Testing a Sass Function in 5 Minutes

Core points

• Sass function testing can be done by creating virtual functions, writing test cases, creating test runners, and custom output results. This can be implemented using the minimalist Sass test engine for testing functions in different scenarios.
• Use Sass mapping to test functions with multiple parameters, which can accept anything as a key, including a list. This allows testing functions with multiple parameters.
• While this minimalist testing system is very convenient for testing a small number of functions, it is more suitable for a full Sass test framework, such as Eric Suzanne's True for more in-depth solutions.

I was using the include-media library of Eduardo Bouças the other day and wanted to quickly test a function I built myself, so I started writing a small mixer to help me test many different scenarios. After a few minutes, I came up with a Sass test engine that was as minimal as possible.

Although this post may be a little more technical, I believe it helps many people, as testing should be the responsibility of every developer. Also, after you understand these one by one, you will find that it is actually not difficult to understand.

Create virtual functions for testing

It all starts with the function to be tested. For our purposes, I suggest we use a very simple function. For example, a function used to double the number.

<code class="language-sass">@function double($value) {
  @return $value * 2;
}</code>

Sounds simple. However, for our demonstration purposes only, we will deliberately introduce an error in the function so that we can actually see one of our tests fail.

<code class="language-sass">@function double($value) {
  // 为了演示目的而故意引入的错误
  @if $value == 3 {
    @return 5;
  }

  @return $value * 2;
}</code>

Writing test cases

You may be surprised, but writing test cases in our system is as simple as writing a Sass map where the keys are function inputs and the values ​​are expected results.

<code class="language-sass">$tests-double: (
  1: 2,
  2: 4,
  3: 6,
  4: 8
);</code>

That's it! We have written test cases. Again: the left is the input, and the right is the expected output.

Test runner

So far, everything went well. We have built our functions and have written test cases. We just need to create a test runner now.

If you are familiar with Sass, you probably already understand this. Our test runner will iterate over the test map, call the function for each input, and make sure it matches the expected output. It will then print our test results.

Here is what our test runner looks like:

<code class="language-sass">/// 在测试套件 ($tests) 上运行函数 ($function)
/// @param {Map} $tests - 测试套件
/// @param {String} $function - 要测试的函数名称
@mixin run-tests($tests, $function) { .. }</code>

Okay. Let's take a deeper look at the inside of this "beast". The idea is to use the results of each test to build a string, and once all operations are completed, print the string using the @error directive. For example, we can also pass it to the content property of the pseudo-element, but this is a little more complicated, so we will stick with @error.

The first thing to do is iterate the test suite. For each test, we call the function dynamically based on its name (using the call(...) function) and check if the results are as expected.

<code class="language-sass">@function double($value) {
  @return $value * 2;
}</code>

At this time, our system is running. Let's run it on our test suite and see what it looks like.

<code class="language-sass">@function double($value) {
  // 为了演示目的而故意引入的错误
  @if $value == 3 {
    @return 5;
  }

  @return $value * 2;
}</code>

<code class="language-sass">$tests-double: (
  1: 2,
  2: 4,
  3: 6,
  4: 8
);</code>

Hey! This is the beginning, right? Now we just need to make the output more useful (and more friendly).

Improved output

This is when you can customize the output to make it look like you want it to look. There is no single way to do this, you can output whatever output you like. Note that according to the CSS specification, you can use a to wrap lines in strings.

In my case, this is what I chose:

<code class="language-sass">/// 在测试套件 ($tests) 上运行函数 ($function)
/// @param {Map} $tests - 测试套件
/// @param {String} $function - 要测试的函数名称
@mixin run-tests($tests, $function) { .. }</code>

If we run it again on the $tests-double function, this is what we get: double

<code class="language-sass">@mixin run-tests($tests, $function) {
  $output: '';

  @each $test, $expected-result in $tests {
    $result: call($function, $test...);

    @if $result == $expected-result {
      // 测试通过
      $output: $output + 'Test passed; ';
    } @else {
      // 测试失败
      $output: $output + 'Test failed; ';
    }
  }

  // 打印输出
  @error $output;
}</code>

It's very neat now, isn't it?

