QUnit Tutorial: Start Unit Testing and Measuring Code Coverage

QUnit Tutorial: Start Unit Testing and Measuring Code Coverage

QUnit Tutorial: Start unit testing and Measuring Code Coverage. Code coverage lets you know the extent to which your source code is tested. Having a code coverage that’s too low is a bad sign; it means your test suite is unable to act as a safety net to developers. In this QUnit tutorial, learn how you can use QUnit to test your JavaScript code.

There are many libraries available to write unit tests for your JavaScript code. One popular option is QUnit. Let's look…

There are many libraries available to write unit tests for your  JavaScript  code. One popular option is QUnit. Let’s look at how you can get started with QUnit.

But let’s not stop there. Measuring code coverage of your tests is a useful metric to gain insights and improve your test suite.

We’ll open the tutorial with a brief overview of QUnit itself, so we’re on the same page in regards to this tool. After that, we’ll talk a little bit about code coverage, explaining why it’s such an important  test metric. Finally, we’ll get to the “tutorial” part of the post, in which we’ll actually walk you through how to get started with QUnit. Let’s begin.

What Is QUnit?

If you’re already familiar with QUnit, you can skip to the next section.

QUnit is a JavaScript library to test your JavaScript code. You can write your tests using certain functions provided by QUnit and then have QUnit run them. QUnit can run your tests in the browser or in a plain Node.js process.

Which way of running your tests depends mainly on where your code will run. If your JavaScript code will run in the browser (i.e. you’re using the  DOM), you might want to run your unit tests in the browser as well. You can run QUnit with Node.js if you’re writing a Node.js application.

If you’re testing logic that will run in the browser but doesn’t interact with the DOM, I recommend running your tests in a Node.js process. It will also be easier to integrate with a  CI/CD pipeline.

What is Code Coverage and Why It Matters?

Code coverage is an important metric related to testing and  quality. It indicates the ratio of source code that is covered—that is, exercised by—at least one automated unit test.

The simplest type of code coverage is line coverage, which it’s calculated just by getting the number of lines covered by a least a single test and dividing by the total number of lines in the application. So, if an application has 100000 lines of code and just 45 thousand of them are covered by tests, we can say the line coverage of this app is 45%.

However, there are more involved—and more useful—forms of coverage, such as branch coverage, which determines the ratio of logical branches in the code that have been exercised by tests.

Code coverage lets you know the extent to which your source code is tested. Having a code coverage that’s too low is a bad sign; it means your test suite is unable to act as a safety net to developers, allowing them to refactor code fearlessly, or add new features at a fast pace without fear of introducing  regressions. A great way to achieve high code coverage is to use  TDD (test-driven development.)

Before we move on, you must keep in mind an important pitfall of code coverage. This metric, alone, can’t really guarantee quality. That’s because code coverage doesn’t say anything about the quality of your unit tests. It’s totally possible to have 100% coverage with your tests still being insufficient or not effective.

In other words, while low code coverage is definitely bad, high coverage isn’t necessarily good. In order to achieve great tests, you need to couple code coverage with practices or tools that ensure quality. TDD is a great practice for that since it encourages you to write simple tests.  Mutation testing is also a practice you can adopt to verify the quality of your test suite.

With that being said, it’s time to roll up our sleeves and get started with QUnit.

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