Node.js Unit Testing: Get Started Quickly With Examples

Node.js unit testing is important to verify the behavior of the smallest units of code in your application. It helps…

Node.js Unit Testing
By Testim,

Node.js unit testing is important to verify the behavior of the smallest units of code in your application. It helps improve the quality of your code and reduces the amount of time and money you spend on bug fixing. Moreover, unit testing helps you find bugs early on in the development life cycle and increases your confidence in the code.

Unit tests also contribute to your documentation. Moreover, they act as self-documentation. A team member joining your team can quickly understand your code’s purpose and functioning by reading the different test cases.

Moreover, unit tests improve your team’s development speed when refactoring code. Here, unit tests help you to validate the implemented changes without writing new tests.

This article will guide you through the concept of Node.js unit testing. You’ll learn about the benefits and discover the anatomy of a Node.js unit test. Then you’ll get some simple tips so you can get started writing your first unit test.

First of all, let’s discuss why testing is necessary.

Why Do You Need Testing?

Node.js unit testing helps you to guarantee the quality of your product. Imagine you launch a new application that requires the user to sign up. First of all, the user has to fill in all their data. Next, they hit the signup button to submit their profile. However, the application returns an error message, and the user isn’t able to sign up.

It’s very likely the user will remove the app and look for a better alternative. There’s no reason the user would wait for a better version of the app when even its first step fails.

This example shows the importance of testing your business logic. The easiest way to test your business logic is by using unit testing because it tests the smallest units of code in your application. Let’s learn more about it.

You can learn more about the software testing life cycle at Testim’s blog.

What Is Node.js Unit Testing?

The concept of unit testing focuses on examining the smallest parts of code by isolating that code. Node.js unit tests are easy to write and can be executed with almost no configuration, as they are often made of just one function call. Besides that, it’s easy to collect and display results from unit tests.

It’s important to write unit tests in a way that tries all possible outcomes of a snippet of code. In each unit test, the returned value of the unit must equal the expected value. If that’s not the case, then your test will fail.

Next, let’s learn about the benefits of Node.js unit testing.

Benefits of Node.js Unit Testing

If you aren’t convinced yet of the benefits of unit testing, here are a few elements you should consider when making your testing decision.

1. Improved Code Quality

It’s obvious that unit testing increases the quality of your code. It gives you the guarantee that your business logic is correct and no unexpected behavior can occur. Besides that, unit testing helps you find bugs and defects in your code.

2. Early Discovery of Code Bugs

Testing helps you find bugs earlier on in the software development life cycle. Unit testing is especially useful for finding bugs in the business logic. By finding bugs earlier in the software development cycle, you can resolve problems earlier so they won’t affect other pieces of code later on.

In addition, the earlier detection of bugs also reduces the overall cost of development because you spend less time on bug fixing in a later stage of the project.

3. Validation of Your Design and Code Structure

When writing tests, you’re indirectly thinking about the design of your code and how it fits into the larger codebase. Unit testing is a second chance to validate your architecture and code structure. In other words, you’ll have to think about writing testable code and make design decisions according to this. You want to avoid designing complex components that are hard to test.

Besides that, when you’re writing unit tests, you’ll review your code a second time. This means you might find problems with your code, such as illogical flows that need to be resolved.

4. Improved Customer Satisfaction

As unit testing contributes to the overall quality of your project, you’ll likely increase customer satisfaction. A product with a higher quality helps you gain trustworthy clients that rely on your quality. Product quality is a key metric that helps you stand out in the market—especially a saturated market.

5. Generate Code Coverage Reports

First, let’s define a code coverage report with a practical example from the Microsoft .NET documentation.

Code coverage is a measurement of the amount of code that is run by unit tests – either lines, branches, or methods. As an example, if you have a simple application with only two conditional branches of code (branch a, and branch b), a unit test that verifies conditional branch a will report branch code coverage of 50%.

In other words, without a code coverage report, you don’t know what percentage of code is covered by tests. This is important to increase confidence in your code but also to have a metric that defines code quality.

