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 and io.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 rules, rule documentation, and test cases come directly from that project.

Related

• markdownlint-cli command-line interface for Node.js
• vscode-markdownlint extension for VS Code
• markdownlint/mdl gem for Ruby

Demonstration

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

Rules / Aliases

• MD001 header-increment - Header levels should only increment by one level at a time
• MD002 first-header-h1 - First header should be a h1 header
• MD003 header-style - Header 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 header
• MD019 no-multiple-space-atx - Multiple spaces after hash on atx style header
• MD020 no-missing-space-closed-atx - No space inside hashes on closed atx style header
• MD021 no-multiple-space-closed-atx - Multiple spaces inside hashes on closed atx style header
• MD022 blanks-around-headers - Headers should be surrounded by blank lines
• MD023 header-start-left - Headers must start at the beginning of the line
• MD024 no-duplicate-header - Multiple headers with the same content
• MD025 single-h1 - Multiple top level headers in the same document
• MD026 no-trailing-punctuation - Trailing punctuation in header
• 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-header - Emphasis used instead of a header
• 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-h1 - First line in file should be a top level header
• MD042 no-empty-links - No empty links

See Rules.md for more details.

Tags

• atx - MD018, MD019
• atx_closed - MD020, MD021
• blank_lines - MD012, MD022, MD031, MD032
• blockquote - MD027, MD028
• bullet - MD004, MD005, MD006, MD007, MD032
• code - MD014, MD031, MD038, MD040
• emphasis - MD036, MD037
• hard_tab - MD010
• headers - MD001, MD002, MD003, MD018, MD019, MD020, MD021, MD022, MD023, MD024, MD025, MD026, MD036, MD041
• hr - MD035
• html - MD033
• indentation - MD005, MD006, MD007, MD027
• language - MD040
• line_length - MD013
• links - MD011, MD034, MD039, MD042
• ol - MD029, MD030, MD032
• spaces - MD018, MD019, MD020, MD021, MD023
• ul - MD004, MD005, MD006, MD007, MD030, MD032
• url - MD034
• whitespace - MD009, MD010, MD012, MD027, MD028, MD030, MD037, MD038, MD039

Configuration

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 within a file, add one of these markers to the appropriate place (HTML comments don't appear in the final markup):

• Disable all rules: <!-- markdownlint-disable -->
• Enable all rules: <!-- markdownlint-enable -->
• Disable one or more rules: <!-- markdownlint-disable MD001 MD002 -->
• Enable one or more rules: <!-- markdownlint-enable MD001 MD002 -->

For example:

<!-- markdownlint-disable MD037 -->
deliberate space * in * emphasis
<!-- markdownlint-enable MD037 -->

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

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

API

Standard asynchronous interface:

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

options

Type: Object

Configures the function.

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.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.

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

Default:

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

Ignores:

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

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:

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

See the style directory for more samples.

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).

Usage

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

var markdownlint = require("markdownlint");

var 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 Hard tabs
bad.string: 1: MD018 No space after hash on atx style header
bad.string: 3: MD018 No space after hash on atx style header
bad.md: 3: MD010 Hard tabs
bad.md: 1: MD018 No space after hash on atx style header
bad.md: 3: MD018 No space after hash on atx style header

Or invoke markdownlint.sync for a synchronous call:

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

Output:

bad.string: 3: no-hard-tabs Hard tabs
bad.string: 1: no-missing-space-atx No space after hash on atx style header
bad.string: 3: no-missing-space-atx No space after hash on atx style header
bad.md: 3: no-hard-tabs Hard tabs
bad.md: 1: no-missing-space-atx No space after hash on atx style header
bad.md: 3: no-missing-space-atx No space after hash on atx style header

To examine the result object directly:

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

Output:

{
  "good.string": {},
  "bad.string": {
    "MD010": [ 3 ],
    "MD018": [ 1, 3 ]
  },
  "good.md": {},
  "bad.md": {
    "MD010": [ 3 ],
    "MD018": [ 1, 3 ]
  }
}

Integration with the gulp build system is straightforward:

var gulp = require("gulp");
var through2 = require("through2");
var 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) {
          var resultString = (result || "").toString();
          if (resultString) {
            console.log(resultString);
          }
          next(err, file);
        });
    }));
});

Output:

[00:00:00] Starting 'markdownlint'...
bad.md: 3: MD010 Hard tabs
bad.md: 1: MD018 No space after hash on atx style header
bad.md: 3: MD018 No space after hash on atx style header
[00:00:00] Finished 'markdownlint' after 10 ms

Integration with the Grunt build system is similar:

var markdownlint = require("markdownlint");

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

  grunt.registerMultiTask("markdownlint", function task() {
    var done = this.async();
    markdownlint(
      { "files": this.filesSrc },
      function callback(err, result) {
        var 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 Hard tabs
bad.md: 1: MD018 No space after hash on atx style header
bad.md: 3: MD018 No space after hash on atx style header
 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:

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

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.