unified lets you inspect and transform content with plugins.

Contents

What is this?
When should I use this?
Install
Use
Overview
API
Types
Compatibility
Contribute
Sponsor
Acknowledgments
License

What is this?

unified is two things:

unified is a collective of 500+ free and open source packages that work with content as structured data (ASTs)
unified (this project) is the core package, used in 1.3m+ projects on GH, to process content with plugins

Several ecosystems are built on unified around different kinds of content. Notably, remark (markdown), rehype (HTML), and retext (natural language). These ecosystems can be connected together.

for more about us, see unifiedjs.com
for updates, see @unifiedjs on Twitter
for questions, see support
to help, see contribute and sponsor below

When should I use this?

In some cases, you are already using unified. For example, it’s used in MDX, Gatsby, Docusaurus, etc. In those cases, you don’t need to add unified yourself but you can include plugins into those projects.

But the real fun (for some) is to get your hands dirty and work with syntax trees and build with it yourself. You can create those projects, or things like Prettier, or your own site generator. You can connect utilities together and make your own plugins that check for problems and transform from one thing to another.

When you are dealing with one type of content (such as markdown), you can use the main package of that ecosystem instead (so remark). When you are dealing with different kinds of content (such as markdown and HTML), it’s recommended to use unified itself, and pick and choose the plugins you need.

Install

This package is ESM only. In Node.js (version 16+), install with npm:

npm install unified

In Deno with esm.sh:

import {unified} from 'https://esm.sh/unified@10'

In browsers with esm.sh:

<script type="module">
  import {unified} from 'https://esm.sh/unified@10?bundle'
</script>

Use

import rehypeDocument from 'rehype-document'
import rehypeFormat from 'rehype-format'
import rehypeStringify from 'rehype-stringify'
import remarkParse from 'remark-parse'
import remarkRehype from 'remark-rehype'
import {unified} from 'unified'
import {reporter} from 'vfile-reporter'

const file = await unified()
  .use(remarkParse)
  .use(remarkRehype)
  .use(rehypeDocument, {title: '👋🌍'})
  .use(rehypeFormat)
  .use(rehypeStringify)
  .process('# Hello world!')

console.error(reporter(file))
console.log(String(file))

Yields:

no issues found

<!doctype html>
<html lang="en">
  <head>
    <meta charset="utf-8">
    <title>👋🌍</title>
    <meta name="viewport" content="width=device-width, initial-scale=1">
  </head>
  <body>
    <h1>Hello world!</h1>
  </body>
</html>

Overview

unified is an interface for processing content with syntax trees. Syntax trees are a representation of content understandable to programs. Those programs, called plugins, take these trees and inspect and modify them. To get to the syntax tree from text, there is a parser. To get from that back to text, there is a compiler. This is the process of a processor.

| ........................ process ........................... |
| .......... parse ... | ... run ... | ... stringify ..........|

          +--------+                     +----------+
Input ->- | Parser | ->- Syntax Tree ->- | Compiler | ->- Output
          +--------+          |          +----------+
                              X
                              |
                       +--------------+
                       | Transformers |
                       +--------------+

Processors

Processors process content. On its own, unified (the root processor) doesn’t work. It needs to be configured with plugins to work. For example:

const processor = unified()
  .use(remarkParse)
  .use(remarkRehype)
  .use(rehypeDocument, {title: '👋🌍'})
  .use(rehypeFormat)
  .use(rehypeStringify)

That processor can do different things. It can:

…parse markdown (parse)
…turn parsed markdown into HTML and format the HTML (run)
…compile HTML (stringify)
…do all of the above (process)

Every processor implements another processor. To create a processor, call another processor. The new processor is configured to work the same as its ancestor. But when the descendant processor is configured in the future it does not affect the ancestral processor.

When processors are exposed from a module (for example, unified itself) they should not be configured directly, as that would change their behavior for all module users. Those processors are frozen and they should be called to create a new processor before they are used.

File

When processing a document, metadata is gathered about that document. vfile is the file format that stores data, metadata, and messages about files for unified and plugins.

There are several utilities for working with these files.

