Add max-len rule to enforce configurable line length limits for Markdown entities

.changeset/max-len.md

@@ -0,0 +1,5 @@
+---
+"eslint-plugin-markdown-preferences": minor
+---
+
+feat: add `markdown-preferences/max-len` rule

README.md

@@ -142,6 +142,8 @@ The rules with the following 💄 are included in the `standard` config.
 
 - Rules to unify the expression and description style of documents.
 
+<!-- eslint-disable markdown-links/no-dead-urls -- Auto generated -->
+
 <!-- prettier-ignore-start -->
 
 | Rule ID                                                                                                                                                           | Description                                                | Fixable | Config |
@@ -157,10 +159,14 @@ The rules with the following 💄 are included in the `standard` config.
 
 <!-- prettier-ignore-end -->
 
+<!-- eslint-enable markdown-links/no-dead-urls -- Auto generated -->
+
 ### Notation Rules
 
 - Rules related to notation styles in Markdown.
 
+<!-- eslint-disable markdown-links/no-dead-urls -- Auto generated -->
+
 <!-- prettier-ignore-start -->
 
 | Rule ID                                                                                                                                                               | Description                                                                                | Fixable | Config |
@@ -185,10 +191,14 @@ The rules with the following 💄 are included in the `standard` config.
 
 <!-- prettier-ignore-end -->
 
+<!-- eslint-enable markdown-links/no-dead-urls -- Auto generated -->
+
 ### Whitespace Rules
 
 - Rules related to whitespace styles in Markdown.
 
+<!-- eslint-disable markdown-links/no-dead-urls -- Auto generated -->
+
 <!-- prettier-ignore-start -->
 
 | Rule ID                                                                                                                                                           | Description                                                                    | Fixable | Config |
@@ -212,10 +222,14 @@ The rules with the following 💄 are included in the `standard` config.
 
 <!-- prettier-ignore-end -->
 
+<!-- eslint-enable markdown-links/no-dead-urls -- Auto generated -->
+
 ### Decorative Rules
 
 - Rules related to visual or stylistic decorations in Markdown.
 
+<!-- eslint-disable markdown-links/no-dead-urls -- Auto generated -->
+
 <!-- prettier-ignore-start -->
 
 | Rule ID                                                                                                                                                                   | Description                                                                               | Fixable | Config |
@@ -234,6 +248,22 @@ The rules with the following 💄 are included in the `standard` config.
 
 <!-- prettier-ignore-end -->
 
+<!-- eslint-enable markdown-links/no-dead-urls -- Auto generated -->
+
+### Undefined Rules
+
+<!-- eslint-disable markdown-links/no-dead-urls -- Auto generated -->
+
+<!-- prettier-ignore-start -->
+
+| Rule ID                                                                                                           | Description                                          | Fixable | Config |
+| :---------------------------------------------------------------------------------------------------------------- | :--------------------------------------------------- | :-----: | :----: |
+| [markdown-preferences/max-len](https://ota-meshi.github.io/eslint-plugin-markdown-preferences/rules/max-len.html) | enforce maximum length for various Markdown entities |         |        |
+
+<!-- prettier-ignore-end -->
+
+<!-- eslint-enable markdown-links/no-dead-urls -- Auto generated -->
+
 <!--RULES_TABLE_END-->
 
 <!--RULES_SECTION_END-->

docs/appendix/comparison-with-markdownlint-rules.md

@@ -159,10 +159,10 @@ Please note that each OSS is constantly evolving, so this list is not exhaustive
 
