The unified npm package is an interface for parsing, inspecting, transforming, and serializing content through syntax trees. It is built on the concept of syntax trees and is often used to work with content in markdown, HTML, and plain text formats. Unified is part of the unified collective which provides a range of plugins and utilities for content processing.
Parsing Markdown to Syntax Trees
This feature allows you to parse Markdown content into an abstract syntax tree (AST) using the remark-parse plugin.
const unified = require('unified');
const markdown = require('remark-parse');
const processor = unified().use(markdown);
const tree = processor.parse('# Hello world');
console.log(tree);Transforming Syntax Trees
This feature demonstrates transforming a Markdown AST to an HTML AST and then serializing it to an HTML string.
const unified = require('unified');
const markdown = require('remark-parse');
const remark2rehype = require('remark-rehype');
const html = require('rehype-stringify');
const processor = unified()
.use(markdown)
.use(remark2rehype)
.use(html);
const file = processor.processSync('# Hello world');
console.log(String(file));Linting and Validating Markdown
This feature shows how to use unified with remark-lint to lint and validate Markdown content.
const unified = require('unified');
const markdown = require('remark-parse');
const remarkLint = require('remark-lint');
const processor = unified()
.use(markdown)
.use(remarkLint);
processor.process('# Hello world', function (err, file) {
console.error(report(err || file));
});Compiling Markdown to HTML
This feature illustrates compiling Markdown to a fully formatted HTML document.
const unified = require('unified');
const markdown = require('remark-parse');
const remark2rehype = require('remark-rehype');
const doc = require('rehype-document');
const format = require('rehype-format');
const html = require('rehype-stringify');
const processor = unified()
.use(markdown)
.use(remark2rehype)
.use(doc)
.use(format)
.use(html);
const file = processor.processSync('# Hello world');
console.log(String(file));markdown-it is a Markdown parser with a focus on speed and extensibility. It is similar to unified in that it can parse and render Markdown, but it does not use an AST or provide the same plugin ecosystem.
remarkable is another Markdown parser and renderer. It offers a similar feature set to markdown-it but also does not use an AST or have the extensive plugin system that unified offers.
showdown is a JavaScript Markdown to HTML converter. It is similar to unified in that it converts Markdown to HTML, but it does not provide a unified interface for parsing, transforming, and serializing content.
unified is an interface for processing text using syntax trees. It’s what powers remark, retext, and rehype, but it also allows for processing between multiple syntaxes.
unifiedjs.github.io, the website for unified provides a less
technical, more practical, introduction to unified. Make sure to visit it
and try its introductionary Guides.
npm:
npm install unified
var unified = require('unified');
var markdown = require('remark-parse');
var remark2rehype = require('remark-rehype');
var doc = require('rehype-document');
var format = require('rehype-format');
var html = require('rehype-stringify');
var reporter = require('vfile-reporter');
unified()
.use(markdown)
.use(remark2rehype)
.use(doc)
.use(format)
.use(html)
.process('# Hello world!', function (err, file) {
console.error(reporter(err || file));
console.log(String(file));
});
Yields:
no issues found
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
</head>
<body>
<h1>Hello world!</h1>
</body>
</html>
unified is an interface for processing text using syntax trees. Syntax trees are a representation understandable to programs. Those programs, called plug-ins, take these trees and modify them, amongst other things. To get to the syntax tree from input text, there’s a parser, and, to get from that back to text, there’s a compiler. This is the process of a processor.
┌──────────────┐
┌─ │ Transformers │ ─┐
▲ └──────────────┘ ▼
└────────┐ ┌────────┘
│ │
┌────────┐ │ │ ┌──────────┐
Input ──▶ │ Parser │ ──▶ Tree ──▶ │ Compiler │ ──▶ Output
└────────┘ └──────────┘
Every processor implements another processor. To create a new processor, invoke another processor. This creates a processor that is configured to function the same as its ancestor. But, when the descendant processor is configured in the future, that configuration does not change the ancestral processor.
Often, when processors are exposed from a library (for example, unified itself), they should not be configured directly, as that would change their behaviour for all users. Those processors are frozen, and new processors should be made from them before they are used, by invoking them.
The syntax trees used in unified are Unist nodes,
which are plain JavaScript objects with a type property. The
semantics of those types are defined by other projects.
