Mastering Jest Mocks: How to Implement and Test Your Code with Examples

Table of content

  1. Introduction
  2. What are Jest Mocks?
  3. Why Use Jest Mocks?
  4. Basic Jest Mocks
  5. Mocking HTTP Requests with Jest
  6. Mocking Database Calls with Jest
  7. Conclusion


Hey there! Are you tired of manually testing every piece of code that you write? Do you want to streamline your testing process and improve the efficiency of your development workflow? Well, have no fear, because mastering Jest mocks is here!

Jest is a nifty testing framework for JavaScript that allows you to easily create and run tests for your code. And one of the coolest features of Jest is its ability to create mock objects, which can simulate the behavior of real objects in your code. This means that you can test your code without actually having to use all of the real objects and dependencies that it relies on.

In this article, I'm going to introduce you to the world of Jest mocks and show you how amazing it can be for improving the quality of your code. We'll cover the basics of creating mocks, using them in your tests, and even more advanced techniques like customizing mock behavior and creating mock modules.

So, buckle up and get ready to take your testing game to the next level with Jest mocks!

What are Jest Mocks?

Jest Mocks are a nifty little tool that can make your life as a developer so much easier! Essentially, Jest Mocks are a way to simulate and test certain aspects of your code without having to run the entire application.

In simpler terms, let's say you have a function that relies on an external API. With Jest Mocks, you can create a fake response from that API so that you can test your function without actually having to call the API. This saves time and resources, and allows you to isolate and focus on specific areas of your code.

What's really amazing about Jest Mocks is how customizable they are. You can create mocks that behave differently based on different inputs, or even create multiple mocks for different scenarios. This gives you a lot of control over your testing process and can help you catch bugs and errors before they cause bigger issues.

In short, Jest Mocks are a great tool for any developer looking to streamline their testing process and improve their code's reliability. So why not give them a try? You may just be surprised at how much time and headache they can save you in the long run.

Why Use Jest Mocks?

Have you ever written code that relies on external APIs or services? Then you know how frustrating it can be to test your code when those outside dependencies are in flux. This is where Jest Mocks come in! They allow you to simulate those dependencies, so you can test your code independently and reliably.

But even if you're not dealing with outside dependencies, mocking can still be super nifty. For example, what if you're building a new feature and need to test it, but it relies on a function that hasn't been built yet? Or what if that function has side effects (like updating a database) that you don't want to happen during your tests? Enter Jest Mocks!

By using Jest Mocks, you can test your code with confidence, without worrying about breaking anything or dealing with flaky test results. Plus, by mocking out external dependencies, you can iterate faster and catch bugs earlier.

So Because they're an amazing tool that can make your life as a developer so much easier. Trust me, once you start using them, you'll wonder how you ever lived without them.

Basic Jest Mocks

So, you wanna learn about Jest mocks? Well, you've come to the right place! Let's start with the basics.

Jest mocks are like stand-ins for functions or modules in your code. They allow you to create predictable scenarios for testing your code without having to actually run the real thing. Pretty nifty, huh?

To implement a basic Jest mock, you'll need to use the jest.fn() method. This will create a mock function that you can then pass as an argument to whatever function or module you're testing. Here's an example:

function add(a, b) {
  return a + b;

const mockAdd = jest.fn();


expect(mockAdd(2, 3)).toBe(5);
expect(mockAdd).toHaveBeenCalledWith(2, 3);

In this example, we're creating a mock function called mockAdd and setting it to return the value 5. Then, we call the function with the arguments 2 and 3 and use Jest's built-in expect function to make sure it returns 5 and was called with the correct arguments.

That's just the tip of the iceberg when it comes to Jest mocks. Imagine how amazing it could be to create complex test scenarios with multiple mock functions for all your different modules. The possibilities are endless!

Mocking HTTP Requests with Jest

So you want to mock HTTP requests with Jest? Well, that's nifty because I've got some tips for you! First things first, let's talk about why you might need to mock an HTTP request. Sometimes, you want to test a component that makes an API call, but you don't want to actually make that call during testing. Maybe the API is down, or maybe you don't want to exceed your API limits. That's where mocking comes in handy.

To mock an HTTP request in Jest, you'll need to install a library called "axios-mock-adapter." This library allows you to intercept HTTP requests and send back mock responses. Here's an example of how you can use it:

import axios from 'axios';
import MockAdapter from 'axios-mock-adapter';

const mock = new MockAdapter(axios);

mock.onGet('/users').reply(200, {
  users: [
    { id: 1, name: 'John' },
    { id: 2, name: 'Jane' },

// Now, when you make a GET request to /users, you'll get back the mock response.

Pretty cool, right? You can also use the "once" method to specify that the mock response should only be returned once:

mock.onGet('/users').once().reply(200, {
  users: [
    { id: 1, name: 'John' },
    { id: 2, name: 'Jane' },

And if you want to simulate an error response, you can do that too:

mock.onGet('/users').reply(500, {
  message: 'Internal Server Error',

There are a ton of possibilities with axios-mock-adapter, so I highly recommend checking out the documentation to see what else you can do. is just one of the many amazing things you can do with this testing framework.

Mocking Database Calls with Jest

Let me tell you about a nifty trick I learned for . It's amazingd how helpful this can be when you're testing asynchronous code that relies on a database!

First, you need to install a package called mock-knex. This package allows you to create a mock Knex instance that you can use to execute queries and test your code. Once you've installed it, you'll need to create a Jest setup file to configure the mock instance.

In your Jest setup file, you can use the mockKnex.fromKnex method to create a mock instance from a real Knex instance. Then, you can use Jest's beforeEach and afterEach hooks to set up and tear down the mock instance before and after each test.

Once you have your mock instance set up, you can use it to mock database calls in your tests. For example, you can use the mockKnex instance to return a specific result when a certain query is executed, or you can test that a query was executed with the correct parameters.

Overall, can be a lifesaver when it comes to testing your code. With just a bit of setup, you can ensure that your tests are reliable and accurate, even when they rely on external dependencies like a database. So give it a try, and see how much easier testing can be!


Well folks, that's it for now! You've made it to the end of our little journey through mastering Jest mocks. I hope this guide has been helpful and that you're feeling confident about implementing and testing your code with mocks.

Remember, using mocks can be a bit of a challenge at first, but with practice and persistence, you'll become a pro in no time. Always keep in mind the basic principles of good testing practices, and you'll be well on your way to creating nifty, robust, and reliable applications.

In , Jest is an amazing tool that can save you a ton of time and effort when testing your code. By leveraging its powerful mocks, you can create comprehensive test suites that cover all possible cases and edge cases. So go forth and experiment, and who knows, you might just uncover some new nuggets of wisdom and clever tricks along the way!

As a senior DevOps Engineer, I possess extensive experience in cloud-native technologies. With my knowledge of the latest DevOps tools and technologies, I can assist your organization in growing and thriving. I am passionate about learning about modern technologies on a daily basis. My area of expertise includes, but is not limited to, Linux, Solaris, and Windows Servers, as well as Docker, K8s (AKS), Jenkins, Azure DevOps, AWS, Azure, Git, GitHub, Terraform, Ansible, Prometheus, Grafana, and Bash.

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