markdownlint

A Node.js style checker and lint tool for Markdown/CommonMark files.

Install

npm install markdownlint --save-dev

Overview

The Markdown markup language is designed to be easy to read, write, and understand. It succeeds - and its flexibility is both a benefit and a drawback. Many styles are possible, so formatting can be inconsistent. Some constructs don't work well in all parsers and should be avoided. The CommonMark specification standardizes parsers - but not authors.

markdownlint is a static analysis tool for Node.js with a library of rules to enforce standards and consistency for Markdown files. It was inspired by - and heavily influenced by - Mark Harrison's markdownlint for Ruby. The initial rules, rule documentation, and test cases came directly from that project.

Demonstration

markdownlint demo, an interactive, in-browser playground for learning and exploring.

Rules / Aliases

MD001 heading-increment/header-increment - Heading levels should only increment by one level at a time
MD002 first-heading-h1/first-header-h1 - First heading should be a top level heading
MD003 heading-style/header-style - Heading style
MD004 ul-style - Unordered list style
MD005 list-indent - Inconsistent indentation for list items at the same level
MD006 ul-start-left - Consider starting bulleted lists at the beginning of the line
MD007 ul-indent - Unordered list indentation
MD009 no-trailing-spaces - Trailing spaces
MD010 no-hard-tabs - Hard tabs
MD011 no-reversed-links - Reversed link syntax
MD012 no-multiple-blanks - Multiple consecutive blank lines
MD013 line-length - Line length
MD014 commands-show-output - Dollar signs used before commands without showing output
MD018 no-missing-space-atx - No space after hash on atx style heading
MD019 no-multiple-space-atx - Multiple spaces after hash on atx style heading
MD020 no-missing-space-closed-atx - No space inside hashes on closed atx style heading
MD021 no-multiple-space-closed-atx - Multiple spaces inside hashes on closed atx style heading
MD022 blanks-around-headings/blanks-around-headers - Headings should be surrounded by blank lines
MD023 heading-start-left/header-start-left - Headings must start at the beginning of the line
MD024 no-duplicate-heading/no-duplicate-header - Multiple headings with the same content
MD025 single-title/single-h1 - Multiple top level headings in the same document
MD026 no-trailing-punctuation - Trailing punctuation in heading
MD027 no-multiple-space-blockquote - Multiple spaces after blockquote symbol
MD028 no-blanks-blockquote - Blank line inside blockquote
MD029 ol-prefix - Ordered list item prefix
MD030 list-marker-space - Spaces after list markers
MD031 blanks-around-fences - Fenced code blocks should be surrounded by blank lines
MD032 blanks-around-lists - Lists should be surrounded by blank lines
MD033 no-inline-html - Inline HTML
MD034 no-bare-urls - Bare URL used
MD035 hr-style - Horizontal rule style
MD036 no-emphasis-as-heading/no-emphasis-as-header - Emphasis used instead of a heading
MD037 no-space-in-emphasis - Spaces inside emphasis markers
MD038 no-space-in-code - Spaces inside code span elements
MD039 no-space-in-links - Spaces inside link text
MD040 fenced-code-language - Fenced code blocks should have a language specified
MD041 first-line-heading/first-line-h1 - First line in file should be a top level heading
MD042 no-empty-links - No empty links
MD043 required-headings/required-headers - Required heading structure
MD044 proper-names - Proper names should have the correct capitalization
MD045 no-alt-text - Images should have alternate text (alt text)
MD046 code-block-style - Code block style
MD047 single-trailing-newline - Files should end with a single newline character
MD048 code-fence-style - Code fence style

See Rules.md for more details.

~~Struck through~~ rules are deprecated, and provided for backward-compatibility.

All rules with heading as part of their name are also available as header aliases (e.g. heading-increment is also available as header-increment). The use of header is deprecated and provided for backward-compatibility.

