Writing unit tests is an essential part of quality software development. In this chapter, we will learn how to write our first unit tests using Mocha. We will explore the basic concepts and look at practical examples.

What is a Unit Test?

A unit test is a form of validation that focuses on small units of code, which are usually specific functions or methods. The goal is to ensure that each unit of code behaves as expected in various scenarios.

Basic Structure of a Test in Mocha

Mocha provides a clear and concise structure for defining and grouping tests. The key components are:

• describe(): Groups multiple related tests.
• it(): Defines an individual test.
• assert: Assertion library (you can use others like Chai).

Here is a basic outline of what a test looks like in Mocha:

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Writing Your First Unit Test

Consider a JavaScript file calculator.js with a simple function:

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Let's write a unit test for this function.

Set Up the Test File

Create a test file called calculator.test.js in your test folder:

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Run the Test

Run the test using the command:

sh

You should see output indicating that both tests have passed, similar to this:

More Examples of Unit Tests

Testing a Function with Conditional Behavior

Suppose we want to add a new function that only sums positive numbers:

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We can write a unit test to ensure this function behaves correctly:

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Testing Asynchronous Functions

To test asynchronous functions, you can use done to indicate the end of a test:

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Tests with Assertion Libraries

Mocha is compatible with various assertion libraries like Chai. Later, we will see in detail how to combine them to improve the readability and functionality of your tests.

Conclusion

You now have a basic understanding of how to write unit tests with Mocha. In the following chapters, we will delve into the combined use of Mocha with other tools and advanced techniques to further enrich your tests.