Writing Your First Unit Tests with Jest

Now that you have installed and configured Jest, it's time to dive into writing unit tests. This chapter will guide you through the basics and teach you how to write and run your first unit tests.

What is a Unit Test?

A unit test is an automated test that verifies the behavior of a small "unit" of code, typically a function or a method. The goal is to ensure that the unit of code produces the expected results for a predefined set of conditions.

Skeleton of a Unit Test with Jest

Tests in Jest are organized using test or it functions. A basic test follows this structure:

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Writing a Simple Unit Test

Let's write a unit test for a simple function that sums two numbers. Suppose we have the following code in a file called sum.js:

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Create a folder for tests and a test file:

Now, let's write the unit test in sum.test.js:

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This test verifies that the sum function returns 3 when passed 1 and 2 as arguments.

Using Matchers

Jest provides multiple matchers to verify different types of values. Here are some common matchers:

• toBe: Exact equality.
• toEqual: Object or array equality.
• toBeNull: Checks if the value is null.
• toBeDefined: Checks if the value is defined.
• toBeTruthy: Checks if the value is truthy in a boolean context.
• toContain: Checks if an array or string contains a specific value.

Example of using different matchers:

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Grouping Tests with Describe

To better organize your tests, you can group them using the describe block. This is useful for grouping related tests:

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Setup and Teardown

Sometimes, you need to perform some setup before a test runs or cleanup after a test finishes. Jest provides beforeEach, afterEach, beforeAll, and afterAll methods for these purposes:

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Running Tests

To run the tests, simply use the command:

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[Jest will search for and execute all defined tests and generate a detailed report with the results].

Generating Code Coverage Reports

To generate a code coverage report, you can use the --coverage flag:

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[This will generate a coverage report showing which parts of your code are covered by the tests].

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With these basic concepts and examples, you are ready to start writing and running your own unit tests with Jest. In the next section, we will cover testing DOM components using Jest and how you can verify the correct manipulation of the user interface.