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Writing plugins

Plugins are custom rules and sets of custom rules. They might support a particular methodology or toolset, apply to non-standard constructs and features, or be for specific use cases.

We recommend your custom rules adhere to our rule conventions for:

  • names
  • options
  • messages
  • tests
  • docs
  • metadata
  • construct-specific parsers

The anatomy of a plugin​

This example plugin disallows the word "foo" in selectors:

import stylelint from "stylelint";

const {
  createPlugin,
  utils: { report, ruleMessages, validateOptions }
} = stylelint;

const ruleName = "foo-org/selector-no-foo";

const messages = ruleMessages(ruleName, {
  rejected: (selector) => `Unexpected "foo" within selector "${selector}"`
});

const meta = {
  url: "https://github.com/foo-org/stylelint-selector-no-foo/blob/main/README.md"
};

/** @type {import('stylelint').Rule} */
const ruleFunction = (primary, secondaryOptions, context) => {
  return (root, result) => {
    const validOptions = validateOptions(result, ruleName, {
      actual: primary,
      possible: [true]
    });

    if (!validOptions) return;

    root.walkRules((ruleNode) => {
      const { selector } = ruleNode;

      if (!selector.includes("foo")) return;

      report({
        result,
        ruleName,
        message: messages.rejected(selector),
        node: ruleNode,
        word: selector
      });
    });
  };
};

ruleFunction.ruleName = ruleName;
ruleFunction.messages = messages;
ruleFunction.meta = meta;

export default createPlugin(ruleName, ruleFunction);
note

The @type JSDoc annotation enables Typescript to autocomplete and type-check.

The usage would be:

{
  "plugins": ["@foo-org/stylelint-selector-no-foo"],
  "rules": {
    "foo-org/selector-no-foo": true
  }
}
$ echo '.foo {}' | stylelint --stdin-filename=test.css

test.css
 1:1  ✖  Unexpected "foo" within selector ".foo"  foo-org/selector-no-foo

1 problem (1 error, 0 warnings)

Your plugin's rule name must be namespaced, e.g. your-namespace/your-rule-name, to ensure it never clashes with the built-in rules. If your plugin provides only a single rule or you can't think of a good namespace, you can use plugin/my-rule. You should document your plugin's rule name (and namespace) because users need to use them in their config.

Use stylelint.createPlugin(ruleName, ruleFunction) to ensure that your plugin is set up properly alongside other rules.

For your plugin rule to work with the standard configuration format, ruleFunction should accept 2 arguments:

  • the primary option
  • optionally, a secondary options object

If your plugin rule supports autofixing, then meta.fixable should be set to true.

ruleFunction should return a function that is essentially a little PostCSS plugin. It takes 2 arguments:

  • the PostCSS Root (the parsed AST)
  • the PostCSS LazyResult

You'll have to learn about the PostCSS API.

Asynchronous rules​

You can write your plugin as an async function to deal with Promise.

const ruleFunction = (primary, secondaryOptions) => {
  return async (root, result) => {
    // validate options...

    // load disallowed words asynchronously
    const disallowedWords = await import("./disallowed-words.js");

    // traverse AST nodes...

    // report a warning if a problem word is detected...
  };
};

Testing​

You can use either:

Both expose a testRule function that you can use to efficiently test your plugin using a schema.

For example:

import { testRule } from "stylelint-test-rule-node";

import plugin from "./index.js";

const {
  rule: { messages, ruleName }
} = plugin;

testRule({
  plugins: [plugin],
  ruleName,
  config: true,
  fix: true,

  accept: [
    {
      code: ".a {}"
    },
    {
      code: ".b {}"
    }
  ],

  reject: [
    {
      code: ".foo {}",
      fixed: ".safe {}",
      message: messages.rejected(".foo"),
      line: 1,
      column: 1,
      endLine: 1,
      endColumn: 8
    }
  ]
});

Alternatively, you'll find more testing options in Awesome Stylelint.

If your plugin involves more than just checking syntax, you can use Stylelint directly.

For example:

import stylelint from "stylelint";

const { lint } = stylelint;

const config = {
  plugins: ["./index.js"],
  rules: {
    "foo-org/selector-no-foo": true
  }
};

it("warns", async () => {
  const {
    results: [{ warnings, parseErrors }]
  } = await lint({
    files: ["fixtures/test.css"],
    config
  });

  expect(parseErrors).toHaveLength(0);
  expect(warnings).toHaveLength(1);

  const [{ text, line, column }] = warnings;

  expect(text).toBe('Unexpected "foo" within selector ".foo"');
  expect(line).toBe(1);
  expect(column).toBe(1);
});

it("doesn't warn", async () => {
  const {
    results: [{ warnings, parseErrors }]
  } = await lint({
    code: ".foo {}",
    config
  });

  expect(parseErrors).toHaveLength(0);
  expect(warnings).toHaveLength(0);
});

stylelint.utils​

Stylelint exposes some useful utilities.

