Best Testing Practices in Node.js

Testing is a critical aspect of software development, as it ensures your application works as intended and meets quality standards.
In Node.js, testing is essential for the early detection of bugs in public endpoints.

However, there are many challenges associated with testing in Node.
External dependencies, asynchronous operations, and several possible input scenarios make writing tests a daunting task. Additionally, determining which components and aspects of your application to prioritize for testing can be challenging.

With some guidelines and best practices, you can tackle testing challenges and ensure that your backend is robust and reliable.

In this tutorial, we'll run through 15 top testing practices to write efficient, effective, and easy-to-maintain tests in Node.

Time to become a master of Node testing!

The Golden Rule of Testing in Node.js

Before diving into Node.js testing best practices, remember this golden rule:

Testing code is not production code.

Keep testing simple and avoid over-engineering. Tests should be intuitive and validate functionality, rather than implement complex design patterns or architectures.

Top 15 Node.js Testing Best Practices

Now that you've learned the golden rule of testing in Node.js, we'll explore 15 top testing best practices. Keep in mind that these tips apply to any Node test runner or testing library.

Let's dive in!

1. Consider the Test Pyramid

The Test Pyramid is a concept devised by Mike Cohn that first appeared in his book Succeeding with Agile, published in 2009.
This pyramid is a visual metaphor that explains how to approach testing.

Source: The Practical Test Pyramid

The Test Pyramid contains three layers. These represent what a test suite should consist of and the amount of testing to be performed on each level. The pyramid also highlights the expected performance and level of isolation required by each layer.

From a modern perspective, though, the test pyramid may appear overly simplistic. Even so, there are two lessons we can learn from it when designing Node.js tests:

1. Write tests with different levels of granularity.
2. The more high-level you get, the fewer tests you should have.

2. Define Easy-to-Understand Test Titles

Engineers or DevOps experts who take care of deployments may not be familiar with code. When a deployment fails because of a failed test, their first reaction will be to look at the logs or test reports. In this case, the only thing they might see is the name of the failed test.

That's why each test should have a title that is informative enough to explain exactly what the test does. Specifically, a test name should answer these three questions:

1. What is being tested?
2. What is the expected result?
3. Under what circumstances? / In what scenario?

As a reference, take a look at the Node.js test below:

// unit/component under test (1.)
describe("user authentication service", function () {
  describe("login functionality", function () {
    // expected result (2.) and circumstance/scenario (3.)
    it("should reject login with incorrect password", () => {
      // ...
    });
  });
});

3. Include Tags with Your Test Titles

Adding tags to test titles improves test organization. Tags such as "#api" or "#authentication" help categorize tests and allow you to run similar tests in batches, because Node test runners can usually execute tests that contain a specific string in their titles. Also, by using tags, it's easier to find tests using the search capabilities of your IDE.

4. Follow the Arrange, Act, Assert (AAA) Pattern in Your Tests

The Arrange, Act, Assert pattern ensures that each test has a clear scope and is well-structured. A test written according to this pattern should be based on these three phases:

• Arrange: Set up the test environment. This also includes adding records to the test database and defining any necessary stubs and/or mocks.
• Act: Execute the code that will be tested. That generally boils down to calling a single function or method.
• Assert: Verifies the expected outcomes.

This approach to test writing enhances readability and maintainability.

5. Focus on Testing Public Behavior

A well-known and effective strategy for API testing is to focus on public behavior. At the end of the day, the main goal of your testing operation is to make sure that your Node endpoints produce the expected results when given specific inputs. This contract-based approach to testing, also known as black-box testing, aims to test the expected input-output relationships without delving into internal implementation details.

Libraries like node-mocks-http enable the creation of req and res objects to call routing functions and simulate incoming requests.
You can then inspect the resulting res object to check API behavior, such as via JSON schema validation. Additionally, you can assert whether the HTTP status or headers set on the res object align with expectations.

By targeting public behavior, you can verify that your APIs adhere to defined contracts with just a few tests. This approach checks that your Node.js endpoints behave as intended for end users, without you having to test dozens of functions.

6. Use a Dedicated Database in Each Test

Using dedicated databases in every test is key to ensuring isolation and reproducibility. The biggest objection you might have is that this approach leads to data duplication. Well, don't forget the golden rule!

Employing shared databases for multiple tests can be the source of flaky behavior. A test might fail because the record it's supposed to operate on has been modified or deleted by another test running at the same time.

Setting up separate databases for each test prevents data contamination and interference between tests. This leads to robust test results and opens the door to safe test parallel execution.

7. Define an Effective Data Clean Strategy

The AAA pattern and previous best practices recommend that each test sets up its own database. To prevent filling your memory or disk with unnecessary data, you must define an effective data-cleaning strategy.

There are a few possible options for data cleaning, including cleaning the database:

• After every single test.
• After all tests have finished.
• Periodically, at set intervals.

The right strategy depends on your specific project requirements and architecture. Regardless of the approach selected, don't clean the database on test failures. Having access to the data associated with a failed test is essential for debugging and understanding what went wrong.