Syntax tree

The syntax trees used in unified are unist nodes. A tree represents a whole document and each node is a plain JavaScript object with a type field. The semantics of nodes and the format of syntax trees is defined by other projects:

esast — JavaScript
hast — HTML
mdast — markdown
nlcst — natural language
xast — XML

There are many utilities for working with trees listed in each aforementioned project and maintained in the syntax-tree organization. These utilities are a level lower than unified itself and are building blocks that can be used to make plugins.

Ecosystems

Around each syntax tree is an ecosystem that focusses on that particular kind of content. At their core, they parse text to a tree and compile that tree back to text. They also provide plugins that work with the syntax tree, without requiring that the end user has knowledge about that tree.

rehype (hast) — HTML
remark (mdast) — markdown
retext (nlcst) — natural language

Plugins

Each aforementioned ecosystem comes with a large set of plugins that you can pick and choose from to do all kinds of things.

There are also a few plugins that work in any ecosystem:

unified-diff — ignore unrelated messages in GitHub Actions and Travis
unified-infer-git-meta — infer metadata of a document from Git
unified-message-control — enable, disable, and ignore messages from content

Configuration

Processors are configured with plugins or with the data method. Most plugins also accept configuration through options. See each plugin’s readme for more info.

Integrations

unified can integrate with the file system through unified-engine. CLI apps can be created with unified-args, Gulp plugins with unified-engine-gulp, and language servers with unified-language-server. A streaming interface can be created with unified-stream.

Programming interface

The API provided by unified allows multiple files to be processed and gives access to metadata (such as lint messages):

import rehypeStringify from 'rehype-stringify'
import remarkParse from 'remark-parse'
import remarkPresetLintMarkdownStyleGuide from 'remark-preset-lint-markdown-style-guide'
import remarkRehype from 'remark-rehype'
import remarkRetext from 'remark-retext'
import retextEnglish from 'retext-english'
import retextEquality from 'retext-equality'
import {unified} from 'unified'
import {reporter} from 'vfile-reporter'

const file = await unified()
  .use(remarkParse)
  .use(remarkPresetLintMarkdownStyleGuide)
  .use(remarkRetext, unified().use(retextEnglish).use(retextEquality))
  .use(remarkRehype)
  .use(rehypeStringify)
  .process('*Emphasis* and _stress_, you guys!')

console.error(reporter(file))
console.log(String(file))

Yields:

1:16-1:24 warning Emphasis should use `*` as a marker                                 emphasis-marker remark-lint
1:30-1:34 warning `guys` may be insensitive, use `people`, `persons`, `folks` instead gals-man        retext-equality

⚠ 2 warnings

<p><em>Emphasis</em> and <em>stress</em>, you guys!</p>

Transforming between ecosystems

Ecosystems can be combined in two modes.

Bridge mode transforms the tree from one format (origin) to another (destination). A different processor runs on the destination tree. Afterwards, the original processor continues with the origin tree.

Mutate mode also transforms the syntax tree from one format to another. But the original processor continues transforming the destination tree.

In the previous example (“Programming interface”), remark-retext is used in bridge mode: the origin syntax tree is kept after retext is done; whereas remark-rehype is used in mutate mode: it sets a new syntax tree and discards the origin tree.

The following plugins lets you combine ecosystems:

remark-retext — turn markdown into natural language
remark-rehype — turn markdown into HTML
rehype-retext — turn HTML into natural language
rehype-remark — turn HTML into markdown

API

This package exports the identifier unified (the root processor). There is no default export.

`processor()`

Create a new processor.

Returns

New unfrozen processor (processor).

This processor is configured to work the same as its ancestor. When the descendant processor is configured in the future it does not affect the ancestral processor.

Example

This example shows how a new processor can be created (from remark) and linked to stdin(4) and stdout(4).

import process from 'node:process'
import concatStream from 'concat-stream'
import {remark} from 'remark'

process.stdin.pipe(
  concatStream(function (buf) {
    process.stdout.write(String(remark().processSync(buf)))
  })
)

`processor.Compiler`

Compiler to use (Compiler, optional).

