41. Unit and integration tests in Javascript

Unit and integration testing are crucial parts of the software development process. They ensure that the code works as expected and that new changes do not break existing functionality. In this chapter, we'll explore how to write and run unit and integration tests in JavaScript, a programming language that is a fundamental part of front-end development.

Unit tests are tests that verify the functionality of a single unit of code, such as a function or a method. They are isolated from the rest of the system and only test the unit of code under test. For example, if you have a function that adds two numbers, a unit test for that function can verify that the function returns the correct sum of the numbers.

Integration tests verify the functionality of several code units working together. They test the interaction between different parts of the system to ensure they work correctly together. For example, if you have a system that includes a user interface, a server, and a database, an integration test can verify that a user action in the interface results in the correct change to the database.

In JavaScript, there are several libraries and frameworks that can be used to write and run unit and integration tests. Some of the most popular include Jest, Mocha, Jasmine and Karma. These tools provide an easy-to-use syntax for writing tests and also include features like mocking, which allow you to isolate the unit of code under test.

Let's use Jest as an example to demonstrate how to write a unit test in JavaScript. Suppose we have the following function:

function sum(a, b) {
  return a + b;
}

We can write a unit test for this function as follows:

test('sum 1 + 2 to equal 3', () => {
  expect(sum(1, 2)).toBe(3);
});

The above test verifies that the sum function returns the correct value when 1 and 2 are passed as arguments. If the function returns 3, the test passes. If not, the test will fail.

For integration testing, we can use the same Jest library, but we need to ensure that we are testing the interaction between different parts of the system. For example, we might have a test that checks whether a user action results in the correct call to an API:

test('user action results in correct API call', async () => {
  // Simulate the API
  axios.get.mockResolvedValue({ data: { result: 'success' } });

  // Performs user action
  await usuario.realizaAcao();

  // Checks if the API was called correctly
  expect(axios.get).toHaveBeenCalledWith('/api/acao');
});

This test simulates the API using the axios-mock-adapter library, performs a user action, and then verifies that the API was called correctly. If the API was called with the correct URL, the test will pass. If not, the test will fail.

In summary, unit and integration tests are valuable tools for ensuring the quality of JavaScript code. They allow developers to verify the functionality of code and ensure that changes do not break existing functionality. With libraries and frameworks like Jest, Mocha, Jasmine, and Karma, it's easy to write and run these tests in JavaScript.

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