#Working on rules

Please help us create, enhance, and debug stylelint rules!

There are well over a hundred rules already, so stylelint needs community contributions to continue to improve.

If you like stylelint and open source software (since you're reading this, you almost certainly do), please consider taking some time to pitch in. Not only will you help stylelint thrive, you will also learn a thing or two — about CSS, PostCSS, Node, ES2015, unit testing, open source software, and more.

We want to do everything we can to encourage contributions! So if you want to participate but don't end up doing it for one reason or another, please file an issue and give us feedback about what we could do to better encourage you.

Also: we hope that your participation in the project isn't a one-off. We'd love to add more members to the organization and see more regulars pop up in issues and pull requests!

#Creating a new rule

First, open an issue with your idea for the new rule.

Usually we have some discussion about the rule's purpose, name, options, and suitability as a rule.

#Criteria for inclusion

We discuss whether the rule meets the following criteria for inclusion in stylelint:

  • Applicable to standard CSS syntax only.
  • Generally useful; not tied to idiosyncratic patterns.
  • Has a clear and unambiguous finished state.
  • Has a singular purpose.
  • Is standalone, and doesn't rely on another rule.
  • Does not contain functionality that overlaps with another rule.

Otherwise, it should be a plugin. However, plugins should also try to adhere to the latter three criteria.

#Naming a rule

Have a look at the rules user guide to familiarize yourself the rule naming conventions.

We take care to ensure that all the rules are named accurately and consistently. Our goals in that effort are to ensure that rules are easy to find and understand, and to prevent us from wanting to change the name later.

Rules are named to encourage explicit, rather than implicit, options. For example, color-hex-case: "upper"|"lower" rather than color-hex-uppercase: "always"|"never". As color-hex-uppercase: "never" implies always lowercase, whereas color-hex-case: "lower" makes it explicit.

#Determining options

#Primary

Every rule must have a primary option.

  • In "color-hex-case": "upper", the primary option is "upper".
  • In "indentation": [2, { "except": ["block"] }], the primary option is 2.

If your rule can accept an array as its primary option, you must designate this by setting the property primaryOptionArray = true on your rule function. For example:

function rule(primary, secondary) {
  return (root, result) => {..}
}
rule.primaryOptionArray = true
export default rule
// or, for plugins: stylelint.createPlugin(ruleName, rule)

There is one caveat here: If your rule accepts a primary option array, it cannot also accept a primary option object. Whenever possible, if you want your rule to accept a primary option array, you should just make an array the only possibility, instead of allowing for various data structures.

#Secondary

Some rules require extra flexibility to address a variety of use-cases. These can use an optional secondary options object.

  • In "color-hex-case": "upper", there is no secondary options object.
  • In "indentation": [2, { "except": ["block"] }], the secondary options object is { "except": ["block"] }.

The most typical secondary options are "ignore": [] and "except": []; but anything is possible.

A rule's secondary option can be anything if you're not ignoring or making exceptions. As an example, resolveNestedSelectors: true|false is used within some selector-* rules to change how the rule processes nested selectors.

#Keyword "ignore" and "except"

"ignore" and "except" accept an array of predefined keyword options e.g. ["relative", "first-nested", "descendant"].

  • Use "ignore" when you want the rule to simply skip-over a particular pattern.
  • Use "except" when you want to invert the primary option for a particular pattern.
#User-defined "ignore*"

Use a more specific secondary option name when accepting a user-defined list of things to ignore. This takes the form of "ignore<Things>": [] e.g. use "ignoreAtRules": [] if a rule checks at-rules and you want to allow a user to specify which particular at-rule types to ignore.

#Determine violation messages

Messages take one of these forms:

  • "Expected [something] [in some context]".
  • "Unexpected [something] [in some context]."

Look at the messages of other rules to glean more conventions and patterns.

#Write the rule

When writing the rule, always look to other similar rules for conventions and patterns to start from and mimic.

You will use the simple PostCSS API to navigate and analyze the CSS syntax tree. We recommend using the walk iterators (e.g. walkDecls), rather than using forEach to loop through the nodes.

Depending on the rule, we also recommend using postcss-value-parser and postcss-selector-parser. There are significant benefits to using these parsers instead of regular expressions or indexOf searches (even if they aren't always the most performant method).

stylelint has a number of utility functions that are used in existing rules and might prove useful to you, as well. Please look through those so that you know what's available. (And if you have a new function that you think might prove generally helpful, let's add it to the list!)

In particular, you will definitely want to use validateOptions() so that users are warned about invalid options. (Looking at other rules for examples of options validation will help a lot.)

#Adding autofixing

Depending on the rule, it might be possible to automatically fix the rule's violations by mutating the PostCSS AST (Abstract Syntax Tree) using the PostCSS API.

Add context variable to rule parameters:

function rule(primary, secondary, context) {
  return (root, result) => {..}
}

context is an object which could have two properties:

  • fix(boolean): If true, your rule can apply autofixes.
  • newline(string): Line-ending used in current linted file.

If context.fix is true, then change root using PostCSS API and return early before report() is called.

if (context.fix) {
  // Apply fixes using PostCSS API
  return // Return and don't report a problem
}

report(...)

#Write tests

Each rule must be accompanied by tests that contain:

  • All patterns that are considered violations.
  • All patterns that should not be considered violations.