`processor.Parser`

Parser to use (Parser, optional).

`processor.data([key[, value]])`

Configure the processor with info available to all plugins. Information is stored in an object.

Typically, options can be given to a specific plugin, but sometimes it makes sense to have information shared with several plugins. For example, a list of HTML elements that are self-closing, which is needed during all phases.

👉 Note: setting information cannot occur on frozen processors. Call the processor first to create a new unfrozen processor.

👉 Note: to register custom data in TypeScript, augment the Data interface.

Signatures

processor = processor.data(key, value)
processor = processor.data(dataset)
value = processor.data(key)
dataset = processor.data()

Parameters

key (keyof Data, optional) — field to get
value (Data[key]) — value to set
values (Data) — values to set

Returns

The current processor when setting (processor), the value at key when getting (Data[key]), or the entire dataset when getting without key (Data).

Example

This example show how to get and set info:

import {unified} from 'unified'

const processor = unified().data('alpha', 'bravo')

processor.data('alpha') // => 'bravo'

processor.data() // => {alpha: 'bravo'}

processor.data({charlie: 'delta'})

processor.data() // => {charlie: 'delta'}

`processor.freeze()`

Freeze a processor.

Frozen processors are meant to be extended and not to be configured directly.

When a processor is frozen it cannot be unfrozen. New processors working the same way can be created by calling the processor.

It’s possible to freeze processors explicitly by calling .freeze(). Processors freeze automatically when .parse(), .run(), .runSync(), .stringify(), .process(), or .processSync() are called.

Returns

The current processor (processor).

Example

This example, index.js, shows how rehype prevents extensions to itself:

import rehypeParse from 'rehype-parse'
import rehypeStringify from 'rehype-stringify'
import {unified} from 'unified'

export const rehype = unified().use(rehypeParse).use(rehypeStringify).freeze()

That processor can be used and configured like so:

import {rehype} from 'rehype'
import rehypeFormat from 'rehype-format'
// …

rehype()
  .use(rehypeFormat)
  // …

A similar looking example is broken as operates on the frozen interface. If this behavior was allowed it would result in unexpected behavior so an error is thrown. This is not valid:

import {rehype} from 'rehype'
import rehypeFormat from 'rehype-format'
// …

rehype
  .use(rehypeFormat)
  // …

Yields:

~/node_modules/unified/index.js:426
    throw new Error(
    ^

Error: Cannot call `use` on a frozen processor.
Create a new processor first, by calling it: use `processor()` instead of `processor`.
    at assertUnfrozen (~/node_modules/unified/index.js:426:11)
    at Function.use (~/node_modules/unified/index.js:165:5)
    …

`processor.parse(file)`

Parse text to a syntax tree.

👉 Note: parse freezes the processor if not already frozen.

👉 Note: parse performs the parse phase, not the run phase or other phases.

Parameters

file (Compatible) — file to parse; typically string or VFile; any value accepted as x in new VFile(x)

Returns

Syntax tree representing file (Node).

Example

This example shows how parse can be used to create a tree from a file.

import remarkParse from 'remark-parse'
import {unified} from 'unified'

const tree = unified().use(remarkParse).parse('# Hello world!')

console.log(tree)

Yields:

{
  type: 'root',
  children: [
    {type: 'heading', depth: 1, children: [Array], position: [Object]}
  ],
  position: {
    start: {line: 1, column: 1, offset: 0},
    end: {line: 1, column: 15, offset: 14}
  }
}

`processor.process(file[, done])`

Process the given file as configured on the processor.

👉 Note: process freezes the processor if not already frozen.

👉 Note: process performs the parse, run, and stringify phases.

Signatures

processor.process(file, done)
Promise<VFile> = processor.process(file?)

Parameters

file (Compatible, optional) — file; typically string or VFile; any value accepted as x in new VFile(x)
done (ProcessCallback, optional) — callback

Returns

Nothing if done is given (undefined). Otherwise a promise, rejected with a fatal error or resolved with the processed file (Promise<VFile>).

The parsed, transformed, and compiled value is available at file.value (see note).