 ## Rules Related to Documents
 
-| Description                   | [markdownlint] Rules                                                          | [@eslint/markdown] Rules | `eslint-plugin-markdown-preferences` Rules |
-| ----------------------------- | ----------------------------------------------------------------------------- | ------------------------ | ------------------------------------------ |
-| Enforce a maximum line length | [MD013] _line-length_<br>Line length                                          | --                       | --                                         |
-| Proper names                  | [MD044] _proper-names_<br>Proper names should have the correct capitalization | --                       | --                                         |
+| Description                   | [markdownlint] Rules                                                          | [@eslint/markdown] Rules | `eslint-plugin-markdown-preferences` Rules                                             |
+| ----------------------------- | ----------------------------------------------------------------------------- | ------------------------ | -------------------------------------------------------------------------------------- |
+| Enforce a maximum line length | [MD013] _line-length_<br>Line length                                          | --                       | [markdown-preferences/max-len]<br>enforce maximum length for various Markdown entities |
+| Proper names                  | [MD044] _proper-names_<br>Proper names should have the correct capitalization | --                       | --                                                                                     |
 
 ## Rules Related to Syntax
 
@@ -256,6 +256,7 @@ Please note that each OSS is constantly evolving, so this list is not exhaustive
 [markdown-preferences/link-paren-spacing]: ./../rules/link-paren-spacing.md
 [markdown-preferences/link-title-style]: ./../rules/link-title-style.md
 [markdown-preferences/list-marker-alignment]: ../rules/list-marker-alignment.md
+[markdown-preferences/max-len]: ./../rules/max-len.md
 [markdown-preferences/no-heading-trailing-punctuation]: ./../rules/no-heading-trailing-punctuation.md
 [markdown-preferences/no-implicit-block-closing]: ../rules/no-implicit-block-closing.md
 [markdown-preferences/no-laziness-blockquotes]: ../rules/no-laziness-blockquotes.md

docs/rules/index.md

@@ -105,3 +105,13 @@ The rules with the following 💄 are included in the `plugin.configs.standard`
 | [markdown-preferences/thematic-break-sequence-pattern](./thematic-break-sequence-pattern.md)         | enforce consistent repeating patterns for thematic breaks (horizontal rules) in Markdown. |   🔧    |   💄   |
 
 <!-- prettier-ignore-end -->
+
+## Undefined Rules
+
+<!-- prettier-ignore-start -->
+
+| Rule ID                                      | Description                                          | Fixable | Config |
+| :------------------------------------------- | :--------------------------------------------------- | :-----: | :----: |
+| [markdown-preferences/max-len](./max-len.md) | enforce maximum length for various Markdown entities |         |        |
+
+<!-- prettier-ignore-end -->

