Testing is an essential part of software development, and TypeScript provides various tools and libraries to efficiently perform testing. In this chapter, we will learn how to set up a testing environment for TypeScript and how to write unit and integration tests to ensure that our code works correctly.

Setting Up the Testing Environment

The first step to writing tests in TypeScript is to set up a suitable environment. There are several testing libraries that are compatible with TypeScript, such as Jest and Mocha. In this chapter, we will use Jest as an example since it is one of the most popular libraries.

Installing Jest with TypeScript

This image shows a logo of Typescript plus jest

To start, we need to install Jest and the necessary tools to integrate it with TypeScript.

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After installing the dependencies, we need to create a configuration file for Jest:

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Configuring jest.config.js

The jest.config.js file allows customizing the behavior of Jest. Here is an example of a basic configuration:

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Writing Unit Tests

Unit tests verify the behavior of individual units of code, such as functions or classes. Let's see how to write a basic unit test using Jest and TypeScript.

Example of a Unit Test

Suppose we have the following function in a math.ts file:

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Now we will write a test for this function in a math.test.ts file:

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Running the Tests

To run the tests, simply execute the following command:

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Asynchronous Tests

In TypeScript, we can also write tests for asynchronous functions using async and await. Let's see an example of a test for an asynchronous function that makes a request to an API.

Example of an Asynchronous Test

Suppose we have the following function in an api.ts file:

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The test for this function would be as follows:

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Mocking in TypeScript

Mocking is a technique used to simulate external dependencies in tests, which allows isolating the unit being tested. Jest provides tools to mock functions and modules.

Example of Mocking

Suppose we have a function that depends on a call to an external API:

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We can simulate the API response using mocks in our tests:

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Code Coverage

Code coverage measures what percentage of our code is being tested. Jest includes a built-in tool to generate code coverage reports.

To generate a coverage report, execute the following command:

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Conclusion

In this chapter, we have learned how to set up a testing environment in TypeScript using Jest, how to write unit and asynchronous tests, and how to use mocks to simulate external dependencies. We also saw how to measure code coverage to ensure that our project is well tested.