👉 Note: unified typically compiles by serializing: most compilers return string (or Uint8Array). Some compilers, such as the one configured with rehype-react, return other values (in this case, a React tree). If you’re using a compiler that doesn’t serialize, expect different result values.

To register custom results in TypeScript, add them to CompileResultMap.

Example

This example shows how process can be used to process a file:

import rehypeDocument from 'rehype-document'
import rehypeFormat from 'rehype-format'
import rehypeStringify from 'rehype-stringify'
import remarkParse from 'remark-parse'
import remarkRehype from 'remark-rehype'
import {unified} from 'unified'

const file = await unified()
  .use(remarkParse)
  .use(remarkRehype)
  .use(rehypeDocument, {title: '👋🌍'})
  .use(rehypeFormat)
  .use(rehypeStringify)
  .process('# Hello world!')

console.log(String(file))

Yields:

<!doctype html>
<html lang="en">
  <head>
    <meta charset="utf-8">
    <title>👋🌍</title>
    <meta name="viewport" content="width=device-width, initial-scale=1">
  </head>
  <body>
    <h1>Hello world!</h1>
  </body>
</html>

`processor.processSync(file)`

Process the given file as configured on the processor.

An error is thrown if asynchronous transforms are configured.

👉 Note: processSync freezes the processor if not already frozen.

👉 Note: processSync performs the parse, run, and stringify phases.

Parameters

file (Compatible, optional) — file; typically string or VFile; any value accepted as x in new VFile(x)

Returns

The processed file (VFile).

The parsed, transformed, and compiled value is available at file.value (see note).

👉 Note: unified typically compiles by serializing: most compilers return string (or Uint8Array). Some compilers, such as the one configured with rehype-react, return other values (in this case, a React tree). If you’re using a compiler that doesn’t serialize, expect different result values.

To register custom results in TypeScript, add them to CompileResultMap.

Example

This example shows how processSync can be used to process a file, if all transformers are synchronous.

import rehypeDocument from 'rehype-document'
import rehypeFormat from 'rehype-format'
import rehypeStringify from 'rehype-stringify'
import remarkParse from 'remark-parse'
import remarkRehype from 'remark-rehype'
import {unified} from 'unified'

const processor = unified()
  .use(remarkParse)
  .use(remarkRehype)
  .use(rehypeDocument, {title: '👋🌍'})
  .use(rehypeFormat)
  .use(rehypeStringify)

console.log(String(processor.processSync('# Hello world!')))

Yields:

<!doctype html>
<html lang="en">
  <head>
    <meta charset="utf-8">
    <title>👋🌍</title>
    <meta name="viewport" content="width=device-width, initial-scale=1">
  </head>
  <body>
    <h1>Hello world!</h1>
  </body>
</html>

`processor.run(tree[, file][, done])`

Run transformers on a syntax tree.

👉 Note: run freezes the processor if not already frozen.

👉 Note: run performs the run phase, not other phases.

Signatures

processor.run(tree, done)
processor.run(tree, file, done)
Promise<Node> = processor.run(tree, file?)

Parameters

tree (Node) — tree to transform and inspect
file (Compatible, optional) — file associated with node; any value accepted as x in new VFile(x)
done (RunCallback, optional) — callback

Returns

Nothing if done is given (undefined). Otherwise, a promise rejected with a fatal error or resolved with the transformed tree (Promise<Node>).

Example

This example shows how run can be used to transform a tree:

import remarkReferenceLinks from 'remark-reference-links'
import {unified} from 'unified'
import {u} from 'unist-builder'

const tree = u('root', [
  u('paragraph', [
    u('link', {href: 'https://example.com'}, [u('text', 'Example Domain')])
  ])
])

const changedTree = await unified().use(remarkReferenceLinks).run(tree)

console.log(changedTree)

Yields:

{
  type: 'root',
  children: [
    {type: 'paragraph', children: [Array]},
    {type: 'definition', identifier: '1', title: '', url: undefined}
  ]
}

`processor.runSync(tree[, file])`

Run transformers on a syntax tree.

An error is thrown if asynchronous transforms are configured.