Configuration

Text passed to markdownlint is parsed as Markdown, analyzed, and any issues reported. Two kinds of text are ignored:

HTML comments
Front matter (see options.frontMatter below)

Rules can be enabled, disabled, and configured via options.config (described below) to define the expected behavior for a set of inputs. To enable or disable rules at a particular location within a file, add one of these markers to the appropriate place (HTML comments don't appear in the final markup):

Disable all rules: 
Enable all rules: 
Disable one or more rules by name: 
Enable one or more rules by name: 
Capture the current rule configuration: 
Restore the captured rule configuration:

For example:

<!-- markdownlint-disable no-space-in-emphasis -->
deliberate space * in * emphasis
<!-- markdownlint-enable no-space-in-emphasis -->

To temporarily disable rule(s), then restore the former configuration:

<!-- markdownlint-capture -->
<!-- markdownlint-disable -->
any violations you want
<!-- markdownlint-restore -->

The initial configuration is captured by default (as if every document began with ), so the pattern above can be expressed more simply:

<!-- markdownlint-disable -->
any violations you want
<!-- markdownlint-restore -->

Changes take effect starting with the line a comment is on, so the following has no effect:

space * in * emphasis <!-- markdownlint-disable --> <!-- markdownlint-enable -->

To apply changes to an entire file regardless of where the comment is located, the following syntax is supported:

Disable all rules: 
Enable all rules: 
Disable one or more rules by name: 
Enable one or more rules by name:

This can be used to "hide" markdownlint comments at the bottom of a file.

API

Linting

Standard asynchronous interface:

/**
 * Lint specified Markdown files.
 *
 * @param {Object} options Configuration options.
 * @param {Function} callback Callback (err, result) function.
 * @returns {void}
 */
function markdownlint(options, callback) { ... }

Synchronous interface (for build scripts, etc.):

/**
 * Lint specified Markdown files synchronously.
 *
 * @param {Object} options Configuration options.
 * @returns {Object} Result object.
 */
function markdownlint.sync(options) { ... }

options

Type: Object

Configures the function.

options.customRules

Type: Array of Object

List of custom rules to include with the default rule set for linting.

Each array element should define a rule. Rules are typically exported by another package, but can be defined inline.

Example:

const extraRules = require("extraRules");
const options = {
  "customRules": [ extraRules.one, extraRules.two ]
};

See CustomRules.md for details about authoring custom rules.

options.files

Type: Array of String

List of files to lint.

Each array element should be a single file (via relative or absolute path); globbing is the caller's responsibility.

Example: [ "one.md", "dir/two.md" ]

options.strings

Type: Object mapping String to String

Map of identifiers to strings for linting.

When Markdown content is not available as files, it can be passed as strings. The keys of the strings object are used to identify each input value in the result summary.

Example:

{
  "readme": "# README\n...",
  "changelog": "# CHANGELOG\n..."
}

options.config

Type: Object mapping String to Boolean | Object

Configures the rules to use.

Object keys are rule names or aliases and values are the rule's configuration. The value false disables a rule, true enables its default configuration, and passing an object customizes its settings. Setting the special default rule to true or false includes/excludes all rules by default. Enabling or disabling a tag name (ex: whitespace) affects all rules having that tag.

The default rule is applied first, then keys are processed in order from top to bottom with later values overriding earlier ones. Keys (including rule names, aliases, tags, and default) are not case-sensitive.

Example:

{
  "default": true,
  "MD003": { "style": "atx_closed" },
  "MD007": { "indent": 4 },
  "no-hard-tabs": false,
  "whitespace": false
}

Sets of rules (known as a "style") can be stored separately and loaded as JSON.

Example:

const options = {
  "files": [ "..." ],
  "config": require("style/relaxed.json")
};

See the style directory for more samples.

See markdownlint-config-schema.json for the JSON Schema of the options.config object.

For more advanced scenarios, styles can reference and extend other styles. The readConfig and readConfigSync functions can be used to read such styles.