Test functions with multiple parameters

In our example, our function has only one parameter, but we can rely on the fact that the Sass map accepts anything as a key (including a list) to test a function with multiple parameters. It looks like this:

<code class="language-sass">@include run-tests($tests-double);</code>

If you review our mixer, you will see that when calling the function using

we add an ellipsis (...) to the call(...) variable. $test

<code>Test passed; Test passed; Test failed; Test passed;</code>

This means we pass the

value as a parameter list. In other words, if $test is a list (e.g. $test), it will be passed as multiple parameters instead of a list. ('a', red, 42px')

Final Thoughts

That's it, friends: the smallest Sass test engine ever. This tiny testing system can be very convenient to test a small number of functions that may have in your project, especially if you plan to make it available to other developers (frameworks, libraries...). Also, I found it very convenient to quickly test functions on SassMeister. Just put the mixer and your function there and run your tests!

Of course, if you're looking for a more in-depth solution, you might want to check out Eric Suzanne's True. As a complete Sass testing framework, it is more suitable for global unit testing infrastructure.

If you want to view (a slightly more advanced version of the code), I have opened the SassyTester repository to collect everything.

So what do you think?

FAQs about testing SASS functions (FAQ)

What is the importance of testing SASS functions?

Testing SASS functions is critical in web development because it ensures the functionality and efficiency of the CSS preprocessor. It helps identify and fix bugs, improve site performance, and ensures that SASS functions work as expected. By testing the SASS functions, you can ensure that your website design and layout are consistent across different browsers and platforms.

How to test SASS functions?

A variety of tools and methods can be used to test SASS functions. A common approach is to use SASS testing frameworks, such as True. True is a unit testing tool that integrates with your SASS projects. It allows you to write tests directly in a SASS file, which can then be run in Node.js or any web browser.

What are the best practices for testing SASS functions?

Some best practices for testing SASS functions include writing tests for each function, using consistent naming conventions for the tests, and ensuring that the test covers all possible scenarios. It is also important to run tests regularly and update tests as needed when making changes to functions.

Can I test SASS functions using JavaScript?

Yes, you can test SASS functions using JavaScript. You can use tools such as Jest to test your SASS functions. Jest is a JavaScript testing framework that allows you to write tests in simple and clear syntax.

What are the common challenges in testing SASS functions?

Some common challenges in testing SASS functions include handling complex functions, managing dependencies, and ensuring cross-browser compatibility. These challenges can be overcome using a powerful testing framework, writing clear and concise tests, and regularly checking and updating tests.

How to improve my SASS testing skills?

You can improve your SASS testing skills by practicing regularly, reading and learning other people’s code, and understanding the latest trends and best practices of SASS testing. Participating in coding challenges and contributing to open source projects can also help improve your skills.

What is the role of SASS in web development?

SASS plays a crucial role in web development by making CSS stronger and easier to maintain. It provides features such as variables, nesting, mixers, and functions, which helps write more efficient and reusable CSS code.

How to debug SASS functions?

A variety of tools and techniques can be used to debug SASS functions. A common approach is to use the SASS inspect function, which allows you to check the value of a SASS expression. You can also use the SASS @debug directive, which prints the value of the SASS expression to the console.

Can I test SASS functions without a test framework?

While SASS functions can be tested without a test framework, this is not recommended. The test framework provides a structured and systematic approach to testing, making it easier to write, manage, and run tests.

What are the benefits of using SASS testing frameworks?

Using the SASS test framework provides many benefits. It allows you to write tests in a structured and systematic way, making it easier to manage and run tests. It also provides tools and features that can help write more efficient tests, such as assertion functions and test runners.

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