Habilelabs-Logo
Blog

Testing with Jest- Why Jesting Is So Popular?

October 11th, 2021 . 8 minutes read
Blog featured image

Test automation is an important part of the software development process as it runs multiple automatic tests on the code to find bugs, without needing any manual efforts, which leads to saving a lot of time and effort during development.

Gone are the days when the testers had to do even the simplest task of software testing on their own. 

Not that manual testing has lost its charm, since there’s the benefit of having several automated testing tools in the market now. 

Not even at the slightest, in fact, it’s the opposite- now that we have all these tools at our disposal we can put our talented minds elsewhere on more complex tasks which only a human brain can accomplish.

Let’s not go deeper in that discussion and come right where we are now- Jest.

Jest is a testing tool for JavaScript frameworks. Its job is to find bugs and errors in the JavaScript codebase.

But the question here is- out of so many automation tools available right now, why would one go for Jest? Why is it that the entire developers’ community is rolling all around it?

In this blog, we are going to get the answers to all these questions, so let’s get on the train to take a quick ride into it-

Overview about Jest

Jest is a JavaScript testing framework and is widely used to test applications created with Vue.js, Babel, Angular, TypeScript, React, Node, and other JavaScript frameworks.

A wonderful fact about Jest is that it requires minimum efforts on the configuration part and is extendable, which enables you to match it with your requirements accordingly.

Let’s give you a brief look into the highlighting features of Jest that makes jesting so popular –

  • Zero-configuration: Jest requires no-to-little configuration and works freely in JavaScript projects.
  • Parallel tests: It enhances performance by running multiple isolated tests, in parallel, in their own processes.
  • Snapshots: You can easily keep track of the complex projects by keeping snapshots of the ongoing process.
  • Great API: Jest is a complete package and has everything all utility functions in one place, so you can start writing tests immediately. It is well-documented and well maintained.

Getting Started with Jest-ing 👇

As the very first step, you would need to open the command window and initialize a sample project with this command-

It will create a default package.json file, which contains the following contents-

Now that you have your project ready, let’s move forward to install Jest- 

Now, let’s write a test for a sample function, it could be as simple as finding the sum of 2 numbers. For that, we need to create a sum.js file-

Then, create another file name as sum.test.js which will contain the actual test as- 

Now, we need to configure the scripts element in the package.json file to run tests- 

That’s it! Let’s run the test now- 

Jest will print the following output for this test-

Why choose Jest over other testing tools?

Why Jesting?

The programming community is flooded with plenty of testing tools but when it comes to JavaScript testing, the three most popular tools come into the picture, i.e., Mocha, Jest, and Jasmine.

It is in popular opinion that Jasmine is recommended for Angular projects, Mocha for Node applications and Jest for React. However, they are compatible with all JavaScript libraries and frameworks. 

Each of the three testing frameworks has its own set of pros, and it entirely depends upon project requirements as to which one you should choose for testing.

While choosing a testing framework, the focus mainly stands with simplicity, well-documented, easy adoption, and productivity. Jest has them all!

Jest is built on top of Jasmine and therefore carries all the key features of Jasmine with it, which is a huge benefit, considering that Jasmine has got all the good things.

Out of the three, Jest is preferred for its simplicity and easy-to-use features but other than that, it has got a long list of features that makes the developers fall in love with it! 

I’m gonna mention some important ones over here-

✔ Simplicity at its peak

Easy-to-use tag can be used to define Jest as it is the simplest testing framework. It is very easy to learn, use, and practice.

It’s a battery included framework and has built-in features for common functionalities so you don’t have to add third-party plug-ins and external libraries.

✔ Lightening fast speed

With Jest, you can significantly reduce the test runtime to almost one thrice comparing how much it takes to run local tests on Mocha. Yes, Jest is maddeningly fast and the factors that contribute to augment its speed are-

  • It runs multiple isolated tests in parallel that enhances the test performance.
  • Jest is CPU-bound, which cuts short a significant time in test runs.
  • It runs the slowest tests first to optimize the parallel running tests, which overall reduces the total test runtime.
  • Jest has its own order to run tests and is not based on alphabetically or any other predefined standards. It picks the lacking tests and previously failed tests first.

Snapshot testing

Snapshot testing is an amazing functionality of Jest where it captures snapshots of the code component at a specific time to track changes in the test case. This method is highly effective in validating data structures.

Basically, it creates detailed reports and stores snapshots of the output for future reference. 

The functions you have run and the output received; it stores it all in a separate file so the next time you run the test again, it will compare the current output to the stored snapshot. 

In case the snapshot shows any changes, which are visible in terms of the color diff, it means the test is failing. Easy-peasy!

You can also keep the newer snapshot if it’s fine according to you and the older one is outdated; all you have to do is to hit -u and it will upgrade the snapshot, overwriting it with the new one. This process is called upgrading.

You can also use the inline snapshot feature for small snapshots, which allows you to visually edit the code components, and you can see the changes updated in the browser only.

✔ Unit testing

Jest is preferred for unit testing, which is a software testing process where we outline and test the smallest units of the code that can be isolated. This unit can be any functional piece of code- a line, a class, etc. 

The smaller the unit, the better and efficient are the test results. The unit tests are easy and cheap to write with Jest, and can be executed quickly.

✔ Easy and Out-of-box Mocking

Mocking is the literal term for creating mock functions– functions created to mock the original functions- to use in test code instead of executing the actual functions.

But why do we need to create mock functions? Why not just go with the original ones?

Well, here’s the thing- while testing the function calls, the test sometimes changes the state of the application unexpectedly or makes changes in the database, or leads to other unavoidable side effects that crawl through the tests, which is not preferable.

Hence, the mock function! By creating mock functions we can return relevant values, which will give the exact same results and that too, without causing any unwanted changes in the database or application.

Jest makes mocking easy by using a custom resolver for imports and enables you to use the mocked imports with mock function APIs to spy on function calls.

The mock function comes in handy especially in unit testing as they let you test the function logic without having to worry about its dependencies.

✔ Code coverage

Code coverage refers to the total code that was tested. Jest has this built-in feature of generating code coverage reports. 

The more is code coverage, the more productive are the results. This coverage report so conveniently reveals which part of the codebase was not covered during testing.

Inside your file structure, you will see a directory named coverage. When you open it, you will see another directory containing lots and lots of files. However, you are looking for an index.html file and when you copy its path into the browser; you will be able to see exactly how much code has been tested and which lines are left open uncovered.

For business reasons, it is likely that you skip testing for certain parts of the code, however, it is always helpful to have a detailed report of the total test coverage of the project.

✔ Rich context for failed tests

Whenever a test fails, the developer quickly wants to identify the cause behind it so that he can look into the issue and resolve it. 

One of the finest properties of Jest is that it provides complete details as to why and where the test failed.

When the test fails, Jest shows the complete stack trace to the error and where the test failed — it highlights the error part with syntax color difference, the code line where it went wrong, and will point an arrow towards the exact problem.

This is an amazing property that helps increase productivity.


Thanks a ton for reading the blog till here.

We are a leading software development company. Looking for a partner company for your JavaScript project? Meet us here!

Happy Jesting!!

Author: payal
Share: