eslint-plugin-markdown

What is eslint-plugin-markdown?

The eslint-plugin-markdown package allows you to lint JavaScript code blocks within Markdown files using ESLint. This is particularly useful for ensuring that code snippets in documentation are up to standard and free of errors.

What are eslint-plugin-markdown's main functionalities?

Linting JavaScript Code Blocks in Markdown

This configuration allows ESLint to process JavaScript code blocks within Markdown files. By specifying the `markdown/markdown` processor, ESLint will recognize and lint the JavaScript code embedded in Markdown.

```json
{
  "overrides": [
    {
      "files": ["**/*.md"],
      "processor": "markdown/markdown"
    }
  ]
}
```

Custom ESLint Rules for Markdown

This configuration applies custom ESLint rules specifically to JavaScript code blocks within Markdown files. In this example, the `no-console` rule is enforced, which will trigger an error if `console` statements are found in the code blocks.

```json
{
  "overrides": [
    {
      "files": ["**/*.md/*.js"],
      "rules": {
        "no-console": "error"
      }
    }
  ]
}
```

Linting Specific Code Blocks

This configuration allows you to lint only specific code blocks within a Markdown file. By defining the `start` and `end` markers, you can control which parts of the Markdown file are subject to linting.

```json
{
  "overrides": [
    {
      "files": ["**/*.md"],
      "processor": "markdown/markdown",
      "settings": {
        "markdown/code-blocks": [
          {
            "language": "js",
            "start": "<!-- eslint-enable -->",
            "end": "<!-- eslint-disable -->"
          }
        ]
      }
    }
  ]
}
```

Other packages similar to eslint-plugin-markdown

remark-lint

remark-lint is a plugin for remark, a Markdown processor powered by plugins. It provides a set of rules to lint Markdown files, ensuring consistency and quality. Unlike eslint-plugin-markdown, which focuses on linting JavaScript code within Markdown, remark-lint is designed to lint the Markdown content itself.

markdownlint

markdownlint is a Node.js style checker and lint tool for Markdown files. It provides a set of rules to enforce consistent Markdown style and formatting. While eslint-plugin-markdown focuses on JavaScript code within Markdown, markdownlint is dedicated to the Markdown syntax and structure.

markdown-it

markdown-it is a Markdown parser that can be extended with plugins to add custom functionality. While it is not a linter, it can be used in conjunction with other tools to process and validate Markdown content. It is more flexible and general-purpose compared to eslint-plugin-markdown, which is specifically for linting JavaScript code in Markdown.

README

eslint-plugin-markdown

Lint JS, JSX, TypeScript, and more inside Markdown.

🚧 This documentation is for an unfinished v2 release in progress in the master branch. The latest stable documentation is in the v1 branch.

Usage

Installing

Install the plugin alongside ESLint v6 or greater:

npm install --save-dev eslint eslint-plugin-markdown@next

Configuring

Add the plugin to your .eslintrc and use the processor option in an overrides entry to enable the plugin's markdown/markdown processor on Markdown files. Each fenced code block inside a Markdown document has a virtual filename appended to the Markdown file's path. The virtual filename's extension will match the fenced code block's syntax tag, so for example, ```js code blocks in README.md would match README.md/*.js. For more information on configuring processors, refer to the ESLint documentation.

// .eslintrc.js
module.exports = {
    // 1. Add the plugin.
    plugins: ["markdown"],
    overrides: [
        {
            // 2. Enable the Markdown processor for all .md files.
            files: ["**/*.md"],
            processor: "markdown/markdown"
        },
        {
            // 3. Optionally, customize the configuration ESLint uses for ```js
            // fenced code blocks inside .md files.
            files: ["**/*.md/*.js"],
            // ...
            rules: {
                // ...
            }
        }
    ]
};

Migrating from eslint-plugin-markdown v1

eslint-plugin-markdown v1 used an older version of ESLint's processor API. The Markdown processor automatically ran on .md, .mkdn, .mdown, and .markdown files, and it only extracted fenced code blocks marked with js, javascript, jsx, or node syntax. Configuration specifically for fenced code blocks went inside an overrides entry with a files pattern matching the containing Markdown document's filename that applied to all fenced code blocks inside the file.

