Writing rules
Please help us create, enhance, and debug our rules!
Add a rule
You should get yourself ready to contribute code.
Define the rule
A rule must be:
- for standard CSS syntax only
- generally useful; not tied to idiosyncratic patterns
And have a:
- unambiguous finished state
- singular purpose that doesn't overlap with other rules
Its name is split into two parts:
- the thing the rule applies to, e.g.
at-rule
- what the rule is checking, e.g.
disallowed-list
Unless it applies to the whole source, then there is no first part.
Write tests
You should add test cases for all patterns that are:
- considered problems
- not considered problems
You should use:
- realistic CSS, avoiding the use of ellipses
- the minimum amount of code possible, e.g. use an empty rule if targeting selectors
{}
for empty rules, rather than{ }
- the
a
type selector by default - the
@media
at-rules by default - the
color
property by default - the
red
value by default - the
(min-)width
media feature by default - foo, bar and baz for names, e.g.
.foo
,#bar
,--baz
You should:
- vary column and line positions across your tests
- include at least one test that has 2 warnings
- test non-standard syntax in the
isStandardSyntax*
utilities, not in the rule itself
Commonly overlooked edge-cases
You should ask yourself how does your rule handle:
- variables (e.g.
var(--custom-property)
)? - CSS strings (e.g.
content: "anything goes";
)? - CSS comments (e.g.
/* anything goes */
)? - empty functions (e.g.
var()
)? url()
functions, including data URIs (e.g.url(anything/goes.jpg)
)?- vendor prefixes (e.g.
@-webkit-keyframes name {}
)? - case sensitivity (e.g.
@KEYFRAMES name {}
)? - a pseudo-class combined with a pseudo-element (e.g.
a:hover::before
)? - nesting (e.g. do you resolve
& a {}
, or check it as is?)? - whitespace and punctuation (e.g. comparing
rgb(0,0,0)
withrgb(0, 0, 0)
)?
Write the rule
When writing the rule, you should:
- make the rule strict by default
- add secondary
ignore
options to make the rule more permissive - not include code specific to language extensions like SCSS
You should make use of the:
- PostCSS API
- construct-specific parsers
- utility functions
PostCSS API
Use the 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.
When using array methods on nodes, e.g. find
, some
, filter
etc, you should explicitly check the type
property of the node before attempting to access other properties. For example:
const hasProperty = nodes.find(
({ type, prop }) => type === "decl" && prop === propertyName
);
Use node.raws
instead of node.raw()
when accessing raw strings from the PostCSS AST.
Construct-specific parsers
Depending on the rule, we also recommend using:
- postcss-value-parser
- postcss-selector-parser
- @csstools/css-parser-algorithms
- @csstools/css-tokenizer
- @csstools/media-query-list-parser
- css-tree
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).
Utility functions
Stylelint has 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!).
Use the:
validateOptions()
utility to warn users about invalid optionsisStandardSyntax*
utilities to ignore non-standard syntax
Add options
Each rule can accept a primary and an optional secondary option.
Only add an option to a rule if it addresses a requested use case to avoid polluting the tool with unused features.
Primary
Every rule must have a primary option. For example, in:
"font-weight-notation": "numeric"
, the primary option is"numeric"
"selector-max-type": [2, { "ignoreTypes": ["custom"] }]
, the primary option is2
Rules are named to encourage explicit primary options. For example, font-weight-notation: "numeric"|"named-where-possible"
rather than font-weight-numeric: "always"|"never"
. As font-weight-named: "never"
implies always numeric, whereas font-weight-notation: "numeric"
makes it explicit.
Secondary
Some rules require extra flexibility to address edge cases. These can use an optional secondary options object. For example, in:
"font-weight-notation": "numeric"
there is no secondary options object"selector-max-type": [2, { "ignore": ["descendant] }]
, the secondary options object is{ "ignore": ["descendant] }
The most typical secondary options are "ignore": []
and "except": []
.
Keyword "ignore"
and "except"
The "ignore"
and "except"
options accept an array of predefined keyword options, e.g. ["relative", "first-nested", "descendant"]
:
"ignore"
skips-over a particular pattern"except"
inverts the primary option for a particular pattern
User-defined "ignore*"
Some rules accept a user-defined list of things to ignore. This takes the form of "ignore<Things>": []
, e.g. "ignoreAtRules": []
.
The ignore*
options let users ignore non-standard syntax at the configuration level. For example, the:
:global
and:local
pseudo-classes introduced in CSS Modules@debug
and@extend
at-rules introduced in SCSS
Methodologies and language extensions come and go quickly, and this approach ensures our codebase does not become littered with code for obsolete things.
