# remark-lint-heading-increment
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[`remark-lint`][github-remark-lint] rule to warn when heading ranks increment with more than
1 at a time.
## Contents
* [What is this?](#what-is-this)
* [When should I use this?](#when-should-i-use-this)
* [Presets](#presets)
* [Install](#install)
* [Use](#use)
* [API](#api)
* [`unified().use(remarkLintHeadingIncrement)`](#unifieduseremarklintheadingincrement)
* [Recommendation](#recommendation)
* [Examples](#examples)
* [Compatibility](#compatibility)
* [Contribute](#contribute)
* [License](#license)
## What is this?
This package checks the increase of headings.
## When should I use this?
You can use this package to check the increase of headings.
## Presets
This plugin is included in the following presets:
| Preset | Options |
| - | - |
| [`remark-preset-lint-markdown-style-guide`](https://github.com/remarkjs/remark-lint/tree/main/packages/remark-preset-lint-markdown-style-guide) | |
## Install
This package is [ESM only][github-gist-esm].
In Node.js (version 16+),
install with [npm][npm-install]:
```sh
npm install remark-lint-heading-increment
```
In Deno with [`esm.sh`][esm-sh]:
```js
import remarkLintHeadingIncrement from 'https://esm.sh/remark-lint-heading-increment@4'
```
In browsers with [`esm.sh`][esm-sh]:
```html
```
## Use
On the API:
```js
import remarkLint from 'remark-lint'
import remarkLintHeadingIncrement from 'remark-lint-heading-increment'
import remarkParse from 'remark-parse'
import remarkStringify from 'remark-stringify'
import {read} from 'to-vfile'
import {unified} from 'unified'
import {reporter} from 'vfile-reporter'
const file = await read('example.md')
await unified()
.use(remarkParse)
.use(remarkLint)
.use(remarkLintHeadingIncrement)
.use(remarkStringify)
.process(file)
console.error(reporter(file))
```
On the CLI:
```sh
remark --frail --use remark-lint --use remark-lint-heading-increment .
```
On the CLI in a config file (here a `package.json`):
```diff
…
"remarkConfig": {
"plugins": [
…
"remark-lint",
+ "remark-lint-heading-increment",
…
]
}
…
```
## API
This package exports no identifiers.
It exports no additional [TypeScript][typescript] types.
The default export is
[`remarkLintHeadingIncrement`][api-remark-lint-heading-increment].
### `unified().use(remarkLintHeadingIncrement)`
Warn when heading ranks increment with more than 1 at a time.
###### Parameters
There are no options.
###### Returns
Transform ([`Transformer` from `unified`][github-unified-transformer]).
## Recommendation
While markdown is not only used for HTML,
HTML accessibility guidelines state that headings should increment by one at
a time.
As in,
say the previous heading had a rank of 2 (so ``),
then the following heading that is to be considered “inside” it should have
a rank of 3 (``).
Due to this,
when HTML output is a goal of the document,
it’s recommended that this rule is turned on.
## Examples
##### `ok.md`
###### In
```markdown
# Mercury
## Nomenclature
```
###### Out
No messages.
##### `also-ok.md`
###### In
```markdown
#### Impact basins and craters
#### Plains
#### Compressional features
```
###### Out
No messages.
##### `not-ok.md`
###### In
```markdown
# Mercury
### Internal structure
### Surface geology
## Observation history
#### Mariner 10
```
###### Out
```text
3:1-3:23: Unexpected heading rank `3`, exected rank `2`
5:1-5:20: Unexpected heading rank `3`, exected rank `2`
9:1-9:16: Unexpected heading rank `4`, exected rank `3`
```
##### `html.md`
###### In
```markdown
# Mercury
Mercury is the first planet from the Sun and the smallest
in the Solar System.
Internal structure
Orbit, rotation, and longitude
```
###### Out
```text
6:1-6:28: Unexpected heading rank `3`, exected rank `2`
```
##### `mdx.mdx`
###### In
> 👉 **Note**: this example uses
> MDX ([`remark-mdx`][github-remark-mdx]).
```mdx
# Mercury
Mercury is the first planet from the Sun and the smallest
in the Solar System.
Internal structure
Orbit, rotation, and longitude
```
###### Out
```text
6:1-6:28: Unexpected heading rank `3`, exected rank `2`
```
## Compatibility
Projects maintained by the unified collective are compatible with maintained
versions of Node.js.
When we cut a new major release, we drop support for unmaintained versions of
Node.
This means we try to keep the current release line,
`remark-lint-heading-increment@4`,
compatible with Node.js 16.
## Contribute
See [`contributing.md`][github-dotfiles-contributing] in [`remarkjs/.github`][github-dotfiles-health] for ways
to get started.
See [`support.md`][github-dotfiles-support] for ways to get help.
This project has a [code of conduct][github-dotfiles-coc].
By interacting with this repository, organization, or community you agree to
abide by its terms.
## License
[MIT][file-license] © [Titus Wormer][author]
[api-remark-lint-heading-increment]: #unifieduseremarklintheadingincrement
[author]: https://wooorm.com
[badge-build-image]: https://github.com/remarkjs/remark-lint/workflows/main/badge.svg
[badge-build-url]: https://github.com/remarkjs/remark-lint/actions
[badge-chat-image]: https://img.shields.io/badge/chat-discussions-success.svg
[badge-chat-url]: https://github.com/remarkjs/remark/discussions
[badge-coverage-image]: https://img.shields.io/codecov/c/github/remarkjs/remark-lint.svg
[badge-coverage-url]: https://codecov.io/github/remarkjs/remark-lint
[badge-downloads-image]: https://img.shields.io/npm/dm/remark-lint-heading-increment.svg
[badge-downloads-url]: https://www.npmjs.com/package/remark-lint-heading-increment
[badge-funding-backers-image]: https://opencollective.com/unified/backers/badge.svg
[badge-funding-sponsors-image]: https://opencollective.com/unified/sponsors/badge.svg
[badge-funding-url]: https://opencollective.com/unified
[badge-size-image]: https://img.shields.io/bundlejs/size/remark-lint-heading-increment
[badge-size-url]: https://bundlejs.com/?q=remark-lint-heading-increment
[esm-sh]: https://esm.sh
[file-license]: https://github.com/remarkjs/remark-lint/blob/main/license
[github-dotfiles-coc]: https://github.com/remarkjs/.github/blob/main/code-of-conduct.md
[github-dotfiles-contributing]: https://github.com/remarkjs/.github/blob/main/contributing.md
[github-dotfiles-health]: https://github.com/remarkjs/.github
[github-dotfiles-support]: https://github.com/remarkjs/.github/blob/main/support.md
[github-gist-esm]: https://gist.github.com/sindresorhus/a39789f98801d908bbc7ff3ecc99d99c
[github-remark-lint]: https://github.com/remarkjs/remark-lint
[github-remark-mdx]: https://mdxjs.com/packages/remark-mdx/
[github-unified-transformer]: https://github.com/unifiedjs/unified#transformer
[npm-install]: https://docs.npmjs.com/cli/install
[typescript]: https://www.typescriptlang.org