docs/rules/max-len.md

@@ -0,0 +1,179 @@
+---
+pageClass: "rule-details"
+sidebarDepth: 0
+title: "markdown-preferences/max-len"
+description: "enforce maximum length for various Markdown entities"
+---
+
+# markdown-preferences/max-len
+
+> enforce maximum length for various Markdown entities
+
+- ❗ <badge text="This rule has not been released yet." vertical="middle" type="error"> **_This rule has not been released yet._** </badge>
+
+## 📖 Rule Details
+
+This rule enforces a configurable maximum length for different Markdown entities such as headings, paragraphs, lists, blockquotes, tables, code blocks, frontmatter, footnote definitions, HTML blocks, and math blocks.
+
+Overly long Markdown elements reduce readability, make diffs harder to review, and complicate maintenance. By enforcing reasonable maximum lengths for various entities, documentation remains cleaner, easier to navigate, and more consistent across the project.
+
+<!-- prettier-ignore-start -->
+
+<!-- eslint-skip -->
+
+```md
+<!-- eslint markdown-preferences/max-len: ['error', { heading: 80, paragraph: 120 }] -->
+
+<!-- ✓ GOOD -->
+
+## This heading is within the limit
+
+This is a paragraph that fits comfortably within the specified maximum length limit.
+
+- A list item that is short enough
+
+<!-- ✗ BAD -->
+
+## This is a very long heading that exceeds the default eighty character maximum length
+
+This is an extremely long paragraph that goes on and on and on well beyond the one hundred and twenty character maximum length limit and should be broken up.
+
+- This is a very long list item that definitely exceeds the default one hundred and twenty character maximum length limit and should be reported
+```
+
+<!-- prettier-ignore-end -->
+
+## 🔧 Options
+
+This rule accepts an object with the following properties:
+
+```json
+{
+  "markdown-preferences/max-len": [
+    "error",
+    {
+      "heading": 80,
+      "paragraph": 120,
+      "list": 120,
+      "blockquote": 120,
+      "table": 120,
+      "footnoteDefinition": 120,
+      "html": 120,
+      "code": "ignore",
+      "frontmatter": "ignore",
+      "math": "ignore",
+      "ignoreUrls": true
+    }
+  ]
+}
+```
+
+### Entity-specific Options
+
+- `heading` (default: `80`): Maximum line length for headings. Set to `"ignore"` to skip checking.
+- `paragraph` (default: `120`): Maximum line length for paragraphs. Set to `"ignore"` to skip checking.
+- `list` (default: `120`): Maximum line length for lists. Set to `"ignore"` to skip checking.
+- `blockquote` (default: `120`): Maximum line length for blockquotes. Set to `"ignore"` to skip checking.
+- `table` (default: `120`): Maximum line length for tables. Set to `"ignore"` to skip checking.
+- `footnoteDefinition` (default: `120`): Maximum line length for footnote definitions. Set to `"ignore"` to skip checking.
+- `html` (default: `120`): Maximum line length for HTML blocks. Set to `"ignore"` to skip checking.
+- `code` (default: `"ignore"`): Maximum line length for code blocks. Set to `"ignore"` to skip checking (recommended). See [notes on code blocks](#notes-on-code-blocks) below.
+- `frontmatter` (default: `"ignore"`): Maximum line length for frontmatter. Set to `"ignore"` to skip checking (recommended).
+- `math` (default: `"ignore"`): Maximum line length for math blocks. Set to `"ignore"` to skip checking.
+
+### Nested Configuration for Containers
+
+For `list`, `blockquote`, and `footnoteDefinition`, you can specify different limits for nested headings and paragraphs:
+
+```json
+{
+  "markdown-preferences/max-len": [
+    "error",
+    {
+      "heading": 80,
+      "paragraph": 120,
+      "blockquote": {
+        "heading": 70,
+        "paragraph": 100
+      }
+    }
+  ]
+}
+```
+
+In this example, headings inside blockquotes are limited to 70 characters, while standalone headings are limited to 80 characters.
+
+### Language-specific Code Block Configuration
+
+For `code` and `frontmatter`, you can specify different limits per language:
+
+```json
+{
+  "markdown-preferences/max-len": [
+    "error",
+    {
+      "code": {
+        "javascript": 100,
+        "python": 80,
+        "shell": "ignore"
+      },
+      "frontmatter": {
+        "yaml": 120,
+        "toml": "ignore"
+      }
+    }
+  ]
+}
+```
+
+### URL Handling
+
+- `ignoreUrls` (default: `true`): When enabled, lines containing URLs are ignored.
+
+This option works similarly to [`@stylistic/eslint-plugin`'s `max-len` rule](https://eslint.style/rules/max-len#ignoreurls).
+
+### Notes on Code Blocks
+
+**Note:** Code blocks are ignored by default (`code: "ignore"`) because:
+
+1. Code inside code blocks should be linted using language-specific linters via ESLint's language plugins
+2. The [`@eslint/markdown`](https://github.com/eslint/markdown) plugin supports linting code blocks as their respective languages
+3. This rule cannot understand the syntax inside code blocks (e.g., comments, strings)
+
+If you need to enforce line length for code blocks in Markdown, you can:
+
+- Use the language-specific ESLint configuration for code blocks (recommended):
+
+  ```js
+  {
+    files: ["**/*.md/*.js"],
+    rules: {
+      "@stylistic/max-len": ["error", { "ignoreComments": true }]
+    }
+  }
+  ```
+
+  **However**, this feature is not currently available with this plugin. It may be possible in the future. The RFC is at: <https://github.com/eslint/rfcs/pull/105>
+
+- Set the `code` option to enforce a simple line length limit (not syntax-aware)
+
+For more information, see the [advanced configuration guide](https://github.com/eslint/markdown/blob/main/docs/processors/markdown.md#advanced-configuration).
+
+## 📚 Further Reading
+
+- [Markdownlint MD013](https://github.com/DavidAnson/markdownlint/blob/v0.38.0/doc/md013.md) - Similar rule in markdownlint
+- [@stylistic/eslint-plugin max-len](https://eslint.style/rules/max-len) - ESLint stylistic rule for code
+
+## 👫 Related Rules
+
+- [no-multiple-empty-lines](./no-multiple-empty-lines.md) - Disallow multiple empty lines
+
+## 🔍 Implementation
+
+<!-- eslint-disable markdown-links/no-dead-urls -- Auto generated -->
+
+- [Rule source](https://github.com/ota-meshi/eslint-plugin-markdown-preferences/blob/main/src/rules/max-len.ts)
+- [Test source](https://github.com/ota-meshi/eslint-plugin-markdown-preferences/blob/main/tests/src/rules/max-len.ts)
+- [Test fixture sources](https://github.com/ota-meshi/eslint-plugin-markdown-preferences/tree/main/tests/fixtures/rules/max-len)
+
+<!-- eslint-enable markdown-links/no-dead-urls -- Auto generated -->