For example, assuming a base.json configuration file:

{
  "default": true
}

And a custom.json configuration file:

{
  "extends": "base.json",
  "line-length": false
}

Then code like the following:

const options = {
  "config": markdownlint.readConfigSync("./custom.json")
};

Merges custom.json and base.json and is equivalent to:

const options = {
  "config": {
    "default": true,
    "line-length": false
  }
};

options.frontMatter

Type: RegExp

Matches any front matter found at the beginning of a file.

Some Markdown content begins with metadata; the default RegExp for this option ignores common forms of "front matter". To match differently, specify a custom RegExp or use the value null to disable the feature.

The default value:

/((^---\s*$[^]*?^---\s*$)|(^\+\+\+\s*$[^]*?^(\+\+\+|\.\.\.)\s*$))(\r\n|\r|\n|$)/m

Ignores YAML and TOML such as:

---
layout: post
title: Title
---

Note: Matches must occur at the start of the file.

options.handleRuleFailures

Type: Boolean

Catches exceptions thrown during rule processing and reports the problem as a rule violation.

By default, exceptions thrown by rules (or the library itself) are unhandled and bubble up the stack to the caller in the conventional manner. By setting handleRuleFailures to true, exceptions thrown by failing rules will be handled by the library and the exception message logged as a rule violation. This setting can be useful in the presence of (custom) rules that encounter unexpected syntax and fail. By enabling this option, the linting process is allowed to continue and report any violations that were found.

options.noInlineConfig

Type: Boolean

Disables the use of HTML comments like  to toggle rules within the body of Markdown content.

By default, properly-formatted inline comments can be used to create exceptions for parts of a document. Setting noInlineConfig to true ignores all such comments.

options.resultVersion

Type: Number

Specifies which version of the result object to return (see the "Usage" section below for examples).

Passing a resultVersion of 0 corresponds to the original, simple format where each error is identified by rule name and line number. This is deprecated.

Passing a resultVersion of 1 corresponds to a detailed format where each error includes information about the line number, rule name, alias, description, as well as any additional detail or context that is available. This is deprecated.

Passing a resultVersion of 2 corresponds to a detailed format where each error includes information about the line number, rule names, description, as well as any additional detail or context that is available. This is the default.

Passing a resultVersion of 3 corresponds to the detailed version 2 format with additional information about how to fix automatically-fixable errors. In this mode, all errors that occur on each line are reported (other versions report only the first error for each rule).

options.markdownItPlugins

Type: Array of Array of Function and plugin parameters

Specifies additional markdown-it plugins to use when parsing input. Plugins can be used to support additional syntax and features for advanced scenarios.

Each item in the top-level Array should be of the form:

[ require("markdown-it-plugin"), plugin_param_0, plugin_param_1, ... ]

callback

Type: Function taking (Error, Object)

Standard completion callback.

result

Type: Object

Call result.toString() for convenience or see below for an example of the structure of the result object. Passing the value true to toString() uses rule aliases (ex: no-hard-tabs) instead of names (ex: MD010).

Config

The options.config configuration object is simple and can be stored in a file for readability and easy reuse. The readConfig and readConfigSync functions load configuration settings and support the extends keyword for referencing other files (see above).

By default, configuration files are parsed as JSON (and named .markdownlint.json). Custom parsers can be provided to handle other formats like JSONC, YAML, and TOML.

Asynchronous interface:

/**
 * Read specified configuration file.
 *
 * @param {String} file Configuration file name/path.
 * @param {Array} [parsers] Optional parsing function(s).
 * @param {Function} callback Callback (err, result) function.
 * @returns {void}
 */
function readConfig(file, parsers, callback) { ... }

Synchronous interface:

/**
 * Read specified configuration file synchronously.
 *
 * @param {String} file Configuration file name/path.
 * @param {Array} [parsers] Optional parsing function(s).
 * @returns {Object} Configuration object.
 */
function readConfigSync(file, parsers) { ... }

file

Type: String

Location of configuration file to read.

The file is resolved relative to the current working directory. If an extends key is present once read, its value will be resolved as a path relative to file and loaded recursively. Settings from a file referenced by extends are applied first, then those of file are applied on top (overriding any of the same keys appearing in the referenced file).

parsers

Type: Optional Array of Function taking (String) and returning Object

Array of functions to parse configuration files.