There are several utilities for working with these nodes.
The following projects process different syntax trees. They parse text to their respective syntax tree, and they compile their syntax trees back to text. These processors can be used as-is, or their parsers and compilers can be mixed and matched with other plug-ins to process between different syntaxes.
When processing documents, metadata is often gathered about that document. VFile is a virtual file format which stores data, and handles metadata and messages for unified and its plug-ins.
There are several utilities for working with these files.
To configure a processor, invoke its use method, supply it a
plug-in, and optionally settings.
unified can integrate with the file-system through unified-engine. On top of that, CLI apps can be created with unified-args, Gulp plug-ins with unified-engine-gulp, and Atom Linters with unified-engine-atom.
The API gives access to processing metadata (such as lint messages), and supports multiple passed through files:
var unified = require('unified');
var markdown = require('remark-parse');
var styleGuide = require('remark-preset-lint-markdown-style-guide');
var remark2retext = require('remark-retext');
var english = require('retext-english');
var equality = require('retext-equality');
var remark2rehype = require('remark-rehype');
var html = require('rehype-stringify');
var reporter = require('vfile-reporter');
unified()
.use(markdown)
.use(styleGuide)
.use(remark2retext, unified().use(english).use(equality))
.use(remark2rehype)
.use(html)
.process('*Emphasis* and _importance_, you guys!', function (err, file) {
console.error(reporter(err || file));
console.log(String(file));
});
Which yields:
1:16-1:28 warning Emphasis should use `*` as a marker emphasis-marker remark-lint
1:34-1:38 warning `guys` may be insensitive, use `people`, `persons`, `folks` instead gals-men retext-equality
⚠ 2 warnings
<p><em>Emphasis</em> and <em>importance</em>, you guys!</p>
The processors can be combined in two modes.
Bridge mode transforms the syntax tree from one flavour (the origin) to another (the destination). Then, transformations are applied on that tree. Finally, the origin processor continues transforming the original syntax tree.
Mutate mode also transforms the syntax tree from one flavour to another. But then the origin processor continues transforming the destination syntax tree.
In the previous example (“Programming interface”), remark-retext is
used in bridge mode: the origin syntax tree is kept after retext is
finished; whereas remark-rehype is used in mutate mode: it sets a
new syntax tree and discards the original.
processor()Object describing how to process text.
Function — A new unfrozen processor which is
configured to function the same as its ancestor. But, when the
descendant processor is configured in the future, that configuration
does not change the ancestral processor.
The following example shows how a new processor can be created (from the remark processor) and linked to stdin(4) and stdout(4).
var remark = require('remark');
var concat = require('concat-stream');
process.stdin.pipe(concat(function (buf) {
process.stdout.write(remark().processSync(buf).toString());
}));
processor.use(plugin[, options])Configure the processor to use a plug-in, and configure that plug-in with optional options.
processor.use(plugin[, options])processor.use(preset)processor.use(list)plugin (Plugin)options (*, optional) — Configuration for pluginpreset (Object) — Object with an optional plugins (set to list),
and/or an optional settings objectlist (Array) — plugins, presets, and arguments (a plugin and options
in an array), in an arrayprocessor — The processor on which use is invoked.
use cannot be called on frozen processors. Invoke the processor
first to create a new unfrozen processor.
There are many ways to pass plugins to .use(). The below example
gives an overview.
var unified = require('unified');
unified()
// Plugin with options:
.use(plugin, {})
// Plugins:
.use([plugin, pluginB])
// Two plugins, the second with options:
.use([plugin, [pluginB, {}]])
// Preset with plugins and settings:
.use({plugins: [plugin, [pluginB, {}]], settings: {position: false}})
// Settings only:
.use({settings: {position: false}});
function plugin() {}
function pluginB() {}
processor.parse(file|value)Parse text to a syntax tree.
file (VFile)
— Or anything which can be given to vfile()Node — Syntax tree representation of input.
parse freezes the processor, if not already frozen.
processor.ParserFunction handling the parsing of text to a syntax tree. Used in the
parse phase in the process and invoked with a string
and VFile representation of the document to parse.
If Parser is a normal parser, it should return a Node: the syntax
tree representation of the given file.