👉 Note: runSync freezes the processor if not already frozen.

👉 Note: runSync performs the run phase, not other phases.

Parameters

tree (Node) — tree to transform and inspect
file (Compatible, optional) — file associated with node; any value accepted as x in new VFile(x)

Returns

Transformed tree (Node).

`processor.stringify(tree[, file])`

Compile a syntax tree.

👉 Note: stringify freezes the processor if not already frozen.

👉 Note: stringify performs the stringify phase, not the run phase or other phases.

Parameters

tree (Node) — tree to compile
file (Compatible, optional) — file associated with node; any value accepted as x in new VFile(x)

Returns

Textual representation of the tree (Uint8Array or string, see note).

👉 Note: unified typically compiles by serializing: most compilers return string (or Uint8Array). Some compilers, such as the one configured with rehype-react, return other values (in this case, a React tree). If you’re using a compiler that doesn’t serialize, expect different result values.

To register custom results in TypeScript, add them to CompileResultMap.

Example

This example shows how stringify can be used to serialize a syntax tree:

import {h} from 'hastscript'
import rehypeStringify from 'rehype-stringify'
import {unified} from 'unified'

const tree = h('h1', 'Hello world!')

const doc = unified().use(rehypeStringify).stringify(tree)

console.log(doc)

Yields:

<h1>Hello world!</h1>

`processor.use(plugin[, options])`

Configure the processor to use a plugin, a list of usable values, or a preset.

If the processor is already using a plugin, the previous plugin configuration is changed based on the options that are passed in. In other words, the plugin is not added a second time.

👉 Note: use cannot be called on frozen processors. Call the processor first to create a new unfrozen processor.

Signatures

processor.use(preset?)
processor.use(list)
processor.use(plugin[, ...parameters])

Parameters

preset (Preset) — plugins and settings
list (PluggableList) — list of usable things
plugin (Plugin) — plugin
parameters (Array<unknown>) — configuration for plugin, typically a single options object

Returns

Current processor (processor).

Example

There are many ways to pass plugins to .use(). This example gives an overview:

import {unified} from 'unified'

unified()
  // Plugin with options:
  .use(pluginA, {x: true, y: true})
  // Passing the same plugin again merges configuration (to `{x: true, y: false, z: true}`):
  .use(pluginA, {y: false, z: true})
  // Plugins:
  .use([pluginB, pluginC])
  // Two plugins, the second with options:
  .use([pluginD, [pluginE, {}]])
  // Preset with plugins and settings:
  .use({plugins: [pluginF, [pluginG, {}]], settings: {position: false}})
  // Settings only:
  .use({settings: {position: false}})

`CompileResultMap`

Interface of known results from compilers (TypeScript type).

Normally, compilers result in text (Value of vfile). When you compile to something else, such as a React node (as in, rehype-react), you can augment this interface to include that type.

import type {ReactNode} from 'somewhere'

declare module 'unified' {
  interface CompileResultMap {
    // Register a new result (value is used, key should match it).
    ReactNode: ReactNode
  }
}

export {} // You may not need this, but it makes sure the file is a module.

Use CompileResults to access the values.

Type

interface CompileResultMap {
  // Note: if `Value` from `VFile` is changed, this should too.
  Uint8Array: Uint8Array
  string: string
}

`CompileResults`

Acceptable results from compilers (TypeScript type).

To register custom results, add them to CompileResultMap.

Type

type CompileResults = CompileResultMap[keyof CompileResultMap]

`Compiler`

A compiler handles the compiling of a syntax tree to something else (in most cases, text) (TypeScript type).

It is used in the stringify phase and called with a Node and VFile representation of the document to compile.

Compiler can be a normal function, in which case it should return the textual representation of the given tree (typically string).

Compiler can also be a constructor function (a function with a compile field in its prototype), in which case it is constructed with new. Instances must have a compile method that is called without arguments and typically returns a string.

👉 Note: unified typically compiles by serializing: most compilers return string (or Uint8Array). Some compilers, such as the one configured with rehype-react, return other values (in this case, a React tree). If you’re using a compiler that doesn’t serialize, expect different result values.