The contents of a configuration file are passed to each parser function until one of them returns a value (vs. throwing an exception). Consequently, strict parsers should come before flexible parsers.

For example:

[ JSON.parse, require("toml").parse, require("js-yaml").safeLoad ]

callback

Type: Function taking (Error, Object)

Standard completion callback.

result

Type: Object

Configuration object.

Usage

Invoke markdownlint and use the result object's toString method:

const markdownlint = require("markdownlint");

const options = {
  "files": [ "good.md", "bad.md" ],
  "strings": {
    "good.string": "# good.string\n\nThis string passes all rules.",
    "bad.string": "#bad.string\n\n#This string fails\tsome rules."
  }
};

markdownlint(options, function callback(err, result) {
  if (!err) {
    console.log(result.toString());
  }
});

Output:

bad.string: 3: MD010/no-hard-tabs Hard tabs [Column: 19]
bad.string: 1: MD018/no-missing-space-atx No space after hash on atx style heading [Context: "#bad.string"]
bad.string: 3: MD018/no-missing-space-atx No space after hash on atx style heading [Context: "#This string fails        some rules."]
bad.string: 1: MD041/first-line-heading/first-line-h1 First line in file should be a top level heading [Context: "#bad.string"]
bad.md: 3: MD010/no-hard-tabs Hard tabs [Column: 17]
bad.md: 1: MD018/no-missing-space-atx No space after hash on atx style heading [Context: "#bad.md"]
bad.md: 3: MD018/no-missing-space-atx No space after hash on atx style heading [Context: "#This file fails      some rules."]
bad.md: 1: MD041/first-line-heading/first-line-h1 First line in file should be a top level heading [Context: "#bad.md"]

Or invoke markdownlint.sync for a synchronous call:

const result = markdownlint.sync(options);
console.log(result.toString());

To examine the result object directly:

markdownlint(options, function callback(err, result) {
  if (!err) {
    console.dir(result, { "colors": true, "depth": null });
  }
});

Output:

{
  "good.md": [],
  "bad.md": [
    { "lineNumber": 3,
      "ruleNames": [ "MD010", "no-hard-tabs" ],
      "ruleDescription": "Hard tabs",
      "ruleInformation": "https://github.com/DavidAnson/markdownlint/blob/v0.0.0/doc/Rules.md#md010",
      "errorDetail": "Column: 17",
      "errorContext": null,
      "errorRange": [ 17, 1 ] },
    { "lineNumber": 1,
      "ruleNames": [ "MD018", "no-missing-space-atx" ],
      "ruleDescription": "No space after hash on atx style heading",
      "ruleInformation": "https://github.com/DavidAnson/markdownlint/blob/v0.0.0/doc/Rules.md#md018",
      "errorDetail": null,
      "errorContext": "#bad.md",
      "errorRange": [ 1, 2 ] },
    { "lineNumber": 3,
      "ruleNames": [ "MD018", "no-missing-space-atx" ],
      "ruleDescription": "No space after hash on atx style heading",
      "ruleInformation": "https://github.com/DavidAnson/markdownlint/blob/v0.0.0/doc/Rules.md#md018",
      "errorDetail": null,
      "errorContext": "#This file fails\tsome rules.",
      "errorRange": [ 1, 2 ] },
    { "lineNumber": 1,
      "ruleNames": [ "MD041", "first-line-heading", "first-line-h1" ],
      "ruleDescription": "First line in file should be a top level heading",
      "ruleInformation": "https://github.com/DavidAnson/markdownlint/blob/v0.0.0/doc/Rules.md#md041",
      "errorDetail": null,
      "errorContext": "#bad.md",
      "errorRange": null }
  ]
}