You're also welcome to copy any of the internal utils into your plugin. You should not import them directly, as they are not part of the public API and may change or be removed without warning.

stylelint.utils.report()​

Adds problems from your plugin to the list of problems that Stylelint will report to the user.

Use stylelint.utils.report() to ensure your plugin respects disabled ranges and other possible future features of Stylelint.

info

Do not use PostCSS's Node#warn() method directly.

You can specify PostCSS's warning options (e.g., word, index, start, etc.) to detail where a problem occurs.

stylelint.utils.ruleMessages()​

Tailors your messages to the format of standard Stylelint rules.

stylelint.utils.validateOptions()​

Validates the options for your rule.

stylelint.utils.checkAgainstRule()​

Checks CSS against a standard or custom Stylelint rule within your own rule. This function provides power and flexibility for plugins authors who wish to modify, constrain, or extend the functionality of existing Stylelint rules.

note

This is an async function. Your custom rule may need to wait until a Promise returned by the function is resolved.

It accepts an options object and a callback that is invoked with warnings from the specified rule. The options are:

  • ruleName: the name of the rule you are invoking
  • ruleSettings: settings for the rule you are invoking
  • root: the root node to run this rule against
  • result?: the PostCSS result for resolving and invoking custom rules
  • context?: the context for the rule you are invoking

Use the warning to create a new warning from your plugin rule that you report with stylelint.utils.report().

For example, imagine you want to create a plugin that runs at-rule-no-unknown with a built-in list of exceptions for at-rules provided by your preprocessor-of-choice:

const {
  utils: { checkAgainstRule, report }
} = stylelint;

const allowableAtRules = [
  /* .. */
];

const ruleName = "your-own/at-rule-no-unknown";

const myPluginRule = (primary, secondaryOptions, context) => {
  return async (root, result) => {
    const ignoreAtRules = allowableAtRules.concat(
      secondaryOptions?.ignoreAtRules ?? []
    );
    const defaultedSecondaryOptions = { ...secondaryOptions, ignoreAtRules };

    await checkAgainstRule(
      {
        ruleName: "at-rule-no-unknown",
        ruleSettings: [primary, defaultedSecondaryOptions],
        root,
        result,
        context
      },
      (warning) => {
        report({
          ruleName,
          result,
          message: warning.text,
          node: warning.node,
          start: { line: warning.line, column: warning.column },
          end: { line: warning.endLine, column: warning.endColumn }
        });
      }
    );
  };
};

stylelint.rules​

All of the rule functions are available at the stylelint.rules object. This allows you to build on top of existing rules for your particular needs.

note

Every value in the stylelint.rules object is a Promise resolving a rule function.

A typical use-case is to build in more complex conditionals that the rule's options allow for. For example, maybe your codebase uses special comment directives to customize rule options for specific stylesheets. You could build a plugin that checks those directives and then runs the appropriate rules with the right options (or doesn't run them at all).

All rules share a common signature. They are a function that accepts two arguments: a primary option and a secondary options object. And that function returns a function that has the signature of a PostCSS plugin, expecting a PostCSS root and result as its arguments.

Here's an example of a plugin that runs declaration-no-important only if there is a special directive @@check-declaration-no-important somewhere in the stylesheet:

createPlugin(ruleName, (primary) => {
  const rulePromise = stylelint.rules["declaration-no-important"];
  const ruleRunnner = rulePromise.then((rule) => rule(primary));

  return async (root, result) => {
    if (!root.toString().includes("@@check-declaration-no-important")) {
      return;
    }

    (await ruleRunnner)(root, result);
  };
});

Allow primary option arrays​

If your plugin can accept an array as its primary option, you must designate this by setting the property primaryOptionArray = true on your rule function. For more information, check out the "Working on rules" doc.

Peer dependencies​

You should express, within the peerDependencies key (and not within the dependencies key) of your plugin's package.json, what version(s) of Stylelint your plugin can be used with. This is to ensure that different versions of Stylelint are not unexpectedly installed.

For example, to express that your plugin can be used with Stylelint versions 14 and 15:

{
  "peerDependencies": {
    "stylelint": "^14.0.0 || ^15.0.0"
  }
}

Plugin packs​

To make a single module provide multiple rules, export an array of plugin objects (rather than a single object).

Sharing plugins and plugin packs​

Use the stylelint-plugin keyword within your package.json. For example:

{
  "keywords": ["stylelint", "stylelint-plugin"]
}

Sharing configurations​

You may want to create a shareable configuration for your plugins to provide a default ruleset. For example:

{
  "plugins": ["stylelint-plugin-foo"],
  "rules": {
    "foo/rule-1": true,
    "foo/rule-2": true
  }
}

When publishing such configurations, use the stylelint-config keyword within your package.json. For example:

{
  "keywords": ["stylelint", "stylelint-config"]
}