# remark-lint-fenced-code-flag
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[`remark-lint`][github-remark-lint] rule to warn when language flags of fenced code
are not used or used incorrectly.
## 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(remarkLintFencedCodeFlag[, options])`](#unifieduseremarklintfencedcodeflag-options)
* [`checkGithubLinguistFlag(value)`](#checkgithublinguistflagvalue)
* [`CheckFlag`](#checkflag)
* [`Options`](#options)
* [Recommendation](#recommendation)
* [Examples](#examples)
* [Compatibility](#compatibility)
* [Contribute](#contribute)
* [License](#license)
## What is this?
This package checks the language flags of fenced code blocks,
whether they exist,
and optionally what values they hold.
Particularly,
it provides a check according to GitHub Linguist.
Which is what GitHub uses to highlight code.
So you can make sure that the language flags you use are recognized by
GitHub (or [`starry-night`][github-starry-night])
## When should I use this?
You can use this package to check that the style of language flags of fenced
code blocks is consistent and known.
## 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) | `{ allowEmpty: false }` |
## Install
This package is [ESM only][github-gist-esm].
In Node.js (version 16+),
install with [npm][npm-install]:
```sh
npm install remark-lint-fenced-code-flag
```
In Deno with [`esm.sh`][esm-sh]:
```js
import remarkLintFencedCodeFlag, {checkGithubLinguistFlag} from 'https://esm.sh/remark-lint-fenced-code-flag@4'
```
In browsers with [`esm.sh`][esm-sh]:
```html
```
## Use
On the API:
```js
import remarkLint from 'remark-lint'
import remarkLintFencedCodeFlag from 'remark-lint-fenced-code-flag'
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(remarkLintFencedCodeFlag)
.use(remarkStringify)
.process(file)
console.error(reporter(file))
```
On the CLI:
```sh
remark --frail --use remark-lint --use remark-lint-fenced-code-flag .
```
On the CLI in a config file (here a `package.json`):
```diff
…
"remarkConfig": {
"plugins": [
…
"remark-lint",
+ "remark-lint-fenced-code-flag",
…
]
}
…
```
## API
This package exports the identifier
[`checkGithubLinguistFlag`][api-check-github-linguist-flag].
It exports the [TypeScript][typescript] types
[`CheckFlag`][api-check-flag] and
[`Options`][api-options].
The default export is
[`remarkLintFencedCodeFlag`][api-remark-lint-fenced-code-flag].
### `unified().use(remarkLintFencedCodeFlag[, options])`
Warn when language flags of fenced code are not used.
###### Parameters
* `options` (`Array`, [`CheckFlag`][api-check-flag], or
[`Options`][api-options], optional)
— check, configuration, or flags to allow
###### Returns
Transform ([`Transformer` from `unified`][github-unified-transformer]).
### `checkGithubLinguistFlag(value)`
Check according to GitHub Linguist.
###### Parameters
* `value` (`string`)
— language flag to check
###### Returns
Whether the flag is valid (`undefined`),
or a message to warn about (`string`).
###### Returns
Transform ([`Transformer` from `unified`][github-unified-transformer]).
### `CheckFlag`
Custom check (TypeScript type).
###### Parameters
* `value` (`string`)
— language flag to check
###### Returns
Whether the flag is valid (`undefined`),
or a message to warn about (`string`).
### `Options`
Configuration (TypeScript type).
###### Fields
* `allowEmpty` (`boolean`, default: `false`)
— allow language flags to be omitted
* `flags` (`Array`, optional)
— flags to allow,
other flags will result in a warning
## Recommendation
While omitting language flags is fine to signal that code is plain text,
it *could* point to a mistake.
It’s recommended to instead use a certain flag for plain text (such as
`txt`) and to turn this rule on.
If possible,
stick with what is supported by GitHub.
## Examples
##### `ok.md`
###### In
````markdown
Some markdown:
```markdown
# Mercury
```
````
###### Out
No messages.
##### `not-ok.md`
###### In
````markdown
```
mercury()
```
````
###### Out
```text
1:1-3:4: Unexpected missing fenced code language flag in info string, expected keyword
```
##### `ok-allow-empty.md`
When configured with `{ allowEmpty: true }`.
###### In
````markdown
```
mercury()
```
````
###### Out
No messages.
##### `not-ok-allow-empty.md`
When configured with `{ allowEmpty: false }`.
###### In
````markdown
```
mercury()
```
````
###### Out
```text
1:1-3:4: Unexpected missing fenced code language flag in info string, expected keyword
```
##### `ok-array.md`
When configured with `[ 'markdown' ]`.
###### In
````markdown
```markdown
# Mercury
```
````
###### Out
No messages.
##### `ok-options.md`
When configured with `{ flags: [ 'markdown' ] }`.
###### In
````markdown
```markdown
# Mercury
```
````
###### Out
No messages.
##### `not-ok-array.md`
When configured with `[ 'markdown' ]`.
###### In
````markdown
```javascript
mercury()
```
````
###### Out
```text
1:1-3:4: Unexpected fenced code language flag `javascript` in info string, expected `markdown`
```
##### `not-ok-long-array.md`
When configured with `[ 'javascript', 'markdown', 'mdx', 'typescript' ]`.
###### In
````markdown
```html
Mercury
```
````
###### Out
```text
1:1-3:4: Unexpected fenced code language flag `html` in info string, expected `javascript`, `markdown`, `mdx`, …
```
##### `not-ok-options.md`
When configured with `'🌍'`.
###### Out
```text
1:1: Unexpected value `🌍` for `options`, expected array or object
```
## 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-fenced-code-flag@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-check-flag]: #checkflag
[api-check-github-linguist-flag]: #checkgithublinguistflagvalue
[api-options]: #options
[api-remark-lint-fenced-code-flag]: #unifieduseremarklintfencedcodeflag-options
[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-fenced-code-flag.svg
[badge-downloads-url]: https://www.npmjs.com/package/remark-lint-fenced-code-flag
[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-fenced-code-flag
[badge-size-url]: https://bundlejs.com/?q=remark-lint-fenced-code-flag
[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-starry-night]: https://github.com/wooorm/starry-night
[github-unified-transformer]: https://github.com/unifiedjs/unified#transformer
[npm-install]: https://docs.npmjs.com/cli/install
[typescript]: https://www.typescriptlang.org