Integration with the gulp build system is straightforward:

const gulp = require("gulp");
const through2 = require("through2");
const markdownlint = require("markdownlint");

gulp.task("markdownlint", function task() {
  return gulp.src("*.md", { "read": false })
    .pipe(through2.obj(function obj(file, enc, next) {
      markdownlint(
        { "files": [ file.relative ] },
        function callback(err, result) {
          const resultString = (result || "").toString();
          if (resultString) {
            console.log(resultString);
          }
          next(err, file);
        });
    }));
});

Output:

[00:00:00] Starting 'markdownlint'...
bad.md: 3: MD010/no-hard-tabs Hard tabs [Column: 17]
bad.md: 1: MD018/no-missing-space-atx No space after hash on atx style heading [Context: "#bad.md"]
bad.md: 3: MD018/no-missing-space-atx No space after hash on atx style heading [Context: "#This file fails      some rules."]
bad.md: 1: MD041/first-line-heading/first-line-h1 First line in file should be a top level heading [Context: "#bad.md"]
[00:00:00] Finished 'markdownlint' after 10 ms

Integration with the Grunt build system is similar:

const markdownlint = require("markdownlint");

module.exports = function wrapper(grunt) {
  grunt.initConfig({
    "markdownlint": {
      "example": {
        "src": [ "*.md" ]
      }
    }
  });

  grunt.registerMultiTask("markdownlint", function task() {
    const done = this.async();
    markdownlint(
      { "files": this.filesSrc },
      function callback(err, result) {
        const resultString = err || ((result || "").toString());
        if (resultString) {
          grunt.fail.warn("\n" + resultString + "\n");
        }
        done(!err || !resultString);
      });
  });
};

Output:

Running "markdownlint:example" (markdownlint) task
Warning:
bad.md: 3: MD010/no-hard-tabs Hard tabs [Column: 17]
bad.md: 1: MD018/no-missing-space-atx No space after hash on atx style heading [Context: "#bad.md"]
bad.md: 3: MD018/no-missing-space-atx No space after hash on atx style heading [Context: "#This file fails      some rules."]
bad.md: 1: MD041/first-line-heading/first-line-h1 First line in file should be a top level heading [Context: "#bad.md"]
 Use --force to continue.

Browser

markdownlint also works in the browser.

Generate normal and minified scripts with:

npm run build-demo

Then reference markdown-it and markdownlint:

<script src="demo/markdown-it.min.js"></script>
<script src="demo/markdownlint-browser.min.js"></script>

And call it like so:

const options = {
  "strings": {
    "content": "Some Markdown to lint."
  }
};
const results = window.markdownlint.sync(options).toString();

Examples

For ideas how to integrate markdownlint into your workflow, refer to the following projects:

.NET Documentation (Search repository)
ally.js (Search repository)
Boostnote (Search repository)
CodiMD (Search repository)
ESLint (Search repository)
Garden React Components (Search repository)
Mocha (Search repository)
Reactable (Search repository)
Sinon.JS (Search repository)
TestCafe (Search repository)
TypeScript Handbook (Search repository)
V8 (Search repository)
webhint (Search repository)
webpack (Search repository)

Contributing

See CONTRIBUTING.md for more information.

