Test-Driven Development in JS

Anton Ioffe - October 30th 2023 - 8 minutes read

Welcome fellow developers! In this insightful exploration of Test-Driven Development (TDD) in the realm of JavaScript, we're going to take you on a detailed journey, touching on everything from the basic concepts and methodologies of TDD to leveraging prominent tools like Jest. We'll probe into the integral types of tests, demystify the role of test doubles, and delve into the exciting world of single-page applications. Armed with real-world examples and practical tips, this article seeks to enhance your mastery of JavaScript TDD, providing a sturdy foundation to build upon and an edge in this fast-paced world of modern web development.

Grasping the Concept of Test-Driven Development in JavaScript

Understanding Test-Driven Development (TDD) is a pivotal part of modern JavaScript development. This approach stands out because it encourages developers to write automated tests before they pen down the actual code. The tests act as a tentative roadmap of the desired outcome, ensuring that the final code aligns with the developer's initial vision, rather than mere validation.

Consider a practical example. Suppose you are tasked with developing an arithmetic module for basic addition operations. Instead of producing the code on the fly, TDD proposes drafting the expected behavior using tests. Let's illustrate this using pseudocode.

function testAddition() {
    // Arrange
    let calculator = new Calculator();

    // Act
    let result = calculator.add(5, 6);

    // Assert
    assert(result == 11);

Please note that this is a piece of pseudocode. The purpose here is to imply a scenario where our not-yet-developed calculator should return 11 when adding 5 and 6 together.

Once this preliminary step is through, you write the actual code, run the tests, and refactor as needed. TDD's efficacy lies in this repetitive cycle of test-code-refactor. This method may initially seem to defy logic, but it proves to be exceptionally beneficial, especially when working with comprehensive projects where quality and maintainability are priorities.

TDD has a learning curve, but once you're past it, the benefits are substantial. It refines code discipline and instills confidence in the codebase you create. As such, TDD is not just a methodology but a philosophy that can significantly enhance how you approach JavaScript development. Yes, it's a shift from the traditional development paradigms, but it's a shift that is worth making.

Leveraging Jest for JavaScript TDD

Setting up your development environment for leveraging Jest in Test-Driven Development (TDD) follows a straightforward process. Begin by installing the latest LTS versions of Node and NPM from the Node.js website. Those preferring to use Yarn must ensure a version of 0.25 or later. Once these requirements are met, proceed to create your project directory, let's say, mkdir jest-tdd-setup. Use the cd projectName command to open your project directory.

Writing tests using Jest is where TDD comes to life. Consider an instance where you're writing tests for a function welcomeMessage. Initially, the function doesn't exist, and naturally, the test would fail. This failure kicks off the Red-Green-Refactor cycle in TDD. After your tests are written - the red phase - write just enough code to make the tests pass, ushering in the green phase.

function welcomeMessage(name) {
    // Initially this function is empty

test('welcome message test', () => {
    // The test will fail as welcomeMessage doesn't return anything yet
    expect(welcomeMessage('John')).toBe('Hello, John!');

Jest's feedback system plays a crucial role in rectifying errors when tests fail. Once all tests pass, consider the code from the perspective of its testability, simplicity, and readability. If your code can be improved, refactor it without changing its external behavior. Remember to rerun your tests after refactoring to guarantee your code still produces the expected outcomes.

In the context of Jest's interaction with ES6 modules, you're likely to encounter an essential function: 'jest.mock()'. This function enables you to replace the actual module with a mock module, playing a pivotal role in isolation for unit tests. Here's an example:

jest.mock('./moduleName', () => {
    return {
        functionName: jest.fn()

However, to optimally use ES6 modules in Jest, there's a need for a transpiler like 'babel-jest' which enables Jest's understanding of ES6 import/export syntax. The step-by-step setup process is as follows:

  1. Install 'babel-jest' and '@babel/core' using npm by typing npm install babel-jest @babel/core into your command line.
  2. Set up a babel.config.js file at the root of your project.
  3. State the presets that your project uses in this file.

Leveraging Jest for implementing JavaScript TDD offers valuable insights at every stage of development, enhancing the precision of your code while promoting an effective workflow. The use of Jest indeed accelerates the early detection and rectification of bugs, enhancing the reliability of your applications and ensuring a smoother development process. Above all else, incorporating Jest in your TDD approach propels you further into best practices rooted in self-testing code, clear specifications, and maintainability, optimally preparing you for ambitious JavaScript projects.

Test Types in TDD: Unit, Integration, and End-to-End

In the world of Test-Driven Development (TDD), tests are the lifeblood of your code verification process and generally fall into three main categories: unit tests, integration tests, and end-to-end (E2E) tests. Each type of test addresses a specific aspect of code verification, ensuring the entire system, from small units to complete user flows, behaves as expected.

Unit tests, at the base level, are focused on individual units of code, be it a single function or a component in isolation. Let's put it into context with an example: say we're working on an e-commerce application and our task is to create a function calculateTaxes(rate, amount). A unit test for this function would verify its basic operation - accurately calculating taxes based on a given amount.

function calculateTaxes(rate, amount){
    return rate * amount / 100;

test('check calculateTaxes()', () => {

Here, we validate the calculateTaxes() function to ensure it functions as expected in isolation.

