jest mock implementation once with code examples

Testing is an essential part of software development. It ensures that the code written is functioning as expected and is free from bugs. And, Jest, a popular testing framework, has made writing tests in JavaScript a lot easier.

When it comes to testing JavaScript applications, we often need to test functions that have side effects. In such cases, we need to mock and stub the functions to isolate the code we want to test from the external dependencies. Jest provides us with a mocking library to do just that.

In this article, we will learn about Jest's mock implementation once feature and see how we can use it to mock a function once and test its behavior.

What Is Jest Mock Implementation Once Feature?

Jest's mock implementation once feature allows us to mock a function's implementation just once. It means that the function's original implementation will be restored after the first call.

By default, Jest's mocking library mocks a function's implementation for the entire test suite. However, there are cases when you need to mock a function's implementation only once, and this is where the mock implementation once feature comes into picture.

Jest Mock Implementation Once Example

Let's say we have a function that makes an HTTP request to fetch some data (we will use the Axios library for this example). Our function looks like this:

import axios from 'axios';

export async function fetchData() {
   const response = await axios.get('https://jsonplaceholder.typicode.com/todos/1');
   return response.data;
}

Now, let's say we want to test this function without making an actual HTTP request. To do this, we need to mock the axios.get() function and return our own data.

Here's how we can do it:

import axios from 'axios';
import { fetchData } from './fetchData';

jest.mock('axios');

test('fetchData should return mock data', async () => {
   const mockData = {
      userId: 1,
      id: 1,
      title: 'delectus aut autem',
      completed: false,
   };

   axios.get.mockImplementationOnce(() => Promise.resolve({ data: mockData }));

   const data = await fetchData();

   expect(data).toEqual(mockData);
   expect(axios.get).toHaveBeenCalledTimes(1);
   expect(axios.get).toHaveBeenCalledWith('https://jsonplaceholder.typicode.com/todos/1');
});

In the code above, we first import axios and the fetchData function we want to test. Then, we mock the axios module using jest.mock(). This will replace the axios module with a mock version.

Next, we define our mock data and use axios.get.mockImplementationOnce() to mock the get function of the axios module. The mockImplementationOnce() function will mock the function only once, and after that, it will return its original implementation.

Inside the mockImplementationOnce() function, we return a Promise that resolves to our mockData object.

Finally, we call our fetchData function and assert that it returns the mockData object we defined, and that the axios.get() function was called exactly once with the correct URL.

Conclusion

In this article, we learned about Jest's mock implementation once feature and saw how we can use it to mock a function once and test its behavior. By using this feature, we can isolate our code and test it without worrying about the implementation of external dependencies. And, with Jest's convenient and intuitive API, writing tests has become a lot easier and quicker.

here are some more details about the previous topics discussed in the article.

Testing & Jest

Testing is a crucial aspect of software development. It helps to ensure that the code works as intended, is free from bugs and errors, and can handle edge cases and exceptions.

Jest is a popular testing framework for JavaScript that makes writing tests easy and fun. It provides an intuitive API, a wide range of features, and thorough and accurate test reporting.

Jest's mocking library is one of its key features, allowing developers to simulate the behavior of external dependencies, like APIs and databases.

Mocking & Stubbing

Mocking and stubbing are techniques used in testing to isolate the code being tested from its dependencies.

Mocking is the process of creating a fake version of an external dependency, such as an API or a database, and using this fake version in place of the real one in the tests. Mocks are useful for testing functions that call external APIs or services, as it allows developers to control the data returned by the mock and simulate different scenarios.

Stubbing is the process of replacing a method or function with a pre-determined response. Stubbing is useful when testing asynchronous functions or function calls that are difficult or time-consuming to set up. It speeds up test execution time and ensures that the function being tested is the focus of the test, rather than the external dependency's implementation.

Axios

Axios is a popular JavaScript library used for making HTTP requests from the browser or Node.js. It provides an easy to use API, supports promises, and works with both JSON and XML-based data.

Axios is widely used in modern web development, particularly in creating web applications that rely on APIs to communicate with servers.

In our example in the article, we used Axios to make an HTTP request to fetch some data from a remote API. Using Jest's mocking library, we were able to mock the Axios library to simulate the HTTP request and return our own data for testing purposes.

Popular questions

  1. What is Jest's mock implementation once feature, and why is it useful for testing functions?

Jest's mock implementation once feature allows developers to mock a function's implementation just once, rather than for the entire test suite. This is useful for testing functions that may be dependent on external resources, such as APIs or databases, as it isolates the function being tested and ensures that the test only focuses on the function's behavior.

  1. How does Jest's mocking library work, and how can it be used to mock external dependencies like APIs?

Jest's mocking library works by creating a fake version of an external dependency, such as an API or database. These mocks can then be used in place of the real dependency in the tests, allowing developers to control the data that is returned and simulate different scenarios. To mock an external dependency, developers can use Jest's mock function and provide it with an implementation that returns the desired data.

  1. What is the difference between mocking and stubbing, and when is each technique appropriate for testing?

Mocking and stubbing are both techniques used to isolate code being tested from its dependencies. The main difference between the two is that mocking creates a fake version of an entire external dependency, while stubbing provides a single pre-determined response from a function or method. Mocking is most useful for testing functions that call external APIs or services, while stubbing is useful when testing asynchronous functions or function calls that are difficult to set up.

  1. Why is testing important in software development, and how does Jest make it easier?

Testing is essential in software development as it helps to ensure that code is functioning as intended and is free from bugs and errors. Testing also allows developers to catch edge cases and exceptions that may not have been considered during development. Jest makes testing easier by providing an intuitive API, a wide range of features, and thorough and accurate test reporting. It also includes a mocking library, making it easier to isolate code being tested from external dependencies.

  1. What is Axios, and how did it feature in the code example in the article?

Axios is a popular JavaScript library used for making HTTP requests from the browser or Node.js. It provides an easy-to-use API, supports promises, and works with both JSON and XML-based data. In the code example in the article, Axios was used to make an HTTP request to fetch data from a remote API. Using Jest's mocking library, the Axios library was mocked to simulate the HTTP request and return mock data for testing purposes.

Tag

"UnrepeatableMocks"

Throughout my career, I have held positions ranging from Associate Software Engineer to Principal Engineer and have excelled in high-pressure environments. My passion and enthusiasm for my work drive me to get things done efficiently and effectively. I have a balanced mindset towards software development and testing, with a focus on design and underlying technologies. My experience in software development spans all aspects, including requirements gathering, design, coding, testing, and infrastructure. I specialize in developing distributed systems, web services, high-volume web applications, and ensuring scalability and availability using Amazon Web Services (EC2, ELBs, autoscaling, SimpleDB, SNS, SQS). Currently, I am focused on honing my skills in algorithms, data structures, and fast prototyping to develop and implement proof of concepts. Additionally, I possess good knowledge of analytics and have experience in implementing SiteCatalyst. As an open-source contributor, I am dedicated to contributing to the community and staying up-to-date with the latest technologies and industry trends.
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