Most often, a code coverage report tells you the following metrics:

  • Branch coverage, such as  a conditional branch
  • Line coverage
  • Function coverage

For Node.js specifically, popular code coverage libraries include:

  • Instanbul: Works with most testing frameworks, such as tap, mocha, AVA, and many more
  • Jest: Offers a CLI option --coverage to generate a code coverage report.
  • C8: Code coverage using Node.js’s built-in functionality
  • Codecov: Code coverage tool for 20+ languages that integrates well with most CI/CD pipelines.

The Anatomy of a Node.js Unit Test

A Node.js unit test consists of three steps. Let’s explore them.

Step 1: Arrange

Make sure that everything’s ready for running your test. This includes setting the right state, mocking I/O operations, or adding spies to objects. (The “Unit Testing Concepts” section later in this post explains mocking and spying.)

Besides creating mocks and spies, test arrangement includes creating fixtures. A fixture includes a prefixed state or set of objects that you’ll use as a baseline for running tests.

Step 2: Act

In the “act” step, you call the function or piece of code you want to test.

Step 3: Assert

During the “assert” step, you want to validate if the code produces the expected output. Besides checking the output of a function, you can also determine if certain functions have been called with the right arguments or if a component contains the correct state.

The assert step generally uses expect statements to make those assertions.

Bringing It All Together

Let’s bring all three elements together to create a unit test. Here’s what one looks like:

Next, let’s explore two common concepts in unit testing.

Node.js Unit Testing Concepts

Let’s explore mocking, stubbing, and spying:

  • Mocking: The concept of mocking is primarily used in unit testing. You’d use mocking for isolating code by simulating the behavior of real objects and replacing the real object with the mocked object or function. For example, you can use a mock to make a function throw an error to evaluate how the function you’re testing handles this error.
  • Spying: A spy allows you to catch function invocations so you can later verify if the object got called with the right arguments. A spy won’t change the behavior of a function.
  • Stubbing: Stubs are similar to spies. Instead of spying on a function, you can use a stub to control the behavior of a function. For example, a function makes an HTTP call to an external API. As you want predictable behavior for unit tests that don’t rely on unpredictable outcomes from an external API, we can stub the API call with a predefined value to make the test predictable again.

Let’s take a look at some practical tips for writing unit tests.

4 Tips for Writing Node.js Unit Tests

Consider these four helpful tips when writing your first unit tests.

1. Set a low number of assertions per unit test. A higher number of assertions indicates you might not be testing the smallest unit of code. Besides that, sometimes it makes sense to split your unit test into two test definitions that each test more specific behavior.

2. Avoid repeating assertions. If you’ve verified in one test that a certain property returns “null,” avoid asserting for this null value in other, similar tests. Too many assertion statements will bloat your test case definition. Besides that, too many assertions will make it harder for other developers to determine what they really want to verify with this test.

3. Stick to a strict usage of arrange, act, and assert. Doing so helps make your tests more readable and easier to understand for other developers.

4. Avoid using assertionless tests. An assertionless test calls a function and wants to verify if the function doesn’t throw any errors. It’s a bad practice to use these kinds of tests because they have a higher chance of returning false positives. The below code snippet is an example of a test without any assertions. It calls a function during the “act” step and has no assertions in the “assert” step. The test can fail only when the call during the “act” step fails.

Conclusion: Node.js Unit Testing Made Simple

In short, unit testing is focused on isolating code in order to verify the logic itself, not the integrations. You can isolate code by using a testing technique like mocking in order to simulate real objects’ behavior.

Unit tests are the most simple type of tests and are easy to write. You can increase the readability of unit tests by using the arrange, act, and assert pattern.

Good luck writing your first tests!

This post was written by Michiel Mulders. Michiel is a passionate blockchain developer who loves writing technical content. Besides that, he loves learning about marketing, UX psychology, and entrepreneurship. When he’s not writing, he’s probably enjoying a Belgian beer!

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