Moving up the hierarchy of complexity, integration tests scrutinize how different units of your code interact and function together. Consider another function calculateTotal(amount, rate). This function uses the tax calculated by calculateTaxes() to compute a total payable amount.

function calculateTotal(amount, rate){
    return amount + calculateTaxes(rate, amount);

test('check calculateTotal()', () => {

The integration test here checks the interaction between calculateTotal() and calculateTaxes(), verifying that they integrate correctly.

Lastly, to ensure that the whole application runs smoothly, End-to-End (E2E) tests step in. They perform a full-cycle check, from the user initiating an action on the interface to the expected result. For instance, in our e-commerce application, an E2E test might simulate a user adding items to the cart, applying a coupon, proceeding to checkout, and completing the payment procedure to confirm that the entire flow works in harmony.

In sum, thoroughly understanding and appropriately utilizing these different types of tests cab bolster the robustness and reliability of your JavaScript applications. While the initial effort needed to get onboard with TDD may seem daunting, it soon pays off, leading to efficient and robust code that stands up to the test of time.

Demystifying Test Doubles in JavaScript TDD

Understanding Test Doubles in the context of JavaScript TDD allows us to write precise, versatile test cases that evaluate our code's functionality under various conditions. A Test Double acts as a placeholder, mimicking a portion of the code during testing, effectively minimizing direct dependencies and potential malfunctions from these dependencies. In essence, this simplifies the environment in which code runs during a test, ensuring more accurate results.

There are four main types of Test Doubles employed in TDD: dummy, mock, fake, and stub. Dummies are simple placeholders, primarily used to fill parameters in functions but lack substantial functionality. For instance, when testing a function that interacts with an API and requires a token, a dummy token serves the purpose effectively, allowing the test to proceed without actual API interaction.

function testFunction(apiToken) {
    return apiToken; 

let dummyToken = 'xyz';
testFunction(dummyToken); // The function can now execute

In contrast to dummies, a mock goes a step further; it verifies if the code interacts as planned. Mocks in JavaScript TDD are especially handy in testing whether certain function invocations occur with the expected parameters. This can be done using various available libraries, an example being sinon.js.

let mock = sinon.mock(myObject);

Fakes and stubs provide partial, customizable implementations for the objects they replace. A stub completely overwrites a portion of the code and delivers predefined results, providing a fixed path for code execution. Fakes, like stubs, offer controlled behavior but provide more extensive implementation, reproducing the behavior of real objects in a controlled manner.

let stub = sinon.stub(database, 'save') // Assuming sinon is set up
stub.returns('Data saved')
let result = myFunction()  // This will call but use the stub we created

Overall, Test Doubles, whether dummy, mock, fake, or stub, can significantly increase the reliability and flexibility of your unit tests in JavaScript TDD, acting like precise control levers for code execution during testing. Important to note is how each type serves its unique purpose and fits in different test scenarios, hence understanding them and their suitable usage are essential in maximizing the efficacy of TDD.

Single-Page Applications and Advanced TDD Concepts in JavaScript

When it comes to single-page applications (SPAs) like those built with React or Angular, the testing strategy becomes a bit more intricate. The components in these SPAs are JavaScript-based, which makes them perfect candidates for a TDD approach. When writing a test for a React or Angular component, consider a few things. First, ensure the component renders correctly. If the component relies on props or state, modify these factors in the test to ensure the component behaves as expected. Second, test event handling in the component. For instance, if there's a button that toggles a piece of state when clicked, make a test that simulates this click and checks if the state changed.

Moving on to services - they usually handle communication with external APIs and return data to the components. The challenge of testing them lies in ensuring that they correctly call the required endpoint and handle the response appropriately. Jest provides a range of options for handling API requests within a test. The jest.fn() function can be used to create a mock function that returns a resolved promise, representing a successful API call. By combining this with the fetch.mockImplementation() method, it's possible to simulate API calls without having to connect to a real API during the test.

Our JavaScript-based applications tend to grow in complexity, necessitating the increased usage of mocking. The fundamental principle of mocking within TDD is to isolate each unit test from others, thus ensuring the correct functioning of individual units. This approach provides a means to control the application's behavior during testing. For instance, when using Jest, you might mock a module that makes API calls to ensure your tests aren't making network requests. Instead, you simulate the module behavior, ensuring that the function returns the correct data when the API call is successful and handles errors appropriately.

It's worth noting that single-page applications also bring unique challenges to TDD. As developers, we need to be continuously learning and evolving our techniques to conquer these hurdles. Considering the application of advanced TDD concepts within your SPA, what particular challenges have you experienced, and how have you overcome them?


In this article about Test-Driven Development (TDD) in JavaScript, the author explores the concept of TDD and its benefits in modern web development. They discuss the use of Jest as a prominent testing tool and explain the different types of tests in TDD: unit, integration, and end-to-end. The article also demystifies the concept of test doubles in JavaScript TDD. Finally, the author discusses the challenges and considerations of applying TDD in single-page applications. The key takeaways from the article are the importance of writing automated tests before coding, leveraging tools like Jest for TDD, understanding the different types of tests, and utilizing test doubles effectively. The challenging task for readers is to consider the advanced TDD concepts in single-page applications and share their experiences and techniques for overcoming challenges in this context.