

It’s a development practice that involves automatically testing and integrating code changes, and it pairs exceptionally well with unit testing.

Why Integrate Unit Testing with CI?

  1. Immediate Feedback: CI systems run your unit tests automatically whenever you make changes to the codebase, providing instant feedback on potential issues.
// Jenkinsfile for continuous integration.
pipeline {
  agent any
  stages {
    stage('Build and Test') {
      steps {
        script {
          checkout scm
          sh 'npm install'
          sh 'npm test'

  1. Consistent Build Environment: CI ensures that your tests run in a consistent environment, reducing the chances of “it works on my machine” scenarios.
# Travis CI configuration file.
language: node_js
  - 14
  - npm test

  1. Integration into Development Workflow: By integrating unit testing into CI, you make it an integral part of your development workflow, fostering a culture of code quality.
# GitLab CI configuration file.
  - test
    - pytest

By integrating unit testing into your CI process, you can ensure that your code is continuously validated, resulting in a stronger and more dependable application.

Principle 7: Mocking and Stubbing

Imagine a theater performance where actors employ substitutes for specific scenes.

In unit testing, mocking and stubbing act as these substitutes, assisting you in constructing controlled settings for your tests.

Understanding Mocking and Stubbing:

  1. Mocking: Involves creating fake objects that mimic the behavior of real ones. This allows you to isolate the unit being tested and observe how it interacts with its dependencies.
// Mocking an API call in Jest.

test('Fetches user data from an API', async () => {
  apiService.getUser.mockResolvedValue({ id: 1, name: 'John Doe' });

  const userData = await userService.fetchUser(1);

  expect(userData).toEqual({ id: 1, name: 'John Doe' });

  1. Stubbing: Involves replacing certain parts of a system with predefined responses. This is useful for creating specific scenarios, such as simulating network errors or testing error-handling behavior.
// Stubbing a network request in Sinon.
sinon.stub(requestModule, 'send').throws(new Error('Network error'));

try {
  const result = myFunctionThatUsesNetworkRequest();
} catch (error) {
  expect(error.message).toBe('Network error');

By including mocking and stubbing in your tests, you can have control over external factors and make sure that your units function as intended, even in difficult situations.

Principle 8: Handling Dependencies in Unit Testing

Dependencies can be both a developer’s best friend and worst enemy.

When it comes to unit testing, effectively managing dependencies is crucial to keep the focus on the specific unit being tested, without getting sidetracked by external factors.

Strategies for Handling Dependencies:

  1. Dependency Injection: Pass dependencies as parameters to the functions or objects that require them. This allows you to substitute real dependencies with mocks or stubs during testing.
// Using dependency injection in Java.
public class OrderProcessor {
  private PaymentGateway paymentGateway;

  public OrderProcessor(PaymentGateway paymentGateway) {
    this.paymentGateway = paymentGateway;

  public void processOrder(Order order) {
    // Process the order using the injected payment gateway.

  1. Dependency Inversion Principle (DIP): Abide by the Dependency Inversion Principle, which states that high-level modules (e.g., business logic) should not depend on low-level modules (e.g., data access), but both should depend on abstractions (e.g., interfaces).
# Applying Dependency Inversion Principle in Python.
class PaymentProcessor:
    def __init__(self, payment_gateway):
        self.payment_gateway = payment_gateway

    def process_payment(self, order):
        # Process the order using the injected payment gateway.

By implementing these techniques, you enhance the adaptability and testability of your code.

Principle 9: Test Coverage and Metrics

Test coverage and metrics provide valuable insights into the effectiveness of your unit tests, helping you identify areas that may need more attention.

You can’t improve what you don’t measure.

Peter Drucker

Importance of Test Coverage:

  1. Identifying Untested Code: Test coverage reveals which parts of your codebase are covered by tests and which ones are not. This helps you pinpoint areas that may be prone to bugs.
// Checking test coverage using Istanbul.
// Run your tests with coverage analysis.
npx jest --coverage

// View detailed coverage report.
open coverage/lcov-report/index.html

  1. Setting Targets: Establishing a target test coverage percentage can be a helpful goal. It ensures that a significant portion of your codebase is rigorously tested.
# Specifying a target test coverage percentage in
branch = True
source = my_project
omit = tests/*
fail_under = 80

Additional Metrics:

  1. Code Complexity: Metrics like Cyclomatic Complexity highlight complex areas of your code that may require more thorough testing.
// Checking code complexity using ESLint.
// Run ESLint with complexity analysis.
npx eslint --max-complexity 10

  1. Mutation Testing: Introducing intentional bugs into your code and ensuring your tests catch them is a powerful metric provided by mutation testing frameworks.
// Running mutation tests using Stryker.
// Execute mutation tests.
npx stryker run

Regularly evaluating test coverage and pertinent metrics guarantees the robustness and dependability of your codebase.


The article discusses the importance of unit testing and highlights nine essential principles that every aspiring full-stack developer should know in order to master unit testing.

Unit testing is more than just a practice; it’s a mindset that enhances your coding abilities.

Embrace these principles, incorporate them into your daily routine, and observe the beneficial effects on your development process.

Happy coding!