// .eslintrc.js for eslint-plugin-markdown v1
module.exports = {
    plugins: ["markdown"],
    overrides: [
        {
            files: ["**/*.md"],
            // In v1, configuration for fenced code blocks went inside an
            // `overrides` entry with a .md pattern, for example:
            parserOptions: {
                ecmaFeatures: {
                    impliedStrict: true
                }
            },
            rules: {
                "no-console": "off"
            }
        }
    ]
};

RFC3 designed a new processor API to remove these limitations, and the new API was implemented as part of ESLint v6. eslint-plugin-markdown v2 uses this new API.

$ npm install --save-dev eslint@latest eslint-plugin-markdown@next

All of the Markdown file extensions that were previously hard-coded are now fully configurable in .eslintrc.js. Use the new processor option to apply the markdown/markdown processor on any Markdown documents matching a files pattern. Each fenced code block inside a Markdown document has a virtual filename appended to the Markdown file's path. The virtual filename's extension will match the fenced code block's syntax tag, so for example, ```js code blocks in README.md would match README.md/*.js.

// eslintrc.js for eslint-plugin-markdown v2
module.exports = {
    plugins: ["markdown"],
    overrides: [
        {
            // In v2, explicitly apply eslint-plugin-markdown's `markdown`
            // processor on any Markdown files you want to lint.
            files: ["**/*.md"],
            processor: "markdown/markdown"
        },
        {
            // In v2, configuration for fenced code blocks is separate from the
            // containing Markdown file. Each code block has a virtual filename
            // appended to the Markdown file's path.
            files: ["**/*.md/*.js"],
            // Configuration for fenced code blocks goes with the override for
            // the code block's virtual filename, for example:
            parserOptions: {
                ecmaFeatures: {
                    impliedStrict: true
                }
            },
            rules: {
                "no-console": "off"
            }
        }
    ]
};

If you need to precisely mimic the behavior of v1 with the hard-coded Markdown extensions and fenced code block syntaxes, you can use those as glob patterns in overrides[].files:

// eslintrc.js for v2 mimicking v1 behavior
module.exports = {
    plugins: ["markdown"],
    overrides: [
        {
            files: ["**/*.{md,mkdn,mdown,markdown}"],
            processor: "markdown/markdown"
        },
        {
            files: ["**/*.{md,mkdn,mdown,markdown}/*.{js,javascript,jsx,node}"]
            // ...
        }
    ]
};

Running

ESLint v7

You can run ESLint as usual and do not need to use the --ext option. ESLint v7 automatically lints file extensions specified in overrides[].files patterns in config files.

ESLint v6

Use the --ext option to include .js and .md extensions in ESLint's file search:

eslint --ext js,md .

Autofixing

With this plugin, ESLint's --fix option can automatically fix some issues in your Markdown fenced code blocks. To enable this, pass the --fix flag when you run ESLint:

eslint --fix .

What Gets Linted?

With this plugin, ESLint will lint fenced code blocks in your Markdown documents:

```js
// This gets linted
var answer = 6 * 7;
console.log(answer);
```

Here is some regular Markdown text that will be ignored.

```js
// This also gets linted

/* eslint quotes: [2, "double"] */

function hello() {
    console.log("Hello, world!");
}
hello();
```

```jsx
// This can be linted too if you add `.jsx` files to `overrides` in ESLint v7
// or pass `--ext jsx` in ESLint v6.
var div = <div className="jsx"></div>;
```

Blocks that don't specify a syntax are ignored:

```
This is plain text and doesn't get linted.
```

Unless a fenced code block's syntax appears as a file extension in overrides[].files in ESLint v7, it will be ignored. If using ESLint v6, you must also include the extension with the --ext option.

```python
print("This doesn't get linted either.")
```

Configuration Comments

The processor will convert HTML comments immediately preceding a code block into JavaScript block comments and insert them at the beginning of the source code that it passes to ESLint. This permits configuring ESLint via configuration comments while keeping the configuration comments themselves hidden when the markdown is rendered. Comment bodies are passed through unmodified, so the plugin supports any configuration comments supported by ESLint itself.