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;
module.exports = 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 make an array the only possibility, instead of allowing for various data structures.
Add problem messages
Add problem messages in form of:
- "Expected [something] [in some context]"
- "Unexpected [something] [in some context]"
If the rule has autofix use:
- 'Expected "[unfixed]" to be "[fixed]"' for short strings
- 'Expected "[primary]" ... notation' for long strings
Add autofix
Depending on the rule, it might be possible to automatically fix the rule's problems by mutating the PostCSS AST (Abstract Syntax Tree) using the PostCSS API.
Set meta.fixable = true
to the rule:
const meta = {
url: /* .. */,
+ fixable: true,
};
Pass fix
callback to the report
utility:
function rule(primary, secondary) {
return (root, result) => {
/* .. */
+ const fix = () => { /* put your mutations here */ };
report({
result,
ruleName,
message,
node,
+ fix
});
};
}
Context
context
is an object which could have three properties:
configurationComment
(string): String that prefixes configuration comments like/* stylelint-disable */
.fix
(boolean): Iftrue
, your rule can apply autofixes.newline
(string): Line-ending used in current linted file.
The convention of restricting the appliance of fixes based on the context.fix
property is deprecated in favour of recommending the fix
callback which properly handles configuration comments.
If context.fix
is true
, then change root
using PostCSS API and return early before report()
is called.
function rule(primary, secondary, context) {
return (root, result) => {
if (context.fix) {
// Apply fixes using PostCSS API
return; // Return and don't report a problem
}
report(/* .. */);
};
}
Write the README
Each rule is accompanied by a README in the following format:
- Rule name.
- Single-line description.
- Prototypical code example.
- Expanded description (if necessary).
- Options.
- Example patterns that are considered problems (for each option value).
- Example patterns that are not considered problems (for each option value).
- Optional options (if applicable).
The single-line description is in the form of:
- "Disallow ..." for
no
rules - "Limit ..." for
max
rules - "Require ..." for rules that accept
"always"
and"never"
options - "Specify ..." for everything else
You should:
- pick examples from the tests
- only use standard CSS syntax in examples and options
- add the fewest examples possible to communicate the intent of the rule, rather than show edge cases
- use
<!-- prettier-ignore -->
beforecss
code fences - use "this rule" to refer to the rule, e.g. "This rule ignores ..."
- align the arrows within the prototypical code example with the beginning of the highlighted construct
- align the text within the prototypical code example as far to the left as possible
For example:
@media screen and (min-width: 768px) {}
/** ↑ ↑
* These names and values */
Look at the READMEs of other rules to glean more conventional patterns.
Wire up the rule
The final step is to add references to the new rule in the following places:
Add an option to a rule
You should:
- Get ready to contribute code.
- Add new unit tests to test the option.
- Change the rule's validation to allow for the new option.
- Add (as little as possible) logic to the rule to make the tests pass.
- Add documentation about the new option.
Fix a bug in a rule
You should:
- Get ready to contribute code.
- Write failing unit tests that exemplify the bug.
- Fiddle with the rule until those new tests pass.
Deprecate a rule
Deprecating rules doesn't happen very often. When you do, you must:
- Add the appropriate metadata to mark the rule as deprecated like so:
rule.meta = { deprecated: true }
. - Set the
stylelintType
to'deprecation'
. - Optionally set
stylelintReference
to a link that points to a specific version of the rule's document so that it always remains accessible.
For example:
result.warn(
`"your-namespace/old-rule" has been deprecated and will be removed in 7.0. Use "your-namespace/new-rule" instead.`,
{
stylelintType: "deprecation",
stylelintReference:
"https://github.com/your-org/your-stylelint-plugin/blob/v6.3.0/src/rules/old-rule/README.md"
}
);
Improve the performance of a rule
You can run a benchmark on any given rule with any valid config using:
npm run benchmark-rule -- ruleName ruleOptions [config]
If the ruleOptions
argument is anything other than a string or a boolean, it must be valid JSON wrapped in quotation marks.
npm run benchmark-rule -- value-keyword-case lower
npm run benchmark-rule -- value-keyword-case '["lower", {"camelCaseSvgKeywords": true}]'
If the config
argument is specified, the same procedure would apply:
npm run benchmark-rule -- value-keyword-case '["lower", {"camelCaseSvgKeywords": true}]' '{"fix": true}'
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
When writing new rules or refactoring existing rules, use these measurements to determine the efficiency of your code.
A Stylelint rule can repeat its 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!
Improving the performance of a rule is a great way to contribute if you want a quick little project. Try picking a rule and seeing if there's anything you can do to speed it up.
Make sure you include benchmark measurements in your pull request!