eslint.config.mjs

@@ -197,8 +197,12 @@ export default defineConfig([
             "/^https:\\/\\/www\\.npmtrends\\.com\\//u",
           ],
           allowedAnchors: {
+            // https://eslint-online-playground.netlify.app/
             "/^https:\\/\\/eslint-online-playground\\.netlify\\.app\\//u":
               "/.*/u",
+            // https://github.com/eslint/markdown/blob/main/docs/processors/markdown.md
+            "/^https:\\/\\/github\\.com\\/eslint\\/markdown\\/blob\\/main\\/docs\\/processors\\/markdown\\.md/u":
+              "/.*/u",
           },
         },
       ],

src/rule-types.ts

@@ -139,6 +139,11 @@ export interface RuleOptions {
    * @see https://ota-meshi.github.io/eslint-plugin-markdown-preferences/rules/list-marker-alignment.html
    */
   'markdown-preferences/list-marker-alignment'?: Linter.RuleEntry<MarkdownPreferencesListMarkerAlignment>
+  /**
+   * enforce maximum length for various Markdown entities
+   * @see https://ota-meshi.github.io/eslint-plugin-markdown-preferences/rules/max-len.html
+   */
+  'markdown-preferences/max-len'?: Linter.RuleEntry<MarkdownPreferencesMaxLen>
   /**
    * disallow trailing punctuation in headings.
    * @see https://ota-meshi.github.io/eslint-plugin-markdown-preferences/rules/no-heading-trailing-punctuation.html
@@ -433,6 +438,33 @@ type MarkdownPreferencesLinkTitleStyle = []|[{
 type MarkdownPreferencesListMarkerAlignment = []|[{
   align?: ("left" | "right")
 }]
+// ----- markdown-preferences/max-len -----
+type MarkdownPreferencesMaxLen = []|[{
+  heading?: (number | "ignore")
+  paragraph?: (number | "ignore")
+  list?: ((number | "ignore") | {
+    heading?: (number | "ignore")
+    paragraph?: (number | "ignore")
+  })
+  blockquote?: ((number | "ignore") | {
+    heading?: (number | "ignore")
+    paragraph?: (number | "ignore")
+  })
+  table?: (number | "ignore")
+  footnoteDefinition?: ((number | "ignore") | {
+    heading?: (number | "ignore")
+    paragraph?: (number | "ignore")
+  })
+  html?: (number | "ignore")
+  code?: ((number | "ignore") | {
+    [k: string]: (number | "ignore")
+  })
+  frontmatter?: ((number | "ignore") | {
+    [k: string]: (number | "ignore")
+  })
+  math?: (number | "ignore")
+  ignoreUrls?: boolean
+}]
 // ----- markdown-preferences/no-heading-trailing-punctuation -----
 type MarkdownPreferencesNoHeadingTrailingPunctuation = []|[{
   punctuation?: (string | {