This example enables the browser environment, disables the no-alert rule, and configures the quotes rule to prefer single quotes:

<!-- eslint-env browser -->
<!-- eslint-disable no-alert -->
<!-- eslint quotes: ["error", "single"] -->

```js
alert('Hello, world!');
```

Each code block in a file is linted separately, so configuration comments apply only to the code block that immediately follows.

Assuming `no-alert` is enabled in `.eslintrc`, the first code block will have no error from `no-alert`:

<!-- eslint-env browser -->
<!-- eslint-disable no-alert -->

```js
alert("Hello, world!");
```

But the next code block will have an error from `no-alert`:

<!-- eslint-env browser -->

```js
alert("Hello, world!");
```

Skipping Blocks

Sometimes it can be useful to have code blocks marked with js even though they don't contain valid JavaScript syntax, such as commented JSON blobs that need js syntax highlighting. Standard eslint-disable comments only silence rule reporting, but ESLint still reports any syntax errors it finds. In cases where a code block should not even be parsed, insert a non-standard <!-- eslint-skip --> comment before the block, and this plugin will hide the following block from ESLint. Neither rule nor syntax errors will be reported.

There are comments in this JSON, so we use `js` syntax for better
highlighting. Skip the block to prevent warnings about invalid syntax.

<!-- eslint-skip -->

```js
{
    // This code block is hidden from ESLint.
    "hello": "world"
}
```

```js
console.log("This code block is linted normally.");
```

Unsatisfiable Rules

Since code blocks are not files themselves but embedded inside a Markdown document, some rules do not apply to Markdown code blocks, and messages from these rules are automatically suppressed:

• eol-last
• unicode-bom

Project or directory-wide overrides for code snippets

Given that code snippets often lack full context, and adding full context through configuration comments may be too cumbersome to apply for each snippet, one may wish to instead set defaults for all one's JavaScript snippets in a manner that applies to all Markdown files within your project (or a specific directory).

ESLint allows a configuration property overrides which has a files property which accepts a glob pattern, allowing you to designate files (such as all md files) whose rules will be overridden.

The following example shows the disabling of a few commonly problematic rules for code snippets. It also points to the fact that some rules (e.g., padded-blocks) may be more appealing for disabling given that one may wish for documentation to be more liberal in providing padding for readability.

// .eslintrc.js
module.exports = {
    plugins: ["markdown"],
    overrides: [
        {
            files: ["**/*.md"],
            processor: "markdown/markdown"
        },
        {
            files: ["**/*.md/*.js"],
            rules: {
                "no-undef": "off",
                "no-unused-vars": "off",
                "no-console": "off",
                "padded-blocks": "off"
            }
        }
    ]
};

Overriding strict

The strict rule is technically satisfiable inside of Markdown code blocks, but writing a "use strict" directive at the top of every code block is tedious and distracting. We recommend a glob pattern for .md files containing .js blocks to disable strict and enable the impliedStrict parser option so the code blocks still parse in strict mode:

// .eslintrc.js
module.exports = {
    plugins: ["markdown"],
    overrides: [
        {
            files: ["**/*.md"],
            processor: "markdown/markdown"
        },
        {
            files: ["**/*.md/*.js"],
            parserOptions: {
                ecmaFeatures: {
                    impliedStrict: true
                }
            }
        }
    ]
};

Editor Integrations

VSCode

vscode-eslint has built-in support for the Markdown processor.

Atom

The linter-eslint package allows for linting within the Atom IDE.

In order to see eslint-plugin-markdown work its magic within Markdown code blocks in your Atom editor, you can go to linter-eslint's settings and within "List of scopes to run ESLint on...", add the cursor scope "source.gfm".

However, this reports a problem when viewing Markdown which does not have configuration, so you may wish to use the cursor scope "source.embedded.js", but note that eslint-plugin-markdown configuration comments and skip directives won't work in this context.

Contributing

$ git clone https://github.com/eslint/eslint-plugin-markdown.git
$ cd eslint-plugin-markdown
$ npm install
$ npm test

This project follows the ESLint contribution guidelines.