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markdownlint/doc/CustomRules.md

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# Custom Rules
In addition to its built-in rules, `markdownlint` lets you enhance the linting experience by passing a list of custom rules using the
[`options.customRules` property](../README.md#optionscustomrules).
Custom rules can do everything the built-in rules can and are defined inline or imported from another package
([keyword `markdownlint-rule` on npm](https://www.npmjs.com/search?q=keywords:markdownlint-rule)).
Custom rules can be disabled, enabled, and customized using the same syntax as built-in rules.
## Authoring
Rules are defined by a name (or multiple names), a description, an optional link to more information, one or more tags, and a function that implements
the rule's behavior.
That function is called once for each file/string input and is passed the parsed input and a function to log any violations.
A simple rule implementation looks like:
```js
module.exports = {
"names": [ "any-blockquote" ],
"description": "Rule that reports an error for any blockquote",
"information": new URL("https://example.com/rules/any-blockquote"),
"tags": [ "test" ],
"function": function rule(params, onError) {
params.tokens.filter(function filterToken(token) {
return token.type === "blockquote_open";
}).forEach(function forToken(blockquote) {
var lines = blockquote.map[1] - blockquote.map[0];
onError({
"lineNumber": blockquote.lineNumber,
"detail": "Blockquote spans " + lines + " line(s).",
"context": blockquote.line.substr(0, 7)
});
});
}
};
```
A rule is implemented as an `Object` with one optional and four required properties:
- `names` is a required `Array` of `String` values that identify the rule in output messages and config.
- `description` is a required `String` value that describes the rule in output messages.
- `information` is an optional (absolute) `URL` of a link to more information about the rule.
- `tags` is a required `Array` of `String` values that groups related rules for easier customization.
- `asynchronous` is an optional `Boolean` value that indicates whether the rule returns a `Promise` and runs asynchronously.
- `function` is a required `Function` that implements the rule and is passed two parameters:
- `params` is an `Object` with properties that describe the content being analyzed:
- `name` is a `String` that identifies the input file/string.
- `tokens` is an `Array` of [`markdown-it` `Token` objects](https://markdown-it.github.io/markdown-it/#Token)
with added `line` and `lineNumber` properties.
- `lines` is an `Array` of `String` values corresponding to the lines of the input file/string.
- `frontMatterLines` is an `Array` of `String` values corresponding to any front matter (not present in `lines`).
- `config` is an `Object` corresponding to the rule's entry in `options.config` (if present).
- `onError` is a function that takes a single `Object` parameter with one required and four optional properties:
- `lineNumber` is a required `Number` specifying the 1-based line number of the error.
- `detail` is an optional `String` with information about what caused the error.
- `context` is an optional `String` with relevant text surrounding the error location.
- `range` is an optional `Array` with two `Number` values identifying the 1-based column and length of the error.
- `fixInfo` is an optional `Object` with information about how to fix the error (all properties are optional, but
at least one of `deleteCount` and `insertText` should be present; when applying a fix, the delete should be
performed before the insert):
- `lineNumber` is an optional `Number` specifying the 1-based line number of the edit.
- `editColumn` is an optional `Number` specifying the 1-based column number of the edit.
- `deleteCount` is an optional `Number` specifying the number of characters to delete (the value `-1` is used to delete the line).
- `insertText` is an optional `String` specifying the text to insert. `\n` is the platform-independent way to add
a line break; line breaks should be added at the beginning of a line instead of at the end).
The collection of helper functions shared by the built-in rules is available for use by custom rules in the
[markdownlint-rule-helpers package](https://www.npmjs.com/package/markdownlint-rule-helpers).
### Asynchronous Rules
If a rule needs to perform asynchronous operations (such as fetching a network resource), it can specify the value `true` for its `asynchronous` property.
Asynchronous rules should return a `Promise` from their `function` implementation that is resolved when the rule completes.
(The value passed to `resolve(...)` is ignored.)
Linting violations from asynchronous rules are reported via the `onError` function just like for synchronous rules.
**Note**: Asynchronous rules cannot be referenced in a synchronous calling context (i.e., `markdownlint.sync(...)`).
Attempting to do so throws an exception.