Parser can also be a constructor function, in which case it’s invoked with
new. In that case, instances should have a parse method, which is invoked
(without arguments), and should return a Node.
processor.stringify(node[, file])Compile a syntax tree to text.
string — String representation of the syntax tree file.
stringify freezes the processor, if not already frozen.
processor.CompilerFunction handling the compilation of syntax tree to a text. Used in the
stringify phase in the process and invoked with a
Node and VFile representation of the document to
stringify.
If Compiler is a normal stringifier, it should return a string: the text
representation of the given syntax tree.
Compiler can also be a constructor function, in which case it’s invoked with
new. In that case, instances should have a compile method, which is invoked
(without arguments), and should return a string.
processor.run(node[, file][, done])Transform a syntax tree by applying plug-ins to it.
node (Node)file (VFile, optional)
— Or anything which can be given to vfile()done (Function, optional)Promise, if done is not given. Rejected with an error,
or resolved with the resulting syntax tree.
run freezes the processor, if not already frozen.
function done(err[, node, file])Invoked when transformation is complete. Either invoked with an error, or a syntax tree and a file.
processor.runSync(node[, file])Transform a syntax tree by applying plug-ins to it.
If asynchronous plug-ins are configured, an error is thrown.
Node — The given syntax tree.
runSync freezes the processor, if not already frozen.
processor.process(file|value[, done])Process the given representation of a file as configured on the
processor. The process invokes parse, run, and stringify
internally.
Promise, if done is not given. Rejected with an error,
or resolved with the resulting file.
process freezes the processor, if not already frozen.
function done(err, file)Invoked when the process is complete. Invoked with a fatal error, if any, and the VFile.
err (Error, optional) — Fatal errorfile (VFile)var unified = require('unified');
var markdown = require('remark-parse');
var remark2rehype = require('remark-rehype');
var doc = require('rehype-document');
var format = require('rehype-format');
var html = require('rehype-stringify');
var reporter = require('vfile-reporter');
unified()
.use(markdown)
.use(remark2rehype)
.use(doc)
.use(format)
.use(html)
.process('# Hello world!')
.then(function (file) {
console.log(String(file));
}, function (err) {
console.error(String(err));
});
Yields:
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
</head>
<body>
<h1>Hello world!</h1>
</body>
</html>
processor.processSync(file|value)Process the given representation of a file as configured on the
processor. The process invokes parse, run, and stringify
internally.
If asynchronous plug-ins are configured, an error is thrown.
file (VFile)value (string) — String representation of a fileVFile — Virtual file with modified contents.
processSync freezes the processor, if not already frozen.
var unified = require('unified');
var markdown = require('remark-parse');
var remark2rehype = require('remark-rehype');
var doc = require('rehype-document');
var format = require('rehype-format');
var html = require('rehype-stringify');
var reporter = require('vfile-reporter');
var processor = unified()
.use(markdown)
.use(remark2rehype)
.use(doc)
.use(format)
.use(html);
console.log(processor.processSync('# Hello world!').toString());
Yields:
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
</head>
<body>
<h1>Hello world!</h1>
</body>
</html>
processor.data(key[, value])Get or set information in an in-memory key-value store accessible to all phases of the process. An example is a list of HTML elements which are self-closing (i.e., do not need a closing tag), which is needed when parsing, transforming, and compiling HTML.
key (string) — Identifiervalue (*, optional) — Value to set. Omit if getting keyprocessor — If setting, the processor on which data is invoked* — If getting, the value at keySetting information with data cannot occur on frozen processors.
Invoke the processor first to create a new unfrozen processor.
The following example show how to get and set information:
var unified = require('unified');
console.log(unified().data('alpha', 'bravo').data('alpha'))
Yields:
bravo
processor.freeze()Freeze a processor. Frozen processors are meant to be extended, and not to be configured or processed directly.
Once a processor is frozen, it cannot be unfrozen. But, a new processor functioning just like it can be created by invoking the processor.
It’s possible to freeze processors explicitly, by calling .freeze(), but
.parse(), .run(), .stringify(), and
.process() call .freeze() to freeze a processor too.
Processor — The processor on which freeze is invoked.