To register custom results in TypeScript, add them to CompileResultMap.

Type

type Compiler<
  Tree extends Node = Node,
  Result extends CompileResults = CompileResults
> = CompilerClass<Tree, Result> | CompilerFunction<Tree, Result>

See CompilerClass and CompilerFunction for more info.

`CompilerClass`

Class to compile trees (TypeScript type).

Type

type CompilerClass<
  Tree extends Node = Node,
  Result extends CompileResults = CompileResults
> = {
  new (tree: Tree, file: VFile): CompilerClass<Tree, Result>['prototype']
  prototype: {
    compile(): Result
  }
}

`CompilerFunction`

Regular function to compile a tree (TypeScript type).

Type

type CompilerFunction<
  Tree extends Node = Node,
  Result extends CompileResults = CompileResults
> = (tree: Tree, file: VFile) => Result

`Data`

Interface of known data that can be supported by all plugins (TypeScript type).

Typically, options can be given to a specific plugin, but sometimes it makes sense to have information shared with several plugins. For example, a list of HTML elements that are self-closing, which is needed during all phases.

To type this, do something like:

declare module 'unified' {
  interface Data {
    htmlVoidElements?: Array<string> | undefined
  }
}

export {} // You may not need this, but it makes sure the file is a module.

Type

interface Data {
  settings?: Settings | undefined
}

See Settings for more info.

`Parser`

A parser handles the parsing of text to a syntax tree (TypeScript type).

It is used in the parse phase and is called with a string and VFile of the document to parse.

Parser can be a normal function, in which case it must return the syntax tree representation of the given file (Node).

Parser can also be a constructor function (a function with a parse field in its prototype), in which case it is constructed with new. Instances must have a parse method that is called without arguments and must return a Node.

Type

type Parser<Tree extends Node = Node> = ParserClass<Tree> | ParserFunction<Tree>

See ParserClass and ParserFunction for more info.

`ParserClass`

Class to parse files (TypeScript type).

Type

type ParserClass<Tree extends Node = Node> = {
  new (document: string, file: VFile): ParserClass<Tree>['prototype']
  prototype: {
    parse(): Tree
  }
}

`ParserFunction`

Regular function to parse a file (TypeScript type).

Type

type ParserFunction<Tree extends Node = Node> = (document: string, file: VFile) => Tree

`Pluggable`

Union of the different ways to add plugins and settings (TypeScript type).

Type

type Pluggable =
  | Plugin<Array<any>, any, any>
  | PluginTuple<Array<any>, any, any>
  | Preset

See Plugin, PluginTuple, and Preset for more info.

`PluggableList`

List of plugins and presets (TypeScript type).

Type

type PluggableList = Array<Pluggable>

See Pluggable for more info.

`Plugin`

Single plugin (TypeScript type).

Plugins configure the processors they are applied on in the following ways:

they change the processor, such as the parser, the compiler, or by configuring data
they specify how to handle trees and files

In practise, they are functions that can receive options and configure the processor (this).

👉 Note: plugins are called when the processor is frozen, not when they are applied.

Type

type Plugin<
  PluginParameters extends unknown[] = [],
  Input extends Node | string | undefined = Node,
  Output = Input
> = (
  this: Processor,
  ...parameters: PluginParameters
) => Input extends string // Parser.
  ? Output extends Node | undefined
    ? undefined | void
    : never
  : Output extends CompileResults // Compiler.
  ? Input extends Node | undefined
    ? undefined | void
    : never
  : // Inspect/transform.
      | Transformer<
          Input extends Node ? Input : Node,
          Output extends Node ? Output : Node
        >
      | undefined
      | void

See Transformer for more info.