## Examples
- [Simple rules used by the project's test cases](../test/rules)
- [Code for all `markdownlint` built-in rules](../lib)
- [Package configuration for publishing to npm](../test/rules/npm)
- Packages should export a single rule object or an `Array` of rule objects
- [Custom rules from the Microsoft/vscode-docs-authoring repository](https://github.com/microsoft/vscode-docs-authoring/tree/main/packages/docs-linting/markdownlint-custom-rules)
- [Custom rules from the axibase/docs-util repository](https://github.com/axibase/docs-util/tree/master/linting-rules)
- [Custom rules from the webhintio/hint repository](https://github.com/webhintio/hint/blob/main/scripts/lint-markdown.js)
## References
- [CommonMark documentation and specification](https://commonmark.org/)
- [`markdown-it` Markdown parser project page](https://github.com/markdown-it/markdown-it)
## Params
The Markdown document:
```markdown
# Title
Text *text* text.
```
Yields the `params` object:
```json
{
"name": "doc/example.md",
"tokens": [
{
"type": "heading_open",
"tag": "h1",
"attrs": null,
"map": [ 0, 1 ],
"nesting": 1,
"level": 0,
"children": null,
"content": "",
"markup": "#",
"info": "",
"meta": null,
"block": true,
"hidden": false,
"line": "# Title",
"lineNumber": 1
},
{
"type": "inline",
"tag": "",
"attrs": null,
"map": [ 0, 1 ],
"nesting": 0,
"level": 1,
"children": [
{
"type": "text",
"tag": "",
"attrs": null,
"map": null,
"nesting": 0,
"level": 0,
"children": null,
"content": "Title",
"markup": "",
"info": "",
"meta": null,
"block": false,
"hidden": false,
"lineNumber": 1,
"line": "# Title"
}
],
"content": "Title",
"markup": "",
"info": "",
"meta": null,
"block": true,
"hidden": false,
"line": "# Title",
"lineNumber": 1
},
{
"type": "heading_close",
"tag": "h1",
"attrs": null,
"map": null,
"nesting": -1,
"level": 0,
"children": null,
"content": "",
"markup": "#",
"info": "",
"meta": null,
"block": true,
"hidden": false
},
{
"type": "paragraph_open",
"tag": "p",
"attrs": null,
"map": [ 2, 3 ],
"nesting": 1,
"level": 0,
"children": null,
"content": "",
"markup": "",
"info": "",
"meta": null,
"block": true,
"hidden": false,
"line": "Text *text* text.",
"lineNumber": 3
},
{
"type": "inline",
"tag": "",
"attrs": null,
"map": [ 2, 3 ],
"nesting": 0,
"level": 1,
"children": [
{
"type": "text",
"tag": "",
"attrs": null,
"map": null,
"nesting": 0,
"level": 0,
"children": null,
"content": "Text ",
"markup": "",
"info": "",
"meta": null,
"block": false,
"hidden": false,
"lineNumber": 3,
"line": "Text *text* text."
},
{
"type": "em_open",
"tag": "em",
"attrs": null,
"map": null,
"nesting": 1,
"level": 1,
"children": null,
"content": "",
"markup": "*",
"info": "",
"meta": null,
"block": false,
"hidden": false,
"lineNumber": 3,
"line": "Text *text* text."
},
{
"type": "text",
"tag": "",
"attrs": null,
"map": null,
"nesting": 0,
"level": 1,
"children": null,
"content": "text",
"markup": "",
"info": "",
"meta": null,
"block": false,
"hidden": false,
"lineNumber": 3,
"line": "Text *text* text."
},
{
"type": "em_close",
"tag": "em",
"attrs": null,
"map": null,
"nesting": -1,
"level": 0,
"children": null,
"content": "",
"markup": "*",
"info": "",
"meta": null,
"block": false,
"hidden": false,
"lineNumber": 3,
"line": "Text *text* text."
},
{
"type": "text",
"tag": "",
"attrs": null,
"map": null,
"nesting": 0,
"level": 0,
"children": null,
"content": " text.",
"markup": "",
"info": "",
"meta": null,
"block": false,
"hidden": false,
"lineNumber": 3,
"line": "Text *text* text."
}
],
"content": "Text *text* text.",
"markup": "",
"info": "",
"meta": null,
"block": true,
"hidden": false,
"line": "Text *text* text.",
"lineNumber": 3
},
{
"type": "paragraph_close",
"tag": "p",
"attrs": null,
"map": null,
"nesting": -1,
"level": 0,
"children": null,
"content": "",
"markup": "",
"info": "",
"meta": null,
"block": true,
"hidden": false
}
],
"lines": [
"# Title",
"",
"Text *text* text.",
""
],
"frontMatterLines": [],
"config": {
"customValue1": "abc",
"customValue2": 123
}
}
```
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