8. Use Realistic Input Data

When testing methods and functions that need input data, you might be tempted to use random strings and values like "foo" or "1234."
While this reduces the mental effort of coming up with accurate input, it's a bad practice. Instead, always test functions with realistic inputs. This way, you can ensure that your Node.js tests are more representative of real-world scenarios.

For automatic data generation, consider using libraries like Chance or Faker.
These can generate fake — yet realistic — data resembling production data, such as real-world phone numbers, usernames, credit card numbers, company names, and text.

9. Use Property-Based Testing to Cover All Possible Input Combinations

While you can test a function with realistic data, testing all possible user input combinations may take too much time, especially when accepting many inputs. The problem is that one of those untested combinations might lead to an unexpected result. That's where property-based testing comes in!

This technique involves automatically testing a function with a wide range of input combinations, increasing the chances of discovering bugs. Instead of manually selecting a few input samples, property-based testing generates numerous permutations to thoroughly examine how your code handles different scenarios. This helps identify edge cases and unexpected input interactions, detecting issues that might not emerge with manual testing.

For instance, the function addNewProduct(id, name, isDiscount) can be tested with multiple combinations of (number, string, boolean) such as (1, "iPear", false), (2, "Universe XL", true), or (0, undefined, null). By using libraries like fast-check, you can automate property-based testing within your favorite test runner.

10. Prefer Stubs and Spies Over Mocks

When using test doubles, prefer stubs and spies over mocks to maintain simplicity and clarity. Stubs replace functions and control their behavior, while spies track function calls and arguments. Both are less intrusive and easier to manage compared to mocks, which often involve complex logic to interact with the other components in your application.

Libraries like Sinon.JS provide robust support for stubs and spies, enabling precise control and inspection of your code's interactions. By favoring stubs and spies, you can keep your tests concise, easier to understand, and less prone to errors.

11. Avoid Catching Expected Errors

Some tests verify whether a function throws a specific error on faulty input. A common mistake when writing such tests is surrounding the function call with a try ... catch statement and asserting whether the test entered the catch branch.

Avoid that, as it only makes the test logic verbose and more difficult to read and maintain. Instead, prefer dedicated assertions like expect(<function call>).to.throw(<blank or error type>) in Chai or expect(<function call>).toThrow(<blank or error type>) in Jest. These assertions provide a clearer and more elegant way to test for thrown errors without catching them.

12. Don't Forget to Test Middleware Functions

As mentioned in testing tip 5 above, you should focus on testing public behavior. However, you shouldn't ignore internal code entirely. For instance, dozens of endpoints exposed by your backend may rely on the same middleware. A bug in that middleware would introduce faulty behavior across many APIs!

While tests for endpoints will fail because of that bug, understanding that the cause of all those failures is down to the same middleware may not be easy. Given their vital importance in a Node.js architecture, it makes sense to have tests dedicated to middleware functions.

A middleware can be tested like any other function. Forge req and res objects with libraries like node-mocks-http and verify that it performs the required actions on these objects. This way, you can guarantee that your middleware layer is reliable and doesn't introduce errors to your endpoints.

13. Produce Useful Test Coverage Reports

Most Node.js testing technologies come with reporting capabilities.
This means they can be configured to generate reports in HTML, XML, or other formats after a test suite run. These reports highlight test results and provide useful statistics.

The reports prove invaluable for QA engineers, who may not be familiar with a codebase, by identifying failing tests and areas of an application that require further tests. A key metric in these reports is code or test coverage, which summarizes the percentage of your codebase exercised during testing.

Coverage reports provide insights into the quality and completeness of your tests, highlighting areas and components that require additional attention. By analyzing and monitoring the results of these reports, you can identify untested code paths and monitor the evolution of your testing operation. You should always configure your testing library to produce test coverage reports.

14. Run Performance and Stress Tests

Writing tests in a Node.js application isn't always about test coverage. You also need to guarantee that your backend remains responsive and fast, even under stressful conditions. You can achieve that by running performance and stress tests. That ensures that your application can handle an unexpected workload and maintain optimal performance under various traffic conditions.

Performance tests evaluate response time, throughput, and resource usage to help you identify bottlenecks. Stress tests simulate heavy loads and peak traffic scenarios to assess system stability and resilience. By regularly performing these types of tests, you can proactively identify and address performance issues and prevent downtime.

15. Refer to Other Popular Best Practices

Staying up-to-date on popular best practices is critical.
Refer regularly to resources such as the Node.js docs, trusted blogs, and community forums to make sure that your tests remain robust, secure, and efficient. Don't forget that this, or any other, guide is only an introduction to the complex world of Node.js testing best practices.

Wrapping Up: Improve Your Node.js Testing Process

In this blog post, we took a look at the golden rule of Node.js testing and explored some of the most relevant best testing practices.

You now know:

• To consider testing code not as production code
• What components in your Node.js application you should focus your testing efforts on
• Many tips and tricks to keep your tests simple, effective, and easy to maintain

Thanks for reading!

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