Example

move.js:

/**
 * @typedef Options
 *   Configuration (required).
 * @property {string} extname
 *   File extension to use (must start with `.`).
 */

/** @type {import('unified').Plugin<[Options]>} */
export function move(options) {
  if (!options || !options.extname) {
    throw new Error('Missing `options.extname`')
  }

  return function (_, file) {
    if (file.extname && file.extname !== options.extname) {
      file.extname = options.extname
    }
  }
}

example.md:

# Hello, world!

example.js:

import rehypeStringify from 'rehype-stringify'
import remarkParse from 'remark-parse'
import remarkRehype from 'remark-rehype'
import {read, write} from 'to-vfile'
import {unified} from 'unified'
import {reporter} from 'vfile-reporter'
import {move} from './move.js'

const file = await unified()
  .use(remarkParse)
  .use(remarkRehype)
  .use(move, {extname: '.html'})
  .use(rehypeStringify)
  .process(await read('example.md'))

console.error(reporter(file))
await write(file) // Written to `example.html`.

Yields:

example.md: no issues found

…and in example.html:

<h1>Hello, world!</h1>

`PluginTuple`

Tuple of a plugin and its configuration (TypeScript type).

The first item is a plugin, the rest are its parameters.

Type

type PluginTuple<
  TupleParameters extends unknown[] = [],
  Input extends Node | string | undefined = undefined,
  Output = undefined
> = [
  plugin: Plugin<TupleParameters, Input, Output>,
  ...parameters: TupleParameters
]

See Plugin for more info.

`Preset`

Sharable configuration (TypeScript type).

They can contain plugins and settings.

Fields

plugins (PluggableList, optional) — list of plugins and presets
settings (Data, optional) — shared settings for parsers and compilers

Example

preset.js:

import remarkCommentConfig from 'remark-comment-config'
import remarkLicense from 'remark-license'
import remarkPresetLintConsistent from 'remark-preset-lint-consistent'
import remarkPresetLintRecommended from 'remark-preset-lint-recommended'
import remarkToc from 'remark-toc'

/** @type {import('unified').Preset} */
export const preset = {
  settings: {bullet: '*', emphasis: '*', fences: true},
  plugins: [
    remarkPresetLintRecommended,
    remarkPresetLintConsistent,
    remarkCommentConfig,
    [remarkToc, {maxDepth: 3, tight: true}],
    remarkLicense
  ]
}

example.md:

# Hello, world!

_Emphasis_ and **importance**.

## Table of contents

## API

## License

example.js:

import {remark} from 'remark'
import {read, write} from 'to-vfile'
import {reporter} from 'vfile-reporter'
import {preset} from './preset.js'

const file = await remark()
  .use(preset)
  .process(await read('example.md'))

console.error(reporter(file))
await write(file)

Yields:

example.md: no issues found

example.md now contains:

# Hello, world!

*Emphasis* and **importance**.

## Table of contents

*   [API](#api)
*   [License](#license)

## API

## License

[MIT](license) © [Titus Wormer](https://wooorm.com)

`ProcessCallback`

Callback called when the process is done (TypeScript type).

Called with either an error or a result.

Parameters

error (Error, optional) — fatal error
file (VFile, optional) — processed file

Returns

Nothing (undefined).

Example

This example shows how process can be used to process a file with a callback.

import remarkGithub from 'remark-github'
import remarkParse from 'remark-parse'
import remarkStringify from 'remark-stringify'
import {unified} from 'unified'
import {reporter} from 'vfile-reporter'

unified()
  .use(remarkParse)
  .use(remarkGithub)
  .use(remarkStringify)
  .process('@unifiedjs', function (error, file) {
    if (error) throw error
    if (file) {
      console.error(reporter(file))
      console.log(String(file))
    }
  })

Yields:

no issues found

[**@unifiedjs**](https://github.com/unifiedjs)

`Processor`

Type of a processor (TypeScript type).

`RunCallback`

Callback called when transformers are done (TypeScript type).

Called with either an error or results.

Parameters

error (Error, optional) — fatal error
tree (Node, optional) — transformed tree
file (VFile, optional) — file

Returns

Nothing (undefined).

`Settings`

Interface of known extra options, that can be supported by parser and compilers.

This exists so that users can use packages such as remark, which configure both parsers and compilers (in this case remark-parse and remark-stringify), and still provide options for them.