It is easy to write stylelint tests, so write as many as you can stand to.

#Checklist

Please run through this checklist and ensure each point is covered by your tests. Especially consider the edge-cases. These are where the bugs and shortcomings of rules always arise.

#Best practices
  • Ensure you are testing errors in multiple positions, not the same place every time.
  • Ensure you use realistic (if simple) CSS, and avoid the use of ellipses.
  • Ensure you use standard CSS syntax by default, and only swap parsers when testing a specific piece of non-standard syntax.
  • When accessing raw strings from the PostCSS AST, use node.raws instead of node.raw(). This will ensure string corresponds exactly to the original.
#Commonly overlooked edge-cases
  • How does your rule handle variables ($sass, @less, or var(--custom-property))?
  • How does your rule handle CSS strings (e.g. content: "anything goes";)?
  • How does your rule handle CSS comments (e.g. /* anything goes */)?
  • How does your rule handle url() functions, including data URIs (e.g. url(anything/goes.jpg))?
  • How does your rule handle vendor prefixes (e.g. @-webkit-keyframes name {})?
  • How does your rule handle case sensitivity (e.g. @KEYFRAMES name {})?
  • How does your rule handle a pseudo-class combined with a pseudo-element (e.g. a:hover::before)?
  • How does your rule handle nesting (e.g. do you resolve & a {}, or check it as is?)?
  • How does your rule handle whitespace and punctuation (e.g. comparing rgb(0,0,0) with rgb(0, 0, 0))?

#Running tests

You can run the tests via:

npm test

However, this runs all 25,000+ unit tests and also linting.

You can use the interactive testing prompt to run tests for just a chosen set of rules (which you'll want to do during development). For example, to run the tests for just the color-hex-case and color-hex-length rules:

  1. Use npm run watch to start the prompt.
  2. Press p to filter by a filename regex pattern.
  3. Enter color-hex-case|color-hex-length i.e. each rule name separated by the pipe symbol (|).

#Write the README

Each rule must be accompanied by a README, fitting the following format:

  1. Rule name.
  2. Single line description.
  3. Prototypical code example.
  4. Expanded description (if necessary).
  5. Options.
  6. Example patterns that are considered violations (for each option value).
  7. Example patterns that are not considered violations (for each option value).
  8. Optional options (if applicable).

Look at the READMEs of other rules to glean more conventional patterns.

#Single line descriptions

Take the form of:

  • "Disallow ..." (for no rules).
  • "Limit ..." (for max rules).
  • "Require ..." (for rules that accept "always" and "never" options).
  • "Specify ..." (for everything else).

#Example patterns

  • Use complete CSS patterns i.e. avoid ellipses (...)
  • Use standard CSS syntax (and use css code fences) by default.
  • Use the minimum amount of code possible to communicate the pattern e.g. if the rule targets selectors then use an empty rule e.g. {}.
  • Use {}, rather than { } for empty rules.
  • Use the a type selector by default.
  • Use the @media at-rules by default.
  • Use the color property by default.
  • Use foo, bar and baz for names e.g. .foo, #bar --baz

#Wire up the rule

The final step is to add references to the new rule in the following places:

Once you have something to show, you'll create a pull request to continue the conversation.

#Adding an option to an existing rule

First, open an issue about the option you wish to add. We'll discuss its functionality and name there.

Once we've agreed on the direction, you can work on a pull request. Here are the steps you'll need to take:

  1. Change the rule's validation to allow for the new option.
  2. Add to the rule some logic (as little as possible) to make the option work.
  3. Add new unit tests to test the option.
  4. Add documentation about the new option.

#Fixing a bug in an existing rule

Fixing bugs is usually very easy. Here is a process that works:

  1. Write failing unit tests that exemplify the bug.
  2. Fiddle with the rule until those new tests pass.

That's it! If you are unable to figure out how to fix the bug yourself, it is still extremely helpful to submit a pull request with your failing test cases. It means that somebody else can jump right in and help out with the rule's logic.

#Improving the performance of a new or an existing rule

There's a simple way to run benchmarks on any given rule with any valid config for it:

npm run benchmark-rule -- [rule-name] [config]

If the config argument is anything other than a string or a boolean, it must be valid JSON wrapped in quotation marks.

npm run benchmark-rule -- selector-combinator-space-after never
npm run benchmark-rule -- selector-combinator-space-after always
npm run benchmark-rule -- selector-no-combinator true
npm run benchmark-rule -- block-opening-brace-space-before "[\"always\", {\"ignoreAtRules\": [\"else\"]}]"

The script loads Bootstrap's CSS (from its CDN) and runs it through the configured rule.

It will end up printing some simple stats like this:

Warnings: 1441
Mean: 74.17598357142856 ms
Deviation: 16.63969674310928 ms

What can you do with this? When writing new rules or refactoring existing rules, use these measurements to determine the efficiency of your code.

A stylelint rule can repeat it's core logic many, many times (e.g. checking every value node of every declaration in a vast CSS codebase). So it's worth paying attention to performance and doing what we can to improve it!

This is a great way to contribute if you just want a quick little project. Try picking a rule and seeing if there's anything you can do to speed it up.

Make sure to include benchmark measurements in your PR's!