The following example, index.js, shows how rehype
prevents extensions to itself:
var unified = require('unified');
var parse = require('rehype-parse');
var stringify = require('rehype-stringify');
module.exports = unified().use(parse).use(stringify).freeze();
The below example, a.js, shows how that processor can be used and
configured.
var rehype = require('rehype');
var format = require('rehype-format');
// ...
rehype()
.use(format)
// ...
The below example, b.js, shows a similar looking example which
operates on the frozen rehype interface. If this
behaviour was allowed it would result in unexpected behaviour, so
an error is thrown. This is invalid:
var rehype = require('rehype');
var format = require('rehype-format');
// ...
rehype
.use(format)
// ...
Yields:
~/node_modules/unified/index.js:440
throw new Error(
^
Error: Cannot invoke `use` on a frozen processor.
Create a new processor first, by invoking it: use `processor()` instead of `processor`.
at assertUnfrozen (~/node_modules/unified/index.js:440:11)
at Function.use (~/node_modules/unified/index.js:172:5)
at Object.<anonymous> (~/b.js:6:4)
PluginA unified plugin changes the way the applied-on processor works, in the following ways:
Plug-in’s are a concept which materialise as attachers.
move.js:
module.exports = move;
function move(options) {
var expected = (options || {}).extname;
if (!expected) {
throw new Error('Missing `extname` in options');
}
return transformer;
function transformer(tree, file) {
if (file.extname && file.extname !== expected) {
file.extname = expected;
}
}
}
index.js:
var unified = require('unified');
var parse = require('remark-parse');
var remark2rehype = require('remark-rehype');
var stringify = require('rehype-stringify');
var vfile = require('to-vfile');
var reporter = require('vfile-reporter');
var move = require('./move');
unified()
.use(parse)
.use(remark2rehype)
.use(move, {extname: '.html'})
.use(stringify)
.process(vfile.readSync('index.md'), function (err, file) {
console.error(reporter(err || file));
if (file) {
vfile.writeSync(file); // Written to `index.html`.
}
});
function attacher([options])An attacher is the thing passed to use. It configures the
processor and in turn can receive options.
Attachers can configure processors, such as by interacting with parsers and compilers, linking them to other processors, or by specifying how the syntax tree is handled.
The context object is set to the invoked on processor.
options (*, optional) — Configurationtransformer — Optional.
Attachers are invoked when the processor is frozen: either when
.freeze() is called explicitly, or when .parse(), .run(),
.stringify(), or .process() is called for the first
time.
function transformer(node, file[, next])Transformers modify the syntax tree or metadata of a file.
A transformer is a function which is invoked each time a file is
passed through the transform phase. If an error occurs (either
because it’s thrown, returned, rejected, or passed to next),
the process stops.
The transformation process in unified is handled by trough,
see it’s documentation for the exact semantics of transformers.
Error — Can be returned to stop the processstringifys to be performed on the new
treePromise — If a promise is returned, the function is asynchronous,
and must be resolved (optionally with a Node) or
rejected (optionally with an Error)function next(err[, tree[, file]])If the signature of a transformer includes next (third argument),
the function may finish asynchronous, and must invoke next().
err (Error, optional) — Stop the processnode (Node, optional) — New syntax treefile (VFile, optional) — New virtual filePresetA unified preset provides a potentially sharable way to configure processors. They can contain multiple plugins and optionally settings as well.
preset.js:
exports.settings = {bullet: '*', fences: true};
exports.plugins = [
require('remark-preset-lint-recommended'),
require('remark-comment-config'),
require('remark-preset-lint-markdown-style-guide'),
[require('remark-toc'), {maxDepth: 3, tight: true}],
require('remark-github')
];
index.js:
var remark = require('remark');
var vfile = require('to-vfile');
var reporter = require('vfile-reporter');
var preset = require('./preset');
remark()
.use(preset)
.process(vfile.readSync('index.md'), function (err, file) {
console.error(reporter(err || file));
if (file) {
vfile.writeSync(file);
}
});
FAQs
parse, inspect, transform, and serialize content through syntax trees
The npm package unified receives a total of 8,784,793 weekly downloads. As such, unified popularity was classified as popular.
We found that unified demonstrated a healthy version release cadence and project activity because the last version was released less than a year ago. It has 2 open source maintainers collaborating on the project.
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