When you make parsers or compilers, that could be packaged up together, you should support this.data('settings') as input and merge it with explicitly passed options. Then, to type it, using remark-stringify as an example, do something like:

declare module 'unified' {
  interface Settings {
    bullet: '*' | '+' | '-'
    // …
  }
}

export {} // You may not need this, but it makes sure the file is a module.

Type

interface Settings {}

`TransformCallback`

Callback passed to transforms (TypeScript type).

If the signature of a transformer accepts a third argument, the transformer may perform asynchronous operations, and must call it.

Parameters

error (Error, optional) — fatal error to stop the process
tree (Node, optional) — new, changed, tree
file (VFile, optional) — new, changed, file

Returns

Nothing (undefined).

`Transformer`

Transformers handle syntax trees and files (TypeScript type).

They are functions that are called each time a syntax tree and file are passed through the run phase. When an error occurs in them (either because it’s thrown, returned, rejected, or passed to next), the process stops.

The run phase is handled by trough, see its documentation for the exact semantics of these functions.

👉 Note: you should likely ignore next: don’t accept it. it supports callback-style async work. But promises are likely easier to reason about.

Type

type Transformer<
  Input extends Node = Node,
  Output extends Node = Input
> = (
  tree: Input,
  file: VFile,
  next: TransformCallback<Output>
) =>
  | Promise<Output | undefined>
  | Output
  | Error
  | undefined

Types

This package is fully typed with TypeScript. It exports the additional types CompileResultMap, CompileResults, Compiler, CompilerClass, CompilerFunction, Data, Parser, ParserClass, ParserFunction, Pluggable, PluggableList, Plugin, PluginTuple, Preset, ProcessCallback, Processor, RunCallback, Settings, TransformCallback, and Transformer

For TypeScript to work, it is particularly important to type your plugins correctly. We strongly recommend using the Plugin type with its generics and to use the node types for the syntax trees provided by our packages (as in, @types/hast, @types/mdast, @types/nlcst).

/**
 * @typedef {import('hast').Root} HastRoot
 * @typedef {import('mdast').Root} MdastRoot
 */

/**
 * @typedef Options
 *   Configuration (optional).
 * @property {boolean | null | undefined} [someField]
 *   Some option (optional).
 */

// To type options:
/** @type {import('unified').Plugin<[(Options | null | undefined)?]>} */
export function myPluginAcceptingOptions(options) {
  const settings = options || {}
  // `settings` is now `Options`.
}

// To type a plugin that works on a certain tree, without options:
/** @type {import('unified').Plugin<[], MdastRoot>} */
export function myRemarkPlugin() {
  return function (tree, file) {
    // `tree` is `MdastRoot`.
  }
}

// To type a plugin that transforms one tree into another:
/** @type {import('unified').Plugin<[], MdastRoot, HastRoot>} */
export function remarkRehype() {
  return function (tree) {
    // `tree` is `MdastRoot`.
    // Result must be `HastRoot`.
  }
}

// To type a plugin that defines a parser:
/** @type {import('unified').Plugin<[], string, MdastRoot>} */
export function remarkParse(options) {}

// To type a plugin that defines a compiler:
/** @type {import('unified').Plugin<[], HastRoot, string>} */
export function rehypeStringify(options) {}

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, unified@^10, compatible with Node.js 12.

Contribute

See contributing.md in unifiedjs/.github for ways to get started. See support.md for ways to get help.

This project has a code of conduct. By interacting with this repository, organization, or community you agree to abide by its terms.

For info on how to submit a security report, see our security policy.

Sponsor

Support this effort and give back by sponsoring on OpenCollective!

Vercel	Motif	HashiCorp		American Express		GitBook

Gatsby	Netlify	Coinbase	ThemeIsle	Expo	Boost Note	Markdown Space	Holloway
		You?

Acknowledgments

Preliminary work for unified was done in 2014 for retext and inspired by ware. Further incubation happened in remark. The project was finally externalised in 2015 and published as unified. The project was authored by @wooorm.

Although unified since moved its plugin architecture to trough, thanks to @calvinfo, @ianstormtaylor, and others for their work on ware, as it was a huge initial inspiration.

License

MIT © Titus Wormer

License

kkpan11/unified

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