Testing is a crucial part of application development because it ensures that the code works as expected and that changes do not introduce errors. In Node.js, there are several tools that allow us to perform unit, integration, and acceptance tests. In this chapter, we will learn to write tests using Mocha and Chai, and how to test our APIs with Supertest.

Introduction to Testing in Node.js

In Node.js, application testing can be divided into several categories:

• Unit Tests: Verify if an individual unit of code (e.g., a function) works correctly.
• Integration Tests: Test how different modules or components interact within an application.
• Acceptance Tests: Validate that the application works as required, from the user's perspective.

Installing Mocha and Chai

Mocha is a popular testing framework that provides a flexible way to define and run tests. Chai is an assertion library that works well with Mocha. Let's install them:

bash

Creating a Unit Test with Mocha and Chai

Let's create a simple unit test for a function that adds two numbers. Create a file sum.js with the following content:

javascript

Now, create a test file called test/sum.test.js and define a test for this function using Mocha and Chai:

javascript

Running the Tests

To run the tests, we can add a script in our package.json and then run Mocha from the terminal:

json

Then, run the following command:

bash

Testing APIs with Supertest

Supertest is a tool that allows us to perform integration tests of our APIs in Node.js. Let's see how we can test a basic API using Express.

Installing Supertest

First, let's install Supertest as a development dependency:

bash

Creating an API to Test

Imagine we have a basic API that returns a list of users. Here's the code for app.js:

javascript

Creating an API Test with Supertest

Now, let's write a test to verify that the API returns the user list correctly. Create a test file called test/app.test.js:

javascript

Code Coverage Tools

To ensure that all parts of our code are being tested, we can use a code coverage tool like nyc. Install nyc with the following command:

bash

Then, add the following script in your package.json to run the tests with coverage:

json

Run the tests with:

bash

Best Testing Practices

When testing Node.js applications, it is important to follow some best practices:

• Small and Modular Tests: Tests should focus on a single unit of code to facilitate debugging.
• Test Isolation: Tests should not depend on each other; they should be independent and reproducible in any environment.
• Automation: Integrate tests into your CI/CD flow so they run automatically whenever code changes are made.

Summary

In this chapter, we have learned to write unit and integration tests in Node.js using Mocha, Chai, and Supertest. We have also explored how to measure code coverage with nyc. Testing is a vital part of developing robust applications as it ensures that our code works correctly before moving to production.