History

0.0.1 - Initial release, includes tests MD001-MD032.
0.0.2 - Improve documentation, tests, and code.
0.0.3 - Add synchronous API, improve documentation and code.
0.0.4 - Add tests MD033-MD040, update dependencies.
0.0.5 - Add strings option to enable file-less scenarios, add in-browser demo.
0.0.6 - Improve performance, simplify in-browser, update dependencies.
0.0.7 - Add MD041, improve MD003, ignore front matter, update dependencies.
0.0.8 - Support disabling/enabling rules inline, improve code fence, dependencies.
0.1.0 - Add aliases, exceptions for MD033, exclusions for MD013, dependencies.
- 0.1.1 - Fix bug handling HTML in tables, reference markdownlint-cli.
0.2.0 - Add MD042/MD043, enhance MD002/MD003/MD004/MD007/MD011/MD025/MD041, dependencies.
0.3.0 - More detailed error reporting with resultVersion, enhance MD010/MD012/MD036, fixes for MD027/MD029/MD030, include JSON schema, dependencies.
- 0.3.1 - Fix regressions in MD032/MD038, update dependencies.
0.4.0 - Add MD044, enhance MD013/MD032/MD041/MD042/MD043, fix for MD038, dependencies.
- 0.4.1 - Fixes for MD038/front matter, improvements to MD044, update dependencies.
0.5.0 - Add shareable configuration, noInlineConfig option, README links, fix MD030, improve MD009/MD041, update dependencies.
0.6.0 - resultVersion defaults to 1 (breaking change), ignore HTML comments, TOML front matter, fixes for MD044, update dependencies.
- 0.6.1 - Update markdown-it versioning, exclude demo/test from publishing.
- 0.6.2 - Improve MD013/MD027/MD034/MD037/MD038/MD041/MD044, update dependencies.
- 0.6.3 - Improve highlighting for MD020.
- 0.6.4 - Improve MD029/MD042, update dependencies.
0.7.0 - resultVersion defaults to 2 (breaking change), add MD045, improve MD029, remove trimLeft/trimRight, split rules, refactor, update dependencies.
0.8.0 - Add support for using and authoring custom rules, improve MD004/MD007/MD013, add engines to package.json, refactor, update dependencies.
- 0.8.1 - Update item loop to be iterative, improve MD014, update dependencies.
0.9.0 - Remove support for end-of-life Node versions 0.10/0.12/4, change "header" to "heading" per spec (non-breaking), improve MD003/MD009/MD041, handle uncommon line-break characters, refactor for ES6, update dependencies.
0.10.0 - Add support for non-JSON configuration files, pass file/string name to custom rules, update dependencies.
0.11.0 - Improve MD005/MD024/MD029/MD038, improve custom rule example, add CONTRIBUTING.md, update dependencies.
0.12.0 - Add information link for custom rules, markdownItPlugins for extensibility, improve MD023/MD032/MD038, update dependencies.
0.13.0 - Improve MD013/MD022/MD025/MD029/MD031/MD032/MD037/MD041/, deprecate MD002, improve pandoc YAML support, update dependencies.
0.14.0 - Remove support for end-of-life Node version 6, introduce markdownlint-rule-helpers, add MD046/MD047, improve MD033/MD034/MD039, improve custom rule validation and in-browser demo, update dependencies.
- 0.14.1 - Improve MD033.
- 0.14.2 - Improve MD047, add handleRuleFailures option.
0.15.0 - Add markdownlint-capture/markdownlint-restore inline comments, improve MD009/MD013/MD026/MD033/MD036, update dependencies.
0.16.0 - Add custom rule sample for linting code, improve MD026/MD031/MD033/MD038, update dependencies.
0.17.0 - Add resultVersion 3 to support fix information for default and custom rules, add fix information for 24 rules, update newline handling to match latest CommonMark specification, improve MD014/MD037/MD039, update dependencies.
- 0.17.1 - Fix handling of front matter by fix information.
- 0.17.2 - Improve MD020/MD033/MD044.
0.18.0 - Add MD048/code-fence-style, add fix information for MD007/ul-indent, add markdownlint-disable-file/markdownlint-enable-file inline comments, add type declaration file (.d.ts) for TypeScript dependents, update schema, improve MD006/MD007/MD009/MD013/MD030, update dependencies.
0.19.0 - Remove support for end-of-life Node version 8, add fix information for MD005/list-indent, improve MD007/MD013/MD014, deprecate MD006/ul-start-left, add rationale for every rule, update test runner and code coverage, add more JSDoc comments, update dependencies.

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markdownlint

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.github/workflows		.github/workflows
demo		demo
doc		doc
example		example
helpers		helpers
lib		lib
schema		schema
style		style
test		test
.eslintignore		.eslintignore
.eslintrc.json		.eslintrc.json
.gitignore		.gitignore
.markdownlint.json		.markdownlint.json
.npmignore		.npmignore
CONTRIBUTING.md		CONTRIBUTING.md
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README.md		README.md
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options.handleRuleFailures

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